Dissociative photoionization of N 2 O in the region of the N 2 O + (B 2 Π) state studiedby ion–electron velocity vector correlationM. Lebech, J. C. Houver, D. Dowek, and R. R. Lucchese Citation: The Journal of Chemical Physics 120, 8226 (2004); doi: 10.1063/1.1651087 View online: http://dx.doi.org/10.1063/1.1651087 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/120/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The role of Rydberg states in photoionization of NO2 and (NO+, O−) ion pair formation induced by one VUVphoton J. Chem. Phys. 139, 044311 (2013); 10.1063/1.4811713 NO+ formation pathways in dissociation of N2O+ ions at the C2Σ+ state revealed from thresholdphotoelectron–photoion coincidence velocity imaging J. Chem. Phys. 134, 054312 (2011); 10.1063/1.3549130 Valence and inner-valence shell dissociative photoionization of CO in the 26–33 eV range. I. Ion-electron kineticenergy correlation and laboratory frame photoemission J. Chem. Phys. 130, 194307 (2009); 10.1063/1.3125223 Dissociative photoionization of N 2 O in the region of the N 2 O + (C 2 Σ + ) state, studied by ion–electronvelocity vector correlation J. Chem. Phys. 117, 9248 (2002); 10.1063/1.1515765 Ion–electron velocity vector correlations in dissociative photoionization of simple molecules using electrostaticlenses Rev. Sci. Instrum. 73, 1866 (2002); 10.1063/1.1458063

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Dissociative photoionization of N 2O in the region of the N 2O¿„B 2P… state

studied by ion–electron velocity vector correlationM. Lebech, J. C. Houver, and D. Doweka)

Laboratoire des Collisions Atomiques et Mole´culaires (L.C.A.M., UMR Universite´ Paris Sud et CNRS,No. 8625), Baˆtiment 351, Universite´ Paris Sud, F-91405 Orsay Cedex, France

R. R. LuccheseDepartment of Chemistry, Texas A&M University, College Station, Texas 77843-3255

~Received 7 August 2003; accepted 7 January 2004!

Dissociative direct photoionization of the N2O(X 1S1) linear molecule via the N2O1(B 2P) ionicstate induced by linearly polarized synchrotron radiationP in the 18–22 eV photon energy range isinvestigated using the (VA1 ,Ve,P) vector correlation method, whereVA1 is the nascent velocityvector of the NO1, N2

1 , or O1 ionic fragment andVe that of the photoelectron. The DPI processesare identified by the ion–electron kinetic energy correlation, and theIxA1(ue ,fe) molecular framephotoelectron angular distributions~MFPADs! are reported for the dominant reaction leading toNO1(X 1S1,v)1N(2D)1e. The measured MFPADs are found in satisfactory agreement with thereported multichannel Schwinger configuration interaction calculations, when bending of theN2O1(B 2P) molecular ion prior to dissociation is taken into account. A significant evolution of theelectron scattering anisotropies is observed, in particular in the azimuthal dependence of theMFPADs, characteristic of a photoionization transition between a neutral state ofS symmetry andan ionic state ofP symmetry. This interpretation is supported by a simple model describing thephotoionization transition by the coherent superposition of twoss and dd partial waves and theassociated Coulomb phases. ©2004 American Institute of Physics.@DOI: 10.1063/1.1651087#

I. INTRODUCTION

This paper is the second in a series devoted to the inves-tigation of dissociative photoionization~DPI! of small poly-atomic molecules induced by linearly polarized vacuum ul-traviolet ~VUV ! synchrotron radiation~P! leading to twoheavy fragments and one electron, using the (VA1 ,Ve,P)vector correlation method.1–3 In the first paper,4 referred toas paper I, the method was illustrated by the study of DPI ofthe N2O linear molecule via the N2O1(C 2S1) ionic state,corresponding to ionization of the 6s orbital in the(4s)2(5s)2(6s)2(1p)4(7s)2(2p)4 valence electronicconfiguration of the N2O(X 1S1) ground state,5 in the 20–22.5 eV photon energy range. In this paper, we report theresults of the vector correlation study of DPI of the N2Omolecule involving photoionization into the N2O1(B 2P)valence state, corresponding to ionization of the 1p orbital. Anumber of results related to dissociative photoionization ofN2O involving the N2O1(B 2P) state have been reportedpreviously, as referred to below. Here we focus on the newinformation about the DPI processes emphasized by the vec-tor correlation approach extracted from~i! the ion–electronkinetic energy correlation, in terms of reaction pathways andbranching ratios between the DPI processes on one side, and~ii ! the complete angular analysis, leading to theIx(ue ,fe)molecular frame photoelectron angular distributions~MFPADs! for any orientationx of the molecular axis withrespect to the polarization axis of the exciting light~P! for

each identified process, on the other side. They are obtainedusing the following general form of theI (x,ue ,fe) distribu-tion derived for linear molecules ionized by linearly polar-ized light,2 as discussed in paper I:

I ~x,ue ,fe!5F00~ue!1F20~ue!P20~cosx!

1F21~ue!P21~cosx!cos~fe!

1F22~ue!P22~cosx!cos~2fe!. ~1!

The molecular orientationx with respect to the polarizationaxis is identical to thexA1 polar angle of the ion fragmentvelocity VA1 within the frame of the axial recoilapproximation,6 and (ue ,fe) characterizes the photoelectronemission direction in the molecular frame.

The measured MFPADs are compared with multichannelSchwinger configuration interaction calculations~MCSCI!for photoionization to the N2O1(B 2P) state. This detailedcomparison leads us to discuss the influence of bending ofthe N2O1 ion occuring prior to dissociation. For that pur-pose, the effect of bending is introduced in the general de-scription of the MFPADs in Eq.~1!.

Contrary to the N2O1(X 2P,A 2S1,C 2S1) ionic va-lence states, whose high resolution vibrational distributionsare dominated by the~0,0,0! level and display a very weakexcitation of the~0,1,0! bending mode implying a quasi lin-ear geometry of the molecular ion, the N2O1(B 2P) ionicstate analyzed by photoelectron spectroscopy5,7 gives rise toa complicated pattern of bands extending over 1.5 eV whichis not readily analyzed in terms of simple vibrational pro-gressions. This spectrum has been interpreted as resultinga!Electronic mail: dowek@lcam.u-psud.fr

JOURNAL OF CHEMICAL PHYSICS VOLUME 120, NUMBER 17 1 MAY 2004

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from the vibronic coupling of two electronic states ofPsymmetry, being the in-phase and out-of-phase linear combi-nation of the (1p)21 single-hole and a (2p)22(3p) pseudo-state, through totally symmetric vibrational modes.8 Thecomplexity of the vibronic structure persists when theB bandis investigated by two-photon absorption spectroscopy ofN2O1 ions with a two orders of magnitude higherresolution,9 in line with the predictions of the above men-tioned theoretical description. In this study, the observable isthe NO1 ion yield produced by dissociation of theN2O1(B 2P) state, and the measured peak width corre-sponds to a lifetime of 3310214s. This value, much shorterthan the rotational period of the N2O molecule, indicates thatno significant rotation of the molecular ion occurs prior todissociation and rules out rotation as a source of breakdownof the axial recoil approximation for the dissociation of theN2O1(B 2P) state.

Early high resolution photoionization mass spectro-scopy10 on normal and isotopically substituted N2O has pro-vided a first insight into the mechanisms of photoionizationand ion dissociation involving the N2O1(B 2P) state. TheNO1, N2

1 , O1, and N2O1 photoion yields measured asfunction of photon energy indicate the existence of the fivedipole allowed Rydberg series converging on theC 2S1

state, superimposed on the direct ionization continuum lead-ing to the N2O1(B 2P) state distribution, consistent withphotoabsorption spectroscopy11,12 and observation of neutraldissociation.13 The presence of these resonances is anothercharacteristic of the N2O1(B 2P) state with respect to, e.g.,the N2O1(C 2S1) state. The peaks observed in the N2O1

ion yield are assigned mainly to thends andndp/nss Ry-dberg series and are attributed to autoionization to theN2O1(X 2P) ground state, whereas the peaks in the NO1

and N21 ion yields, assigned dominantly to thendp/nss

series, correspond to autoionization to the N2O1(B 2P)ionic state. The attributions of the autoionizing decay routesof the Rydberg series converging on theC 2S1 state involv-ing ionization to the N2O1 (X 2P, A 2S1, andB 2P) statesare confirmed by two-dimensional photoelectron spectros-copy investigations,14 where the electron yield is measuredas a function of both electron and photon energy. Such spec-tra show that autoionization of the Rydberg states convergingon theC 2S1 state, lying in the same enrgy range as theBstate, proceeds mainly following the FC distribution. An ad-ditional structure observed by threshold photoelectron–photoion coincidence~TPEPICO! spectroscopy15 just belowthe C 2S1 state has been attributed to autoionization statesconverging on theC state.

Outside these resonances theB state is completely pre-dissociated producing NO1 and N2

1 fragments, whosebranching ratios have been determined, and to a very smallamount O1.10,16,17The analysis of the NO1 photoion yieldby Berkowitz and Eland10 includes the determination of the15NO1 –14NO1 ratio when a14N 15NO sample was used, in-terpreted as a signature of the relative importance of directscission of the N–N bond with respect to a situation where‘‘scrambling’’ between the two N atoms occurs. An increaseof this ratio across theB state was observed, which mayreflect an increasing reaction rate with increasing energy.10

The role of bending in the dissociation process of theN2O1(B 2P) state is also invoked in a recent theoreticalstudy of the predissociation mechanisms of theA 2S1 andB 2P states of N2O1.18 Investigations of the kinetic energydistribution or kinetic energy releases of the photoions inTPEPICO studies,16 mass spectrometry of kinetic energy se-lected ions,19 or PEPICO spectroscopy at fixedwavelengths20 have lead to the attribution of dissociationlimits populated by DPI via theB state. To our knowledge,the previously reported angular information about these DPIprocesses concern the angle resolved photoelectron spectros-copy of N2O in the 14–70 eV photon energy range21 whichshows that thebe asymmetry parameter for ionization of the1p orbital increases significantly with the ionization energy,varying from 0.08 to 0.32 in the 18–21 eV presently ex-plored photon energy range. For electron emission with re-spect to the molecular axis, the PEPICO results of Kinmondet al.20 do not show any forward–backward asymmetry be-tween the electron and fragment ion ejection directions.

In this paper we report results obtained in the investiga-tion of DPI in the region of the N2O1(B 2P) state performedat the series of photon energies:hn518.04, 18.45, 19, 19.5,20, and 21.2 eV, where our purpose is to focus on the directphotoionization via theB state. Among these energies,hn519.5 eV is close to the excitation energy of the state as-signed as (5ds) which only weakly autoionizes to theBstate, andhn520 eV is at resonance with the structure ob-served in TPEPICO near the threshold of theC state;15 theother energies are not resonant with members of the Rydbergseries. Therefore, the reported results for the angular analysisof DPI involving photoionization via theB state mainly con-cern direct ionization to theB state. Dissociative photoion-ization involving excitation of some of the Rydberg stateswill be discussed in a forthcoming publication.

II. EXPERIMENT

The double velocity spectrometer, which combines theion and electron time-of-flight resolved coincidence tech-nique with position-sensitive detection yielding a 4p collec-tion of ions and electrons, has been described in detailpreviously.4,22,23For each DPI event and each photon energy,the time of flight~TOF! and position of both ion and electronare recorded; thus the three components of the ionVA1 andelectronVe initial velocity vectors are determined. Briefly,the interaction region is defined at the intersection of thesupersonic molecular beam provided by the SAPHIRSset-up24 and the linearly polarized and pulsed light beamdelivered by the SA63 and/or SU5 beamline at Super-ACOoperated in the two-bunch mode. The rotational temperatureof the molecular beam is estimated to be about 35 K. ThePlinear polarization axis is parallel to the molecular beam. Theions and electrons are extracted from the interaction regionby a dc uniform electric fieldE, and guided to their respec-tive position sensitive detectors through an intermediate re-gion where two focusing electrostatic lens sets (L1 andLe)can be applied. Depending on the kinetic energy of the ionand electron fragmentsL1 , Le , or both lens sets are used.The standard settings of each electrostatic lens are defined by

8227J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Dissociative photoionization of N2O

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(VG15U; VE1520/93U; VE250; VT55/93U), whereVG15U, VE1 , VE2 , andVT are the voltages applied on theG1 final electrode of the extraction region, and theE1 , E2 ,and T electrodes which produce the nonuniform focusingelectrostatic field.23 As discussed previously22,23 each set ofextraction conditions corresponds to instrumental widthswhich govern the uncertainty in the velocity components forboth particles, and subsequently the energy and angular reso-lutions. However, for most of the results presented in thispaper the extraction conditions were such that the effect ofthe instrumental widths, controlled by using Monte Carlosimulations of the experiment, is small and will not be dis-cussed.

III. COMPUTATIONAL METHODS

The FLN(ue) functions for ionization leading to theB 2P state of N2O1 were computed using exactly the samecalculation that is discussed in detail in our study of ioniza-tion leading to theC 2S1 state of N2O1.4 Very briefly, weuse the multichannel Schwinger configuration interactionmethod~MCSCI!,25 in which the initial and final bound statewave functions are described using configuration interaction~CI! wave functions. The orbitals used in the CI wave func-tions are the natural orbitals obtained from a complete-active-space self-consistent-field~CASSCF! calculation ofthe ground state of the N2O molecule. With a maximuml inthe single center expansion ofl max580, we found verticalionization potentials of 12.80 eV for theX 2P state, 16.45eV for theA 2S1 state, 18.55 eV for theB 2P, and 20.65 eVfor theC 2S1 state. The expansion of the photoionized con-tinuum state included these four ion states and a low-lyingion state of2P symmetry that had an ionization potential~IP! of 20.91 eV. The maximum of theB 2P state distribu-tion standing at 18.2 eV,7 we consider this value as represen-tative of its effective ionization potential for theB 2P ofN2O1; thus to directly compare experimental and computedcross sections on the basis of equal photoelectron kineticenergy, one would need to shift the photon energy of thecomputed cross section up by 0.35 eV, i.e., the experimentalresults at 18.5 eV should be compared to the computed crosssections at 18.85 eV. Note, however, that a discrepancy of afew tenths of an eV in the photoelectron energy in the ex-periment and the calculations does not have a significanteffect on the computed MFPADs. Finally, the comparisonmay be refined taking into account the extent of the B statein the FC region over 1 eV, by comparing the MFPADs mea-sured at one given photon energy with the proper superposi-tion of energies in the calculation. The generally good agree-ment between computed and measured MFPADs found inour earlier study of photoionization leading to theC 2S1

state of N2O1, when the effects of rotation were included inthe calculation, leads us to expect a similar level of agree-ment between computed and measured MFPADs for theB 2P state. There is, however, the additional complication inthe B 2P state in that the measured MFPADs may be af-fected by vibronic coupling in the ion state. Such vibroniceffects in the photoionization of N2O have not been includedin our calculations.

IV. KINETIC ENERGY CORRELATION DIAGRAMS

Figure 1 displays the sets of (EA1 ,Ee) kinetic energycorrelation diagrams~KECDs! corresponding to a photon ex-citation energyhn518.45, 19.5, 20, and 21.2 eV, respec-tively. As in paper I, the (EA1 ,Ee) representation is pre-ferred to the alternative possibility of plotting the distributionof events as function of the center-of-mass kinetic energyrelease of the two heavy fragments~KER! and the electronenergy, because it enables a direct evaluation of the actualkinetic energy of each ionic fragment: The (EA1 ,Ee) pre-sentation is then more informative from the perspective ofthe subsequent angular analysis which requires a sufficientkinetic energy of both the ionic fragment and the electron.The magnitude of the extraction fieldE and theLe electrontrajectory focusing lens polarization, specified in the figurecaption for each photon energy, ensure a complete collectionof ionic fragments and electrons for DPI involving ionizationinto the N2O1(B 2P) state. As previously observed4,10,20

predissociation of the B state leads mainly to NO1 and N21

ionic fragments, a weak contribution of O1 being foundwhen ionization takes place to the upper part of theB state.

The (NO1,e) coincident event production, which domi-nates DPI in this energy range, is characterized by a broadpeak in the (ENO1 ,Ee) KECD, PB

NO1 , whoseEe electronenergy distribution fits quite well that deduced from the pho-toelectron spectra.7 The ENO1 ion fragment distribution issmooth: It extends from 0 to 0.5 eV with a maximum aroundENO1'0.18 eV kinetic energy, and remains fixed when thephoton energy varies. This value is consistent with the KERof 0.6 eV measured in PEPICO spectroscopy,20 however, thepresent results show a rather stable behavior along the Bstate, and do not feature any bimodal structure in the KERdistribution as suggested in Ref. 20. The straight lines in theKECDs indicate the position of the dissociation limits, asdefined in paper I: We label asLD,v

A1 the straight line whichcorresponds to the production of the A1 ion, where the indexD numbers the dissociation limit starting from the lowest one(D51) andv labels the vibrational level of the molecularfragment A or B, when specified. A set ofED(v,J) valuescharacterizes a series of dissociation limits of given elec-tronic energy, corresponding to the vibrational and/or rota-tional internal states of the molecular fragment. In the fol-lowing we consider only the vibrational distribution, sincedifferent rotational states of the molecular fragments cannotbe resolved in the current experiment. The relevant dissocia-tion channels are listed in Table I. The characteristics ofPB

NO1 lead to the following identification of the DPI process:

N2O~X 1S1!1hn→N2O1~B 2P!1e

→NO1~X 1S1,v !1N~2D !1e~L2,vNO1!,

~2!

where the vibrational distribution of the NO1(X 1S1,v)state extends fromv50 to v56, with a maximum corre-sponding tov54. Owing to the position of the L3

NO1 disso-ciation limit line, going through the center ofPB

NO1 , we donot exclude a contribution of the DPI process leading to the

8228 J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Lebech et al.

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FIG. 1. KECDs of the (NO1,e), (N21 ,e), and (O1,e) events identifying the DPI processes of N2O at ~a! hn518.45 eV,~b! 19.5 eV,~c! 20 eV, and~d! 21.2

eV. Spectrometer conditions:~a! Extraction fieldEext522 V/cm, field free mode;~b! Eext530 V/cm, Le standard focusing lens set;~c! Eext540 V/cm, Le

standard focusing lens set;~d! Eext555 V/cm, Le standard focusing lens set. The straight linesLD,vA1 indicate the dissociation limits~see Table I!. The black

labels on the left side show the position of theB andC states in the FC region, the gray line between theB andC states complements the ‘‘B-state-region.’’The intensity scale runs from blank~lowest intensity! to black~highest intensity! in linear scale as shown for each KECD, the scales are consistent within theset corresponding to each photon energy; the contour lines are spaced by 10% of the maximum value.

8229J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Dissociative photoionization of N2O

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@NO1(X 1S1,v5O)1N(2P)# limit; however, the smoothdistribution of the events in the peak rules out a major con-tribution of this reaction.

For the photon energyhn520 eV a weak additionalstructure is observed in the (ENO1 ,Ee) KECD for low Ee

energies, consistent with that found in TPEPICO spectros-copy and assigned as an autoionization state labeled‘‘b ~0,0,0!’’ just below theC 2S1.15 The shape of this struc-ture in the KECD is very similar to the one characterizingDPI of N2O into theC 2S1 state seen in Fig. 1~d! and paperI.

The minor DPI process identified as peakPANO1 , in the

(ENO1 ,Ee) KECDs at 18.5 and 19.5 eV corresponds tophotoionization into the N2O1(A 2S1) ionic state; in com-petition with fluorescence, the N2O1(A 2S1) state slowlypredissociates when it is formed in a vibrationnally excitedstate,26 leading to the@NO1(X 1S1,v)1N(4S#(L1,v

NO1) dis-sociation limit27 through a spin–orbit coupling mechanism.

The PBN21 peak in the (EN21 ,Ee) KECD displays a

more complex structure thanPBNO1 , since it is elongated

both along theEN21 axis, and along theEe axis. ThePB1N21

main part corresponding to the central part of theB state,shows a distinct maximum along theEN21 axis for aEN21

kinetic energy of about 0.05 eV, with a tail extending to 0.5eV. These characteristics suggest the identification of the fol-lowing DPI reactions for processPB1

N21 :

N2O~X 1S1!1hn→N2O1~B 2P!1e

→N21~X 2S1,v !1O~3P!1e~L1,v

N21! ~3!

with a maximum of the molecular fragment vibrational dis-tribution corresponding to N2

1(X 2S1,v53), while the@N2

1(A 2Pu ,v)1O(3P)# (L2N21) limit may also contribute

to this structure.The PB2

N21 weaker component ofPBN21 is elongated

along theEe axis in the upper part of theB state. It mayinclude the excitation of the DPI processes leading to the@N2

1(A 2Pu ,v)1O(3P)# and @N21(X 2S1,v)1O(1D)# lim-

its, where a transfer of the vibrational excitation of theN2O1(B 2P) parent ion into the N2

1 molecular fragmentwould lead to the observedEe elongated shape. For the pho-ton energyhn520 eV, excitation of the ‘‘b~0,0,0!’’ autoion-izing state15 probably contributes to the lowEe energy partof PB2

N21 . The ridge extending thePB2N21 structure fills up the

gap between theB 2P and theC 2S1 states at the higherphoton energy of 21.2 eV. It may also involve a weak DPIprocess through another N2O1(4P or 2P) ionic state lying inthe intermediate region between theB and C states.20,28

However, the present results do not show a significant evo-lution of the branching ratios between the different ionicfragments when considering ionization into theB state, orinto this intermediate region, as reported in Ref. 20 for aphoton excitation energy of 40.8 eV.

The very weak production of the O1 fragment in DPI inthe region of the N2O1(B 2P) state10,20 appears as a struc-ture labeledPB

O1 in the (EO1 ,Ee) KECDs in Figs. 1~b!,1~d!. The O1 fragment is only produced when ionizationtakes place to the upper part of theB state, above the openingof the @O1(2D)1N2(X 1S1,v)# (L2

O1) dissociation limit,and then continuously till the excitation of theC 2S1 state;the N2(X 1S1) molecular fragment is formed in vibra-tionally excited states. The weak distinct structure labeledPa

O1 observed more clearly for the photon energieshn518.45 eV andhn519.5 eV does not correspond to photo-ionization to the N2O1(A 2S1) state: It involves the@O1(4S)1N2(X 1S1,v)# (L1

O1) ground-state quartet disso-ciation limit also populated by autoionization of the Rydbergseries converging to the N2O1(A 2S1) state.10,29 This fea-ture may indicate that autoionization of a resonance statebelonging to the series converging to theC state contributesto the DPI at these photon energies.

The branching ratios between the processesPBNO1 ,

PB1N21 , PB2

N21 , and PBO1 determined by integration of the

peaks in the KECDs for DPI in the region of theB state arereported in Table II. The limits corresponding to the integra-tion zone for each process are illustrated in Fig. 1~c! for thehn520 eV photon energy as an example. The branching ra-tios in terms of relative importance of the NO1 and N2

1

fragment ions reported previously10,17,20are consistent withthe present results.

TABLE I. Dissociation limitsLD,vA1 for N2O1 and their potential energies

ED . LDA1 is LD,v

A1 with v50.

Dissociation limit LD,vion ED ~eV! Products of dissociation

12.89 X2P

L1NO1 14.19 NO1(X 1S1)1N(4S)

L1,4NO1 15.33 NO1(X 1S1,v54)1N(4S)

L1O1 15.29 N2(X 1Sg

1)1O1(4S)

L1,2O1 15.86 N2(X 1Sg

1 ,v52)1O1(4S)16.38 A2S1

L2NO1 16.57 NO1(X 1S1)1N(2D)

L2,4NO1 17.71 NO1(X 1S1,v54)1N(4S)

L1N21 17.25 N2

1(X 2Sg1)1O(3P)

L1,3N21 18.06 N2

1(X 1S1,v53)1O(3P)17.65 B2P

L3NO1 17.77 NO1(X 1S1)1N(2P)

L2N21 18.39 N2

1(A 2Pu)1O(3P)

L2O1 18.61 N2(X 1Sg

1)1O1(2D)

L3N21 19.22 N2

1(X 2Sg1)1O(1D)

L3,1N21 19.49 N2

1(X 2Sg1 ,v51)1O(1D)

20.11 C2S1

TABLE II. Branching ratios~BR! for the DPI processes identified in theKECDs ~see text! in the 18.5–21.2 eV photon energy range. The limits ofthe integration zone for each process are illustrated in Fig. 1~c! for hn520 eV. The relative uncertainly is of a few percents. The BR reported forprocessPB

O1 correspond to the ‘‘B-state-region;’’ the values in parenthesesare for an integration zone strictly limited to theB state.

hn ~eV! PBNO1 PB1

N21 PB2N21 PB

O1

18.5 87% 13%19.0 88% 12% 0.3%19.5 85% 11% 2.8% 1.1%20.0 81% 11% 6.6% 1.7%

8230 J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Lebech et al.

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V. MOLECULAR FRAME PHOTOELECTRONANGULAR DISTRIBUTIONS

In this section, we present the spatial analysis of the(VA1 ,Ve,P) vector correlation for thePB

NO1 major processinvolving photoionization into the N2O1(B) state identifiedin the KECDs described by Eq.~2!.

A. Presentation of the measured MFPADs in termsof the four FLN functions

As described in paper I, theFLN(ue) functions are ob-tained by a three-angle fitting of the complete angular distri-butions I (xA1 ,ue ,fe) according to the general analyticalform given in Eq.~1!. The measuredF00

NO1 , F20NO1 , F21

NO1 ,andF22

NO1 functions for the (NO1,e) coincident events cor-responding to Eq.~2! selected in the KECDs, are displayedin Fig. 2 for the photon excitation energieshn518.04, 19,19.5, 20, and 21.2 eV; consistently theue electron polarangle is referred with respect to the NO1 emission direction,i.e., the O end of the N2O molecule in the axial recoil ap-proximation. In order to unify the presentation of the mea-sured FLN(ue) functions at different energies obtained inconsistent arbitrary units, each set ofFLN(ue) functions ispresented in MBarn by adjusting the measured total photo-ionization cross section for the studied reaction to the calcu-lated cross section for photoionization of N2O into theN2O1(B 2P) state, at the corresponding photon energy.

The FLNNO1 functions display significant anisotropies, al-

thoughF00(ue) is dominant: It is rather flat at low energy,and displays a more and more pronounced maximum aroundue590 ° when the photon excitation energy increases. Thepattern of the oscillatory structure ofF20, F21, and F22

evolves with photon energy; in particularF21 is negative for0 °,ue,90 ° and positive for 90 °,ue,180 ° at the lowestenergies, and shows the opposite behavior at higher energies,whereasF22, which presents a single oscillation, is negativeat the lowest energies and becomes positive at higher ener-gies. The sign of theF22 function characterizes the azimuthaldependence of theI x590° (ue ,fe) MFPAD for a moleculealigned perpendicular to the polarization axis of the light.According to the definition of the electron emission directionin the molecular frame,F22.0 corresponds to a preferredelectron emission in the plane defined by the polarizationaxis and the internuclear axis, whereasF22,0 correspondsto a preferred emission in a plane perpendicular to the polar-ization axis. We point out that such an evolution of the signof theF22 function, from a negative value close to the photo-ionization threshold, becoming positive for higher electronenergy, was already observed in our previous investigation ofphotoionization of the O2 molecule, for the O2(X 3S2)→O2

1(3 2P) ionization transition from a neutral state ofSsymmetry to an ionic state ofP symmetry corresponding tothe excitation of a (1p)21 inner-valence orbital.3

The measuredbNO1 photoion andbe photoelectronasymmetry parameters characterizing the photoion and pho-toelectron emission anisotropies with respect to the polariza-tion axis, and corresponding to the angular analysis dis-cussed above in terms of theFLN

NO1 functions, are presentedin Tables III and IV, respectively.bNO1 evolves from20.15to 20.3, indicating that the photoabsorption transition into

the continuum for the studied process has a dominant per-pendicular character, increasing with photon energy. Thebe

asymmetry parameter increases significantly frombe

'20.5 close to the ionization threshold, tobe'0.3 at 21.2eV photon energy, consistent with the previous results ofCarlsonet al.7 We note that similar features were also foundfor the O2(X 3S2)→O2

1(3 2P) photoionization transition.3

Since the transition has a dominant perpendicular character,one may consider that the evolution of the sign of theF22

function with photon energy discussed above is correlatedwith that of thebe asymmetry parameter. The physics gov-erning such a general tendancy is further discussed in Sec.VII. The calculatedbn ~nuclei! and be asymmetry param-eters using the MCSCI method also reported in Tables IIIand IV are in qualitative agreement with the experimentalresults; however, the amplitude of the calculatedbn andbe

values are larger than that found experimentally.The second DPI process involving ionization via the

N2O1(B 2P) state leads to the production of the N21 frag-

ment mainly in the N21(X 2Sg

1 ,v) ground state according toEq. ~3!, with a contribution of the@N2

1(A 2Pu ,v)1O(3P)#dissociation limit. Since the N2

1 photoions are produced atlower kinetic energy than the NO1 ions, the angular analysisof the (VN21 ,Ve,P) vector correlation in terms ofFLN

N21

functions is performed in more reliable conditions at the low-est explored photon excitation energies, which correspond tolower extraction fields. Figure 3 illustrates in the limits of thestatistical uncertainty that the set ofFLN

N21 functions, hereF21

andF22, obtained for reaction Eq.~3! at hn518.5 eV is verysimilar to theFLN

NO1 one at the same photon energy. For thesetwo processes, the MFPADs then directly characterize photo-ionization to the N2O1(B 2P) state, independent on the dis-sociation channel corresponding to the breaking of the N–Oor the N–N bond.

B. Presentation of the MFPADs in termsof the IxA¿„ue ,fe… distributions, comparisonwith the MCSCI calculations

An alternative complete presentation of the MFPADs isto plot the IxA1(ue ,fe) distributions for the three specificorientations of the molecular axis parallel, at the magic angleand perpendicular with respect to the linear polarization ofthe light. Once theFLN(ue) functions have been determined,these distributions are obtained using Eq.~1! as:

I x50°~ue!5F00~ue!1F20~ue!,

I x554.7°~ue ,fe!5F00~ue!121/2F21~ue!cos~fe!

12F22~ue!cos~2fe!, ~4!

I x590°~ue ,fe!5F00~ue!2 12 F20~ue!

13F22~ue!cos~2fe!.

In Fig. 4 we display theI x50°(ue) and I x590°(ue ,fe) ex-perimental distributions, that we labelI i(ue) andI'(ue ,fe),for the five photon excitation energies hn518.04, 19, 19.5,20, and 21.2 eV, compared with the MCSCI calculations atthe corresponding most probable photoelectron energy at the

8231J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Dissociative photoionization of N2O

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maximum of theB state distribution. For clarity we haveomitted here the presentation of theI x554.7°(ue ,fe) MF-PADs which reflects the evolution of theF21 function ~seeSec. VII!. The measuredI i(ue) MFPADs @Figs. 4~a!–4~e!#

display a weak anisotropy when the molecule is alignedalong the polarization axis, indicating the contribution ofseveral partial waves in the photoelectron wave function inthe continuum. Their shape varies smoothly with photon en-

FIG. 2. FLN(ue) functions~left column —d— F00 and —m— F20, right column —L— F21 and —h— F22) for the PBNO1 DPI process of N2O @Eq. ~2!# at

~a! hn518.04 eV,~b! 19 eV, ~c! 19.5 eV,~d! 20 eV, and~e! 21.2 eV.ue is referred to the direction of the NO1 fragment. The extraction and focusing lensconditions for each energy are identical to those specified in the caption of Fig. 1. Each set of fourFLN(ue) functions is presented in MBarn by adjusting themeasured total cross section for processPB

NO1 to the computed cross section for PI of N2O into the N2O1(B 2P) state, at the corresponding photon energy~see text!.

8232 J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Lebech et al.

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ergy; electron emission is favored in the directionue'90 ° atthe lowest energyhn518.04 eV, then atue'45 ° and 135°at higher energy, revealing athn520 eV a molecular frameforward–backward ~MF–FW–BW! asymmetry favoringelectron emission atue'135 ° ~side of the N–N bond of theN2O molecule! with respect toue'45 ° ~side of the N–Obond!. Owing to the S and P symmetries of the initialN2O(X) and final N2O1(B) molecular states, respectively,the photoelectron wavefunction for the parallel transition isof p symmetry. When the molecule is aligned perpendicularto the polarization axis, the shape of theI'(ue ,fe) MFPADsshows a significant evolution with photon energy consistentwith that of the sign of theF22 function discussed in Sec.V A. At low energy, electron emission is preferred in a direc-tion perpendicular to the plane defined by the polarizationand molecular axes (F22,0), whereas it is favored in thedirection parallel to the polarization axis at the highest pho-ton energy (F22.0); at intermediate energies the emissiondiagram is more isotropic. The strong observed azimuthaldependence implies the population of both thes andd sym-metries allowed for the perpendicular transition.

The computed MFPADs at the corresponding energies@Figs. 4~f!–4~j!# display some similarities with those foundexperimentally, although the strong anisotropy of the angulardistributions reveals the dominant contribution of ad partialwave ofp symmetry for the parallel orientation, andss anddd for the perpendicular orientation. The dp shape of theMFPAD for a molecule oriented parallel to the polarizationaxis demonstrates a MF–FW–BW asymmetry which de-pends upon photon energy. On the other hand the shape of

the MFPAD for a molecule oriented perpendicular to thepolarization axis strongly favors electron emission in the di-rection perpendicular to the plane defined by the polarizationand molecular axes, while athn521.45 eV the emission dia-gram is quasi-isotropic evolving towards an orbital alignedparallel to the polarization axis at higher energies. Thesmooth shape of the measured MFPADs, as compared withthe computed ones, cannot be attributed to the angular reso-lution of the experiment in the presently explored experi-mental conditions, in particular for the lowest energies wherethe instrumental widths weakly influence the angular resolu-tion. As stated before, the short lifetime of the N2O1(B)excludesa priori a significant rotation of the molecule priorto dissociation. Two aspects are then considered in the fol-lowing discussion~i! the fact that, due to the extent of theBstate in the FC region over more than 1 eV, a given photonenergy in the experiment corresponds to a superposition ofEe distinct electron scattering energies, and~ii ! the possibil-ity of a significant bending of the N2O1(B) molecular ionoccuring prior to dissociation.

In order to take into account the electron energy distri-bution of the N2O1(B) state in the FC region, we have firstexplored the energy dependence of the measuredFLN func-tions in an experiment performed at a single photon energy.At the hn518.45 eV photon energy, where the intrinsic en-ergy resolution is of the order of 200 meV, theFLN functionsobtained for a selection of fourEe slices with DEe

'200 meV performed in the KECD do not show meaningful

FIG. 3. Comparison of the measured~a! F21 and ~b! F22 functions for the DPI processes of N2O defined in Eq.~2! ~—�— PBNO1), and in Eq.~3! ~—m—

PB1N21) at hn518.5 eV. The cross sections for the two processes are normalized to the MCSCI calculation as in Fig. 2.

TABLE III. bNO1 photoion asymmetry parameter for processPBNO1 , com-

pared with the MCSCI calculations at the corresponding photon energies.

hn ~eV! bNO1 hnMCSCI ~eV) bMCSCI

18.04 20.1660.118.24 20.0860.118.5 20.1060.1 18.65 20.4419.0 20.2060.1 19.25 20.4119.5 20.260.1 19.75 20.3220.0 20.1360.1 20.25 20.2021.2 20.3060.1 21.45 20.47

25 20.70

TABLE IV. be photoelectron asymmetry parameter for processPBNO1 , com-

pared with previous measurements for PI to the N2O1(B 2P) state~Ref.21!, and the MCSCI calculations.

hn ~eV! be be @Car83] hnMCSCI ~eV) bMCSCI

18.04 20.4760.118.24 20.3460.118.5 20.2460.1 18.65 20.6119.0 20.0160.1 0.0960.05 19.25 20.3019.5 0.1260.1 0.1260.05 19.75 20.1120.0 0.1360.1 0.1660.05 20.25 0.0521.2 0.3160.1 0.3260.05 21.45 0.47

25 0.80

8233J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Dissociative photoionization of N2O

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differences within the statistical uncertainty. At this energy,the smooth shape of the measured MFPADs is then not at-tributed to the mixing of different sets ofFLN functions. Onthe other side, we have also compared the MFPADs mea-sured at a given photon energy, with those obtained as alinear combination of the calculated MFPADs for a series ofelectron energies scanning the extent of theB state in the FC

region, using the FC factors as weighting parameters in thelinear combination. This does not significantly improve thecomparison, and does not explain in particular the smoothshape of the measured MFPADs as compared with theMCSCI calculations. The effect of the role of molecularbending occuring prior to dissociation is presented in Sec. VIbelow.

FIG. 4. MeasuredI i(ue) andI'(ue ,fe) MFPADs for a molecule oriented parallel and perpendicular to the polarization axis for processPBNO1 corresponding

to theFLN(ue) functions in Fig. 2 at~a! hn518.04 eV, ~b! 19 eV, ~c! 19.5 eV,~d! 20 eV, and~e! 21.2 eV, compared with the MCSCI calculations at thecorresponding photon energies~f! hn518.65 eV,~g! 19.35 eV,~h! 19.75 eV,~i! 20.25 eV, and~j! 21.45 eV~see text!. The molecular axis is vertical along thez axis and points in the direction of the O end of the molecule at the top of the figure. The polarization axis is parallel~i! to the molecular axis along thezaxis, or perpendicular~'! to the molecular axis along thex axis, as shown in panel~a!.

8234 J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Lebech et al.

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VI. EFFECT OF MOLECULAR BENDING

A. Inclusion of bending in the MFPAD

As is mentioned above, the dissociation lifetime of theBstate of the N2O1 molecule is short compared to the rota-tional period of the ion. Thus the discrepancy between thecomputed MFPADs and the measured MFPADs cannot bedue to the effects of rotation that occurs between the time ofionization and the time of dissociation. In the dissociativeionization of a linear polyatomic molecule such as N2O, it iscertainly possible for the recoil direction of the ion fragmentto be different from the orientation of the molecular axis atthe time of ionization. This is, e.g., the case is the molecularion bends prior to dissociation, a possibility which has beeninvoked for the N2O1(B) state. Thus one possible explana-tion of the discrepancy between the computed and measuredMFPADs may be this difference between the recoil directionand the initial orientation of the molecular axis. In order toassess the effects of a nontrivial recoil direction, we havecomputed the apparent MFPAD that would be measured as-suming a single fixed angle between the molecular axis andthe direction of the ion recoil.

The expression for the MFPAD of a particular processwith the state of the light given by the indexm0, before theinclusion of the recoil effects, can be written as:3

I m0~uk ,fk ,un ,fn!

5 (L8,L,N

HL8,L,N

m0 YL8N~uk ,fk!YLN* ~un ,fn!. ~5!

Then introducing a recoil angle ofuR, defined as the anglebetween the original molecular axis and the recoil velocityvector, leads to an apparent MFPAD given by the same func-tional form as that given in Eq.~1! where theH

L8,L,N

m0 are

replaced byHL8,L,N

m0 ,uR given by

HL8,L,N

m0 ,uR 5 (J,N8

^L8,L,N,2NuJ,0&^L8,L,N8,2N8uJ,0&

3HL8,L,N8

m0 PJ~cosuR!3~21!N82N ~6!

where ^L8,L,N8,NuJ,M & is a Clebsch–Gordon coefficient.To obtain Eq.~6!, we have averaged over the angle whichdefines the azimuthal orientation of the recoil angle in theoriginal molecular reference frame and over the azimuthalorientation of the molecular fragments about the recoil axis.With these modified expansion coefficients,H

L8,L,N

m0 ,uR , one canthen use the previously given formulas to obtain theFLN(uk)functions.3 For a symmetric linear triatomic, YXY, whichbends prior to dissociation, the recoil angle is half of thebending angle, assuming that the force that induces the bondbreaking is impulsive and directed along the XY bond. Thisis not exactly the case for N2O since it is not symmetric,however it gives a good qualitative description of connectionbetween the recoil and bending angles for this nearly sym-metric molecule. Equation~6! could also be applied to the

expansion coefficients,HL8,L,N

m0 ,t, which include the effects of

a finite lifetime,t, of the predissociative state during whichthe molecule can rotate.

B. Comparison of the measured and computedMFPADs: Effect of molecular bending

Figure 5 displays the computedI i(ue) and I'(ue ,fe)MFPADs for the photon excitation energy ofhn518.65 eVfor selected recoil angles (uR50°, 20°, 30°, 40°, 60°, and90°!, using the general expression derived in Sec. VI A. Itillustrates the significant influence of the recoil angle on theshape and relative size of the photoelectron emission patternsfor the parallel and the perpendicular transitions. Comparingthe distributions in Fig. 5 with the measuredI i(ue) andI'(ue ,fe) MFPADs at the photon excitation energy ofhn518.04 eV presented in Fig. 4, shows that the assumptionof a recoil angle of the order of 30° to 40° prior to dissocia-tion, i.e., a bending angle of the N2O1(B) ion of the order of70°, improves significantly the agreement between the mea-sured and calculated MFPADs. In Fig. 6 we present the com-puted I i(ue) and I'(ue ,fe) MFPADs assuming a recoilangle of 30° for the four photon energies (hn518.65, 19.35,19.75, 20.25, and 21.45 eV!, which must be directly com-pared with the measured MFPADs presented in Figs. 4~a!–4~e!. The overall agreement between the measured and cal-culated MFPADs is now significantly improved, although theenergy dependence still displays differences; in particular theazimuthal dependence of theI'(ue ,fe) MFPAD evolvesfrom a distribution characterized by a negativeF22 functioninto one characterized by a positiveF22 function more rap-idly in the experiment than in the calculation. Concerning theMF–FW–BW asymmetry, which remains stronger in the cal-culation after inclusion of the bending effect than in the ex-periment, one can see that performing a weighted sum of thecalculated MFPADs for the electron energies correspondingto one photon energy would reduce the asymmetry.

Figure 7 presents a quantitative comparison between thecalculated and measured MFPADs in terms of the measuredFLN functions for the selected energy of 19.5 eV, the initialMCSCI FLN functions at the corresponding 19.75 eV energy,and the MCSCIFLN functions at 19.75 eV including theeffect of a recoil angle of 30°. Including the recoil anglereduces significantly the amplitude of theF20 oscillationswith respect toF00: In this case it leads to an increase of thebn asymmetry parameter; it reduces also the amplitude of thecalculatedF22 function, whereas the influence onF21 is lim-ited. The experimental and theoretical results are very con-sistent at this level, although the difference of the amplitudesof the F22 functions remains significant.

Further extensions in the calculation could be explored:As mentioned above, theB state can be treated as a vibronicstate that includes more than one electronic state rather thanas a single electronic state; a second extension of the calcu-lation would be explicitly examine the dynamics of dissocia-tion to extract the recoil angle or angular distributions. Fi-nally, the expansion of the continuum state could beextended to include more ion states in particular at the rela-tively low photoelectron energies considered in this study.

8235J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Dissociative photoionization of N2O

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FIG. 5. ComputedI i(ue) and I'(ue ,fe) MFPADs forPI into the N2O1(B 2P) state at hn518.65 eV, for se-lected values of the recoil angleuR .

8236 J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Lebech et al.

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VII. GENERAL CHARACTERISTICS OF A S „neutralstate …\P „ionic state … PHOTOIONIZATIONTRANSITION

One notable feature of the energy dependence of boththe measured and computedFLN functions is that at lowenergy the functionF22 is negative and at higher energyF22

becomes positive. As we have noted previously,3 this general

trend is expected for transitions where the initial state hasS1 symmetry and the final ion state hasP symmetry. Inlarge part this energy dependence is a result of the interfer-ence between thedd andss partial waves and the changingvalues of the Coulomb phase shift associated with thesewaves. To illustrate this point, we have plotted in Fig. 8 thevalue ofF22 obtained in a model calculation where we haveassumed that the magnitude of the matrix elements are con-stant and that Coulomb phase, i.e., the phase shift experi-enced by an electron scattering from a point charge, is theonly part of the matrix element that changes with energy; wehave approximated the dipole matrix elements

I lmmMi M f5^CMi

i udmuFM f

f c lm~2 !&, ~7!

by the values

I lmmMi M f5~ i ! lexp~2 ih l !, ~8!

where h l is the Coulomb phase shift and where we onlyinclude the matrix elements for thedd andss partial waves,i.e., (l 52, m52) and (l 50, m50). We have used in thiscalculation the same ionization potential as that found in thefull computed results discussed above. Also, in order to makea more precise comparison, we have corrected the value ofF22 in the model by the ratios of the total cross sections forphotoionization when the light is perpendicular to the mo-lecular axis and plotted F22(Model)s'(MCSCI)/s'(Model). From Fig. 8 we see that much of the energydependence of theF22 function can be reproduced by thisvery simple model containing only two partial waves withtheir corresponding Coulomb phase shifts. In addition theMCSCI results show the effects of the energy dependence ofthe matrix elements and that of additional partial waves athigher energies.

The effects of the interference between these two partialwaves is also evident in the corresponding energy depen-dence of the value ofbe . However, the value ofbe doesdepend on the strength of the parallel transition. Thus inorder to consider a simple model for the energy dependenceof be , we have extended our model by adding an (l 52, m51) matrix where we have also included an additional phaseof 110° in the (l 52, m51) matrix element relative to the( l 52, m52) matrix element. This additional phase anglewas obtained by examining the matrix elements in theMCSCI calculation. In Fig. 9, we show the value ofbe ob-tained as a function of energy using this model, the values ofbe from the full MCSCI calculation, and the experimentaldata forbe . The behavior found in this very simple modelfor the photoionization matrix elements is seen to be verysimilar to that found in both the MCSCI calculation and inthe experimental data. In Fig. 9, we have also plotted thevalues ofF22 of the model and MCSCI calculations that havebeen normalized to the corresponding values ofs' . Thisfigure shows the common trend of negative values ofF22 andbe at low energy going to positive values at higher energies.It illustrates that this behavior in bothF22 and be can beattributed to the changing values of the Coulomb phase shiftsof the s andd partial waves.

FIG. 6. Computed I i(ue) and I'(ue ,fe) MFPADs for PI into theN2O1(B 2P) state at~a! hn518.65 eV, ~b! 19.35 eV, ~b! 19.75 eV, ~d!20.25 eV, and~e! 21.45 eV, assuming a recoil angleuR530 ° These distri-butions must be compared with the measuredI i(ue) and I'(ue ,fe) MF-PADs for processPB

NO1 in Figs. 4~a!–4~e!, respectively.

8237J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Dissociative photoionization of N2O

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The same Coulomb phase shift difference is also respon-sible for much of the energy dependence of theF21 func-tions. This function is computed as a sum of products ofparallel and perpendicular photoionization matrix elements.In Fig. 10 we compare the results of theF21 functions fromthe MCSCI calculations with theF21 functions from themodel used to obtain thebe results given in Fig. 9. As in Fig.8, we have adjusted theF21 function of the model by ratiosof appropriate cross sections. It shows that much of the en-ergy dependence can be explained by this simple model: Atlow energy,F21 has two periods between 0° and 180°, with anegative value for the forward angles~0°–30°!. At high en-ergy, the dominant structure has a single period between 0°and 180° with positive values in the forward direction.Again, the MCSCI cross sections have additional higher har-monics that have not been included in the simple model.

VIII. CONCLUSION

In this second paper of a series studying dissociativephotoionization of the N2O(X 1S1) linear molecule, the(VA1 , Ve, P! vector correlation method has been applied tothe investigation of DPI processes involving direct photoion-ization via the N2O1(B 2P) ionic state induced by linearlypolarized light. The ion–electron kinetic energy correlationdiagrams enable identification of the DPI processes produc-ing the NO1, N2

1 , and O1 fragments. The measuredMFPADs for the dominant reaction leading toNO1(X 1S1,v)1N(2D)1e, presented in terms of the fourFLN(ue) functions describing theI (x,ue ,fe) distributionand of theIxA1(ue ,fe) photoelectron emission diagram fora fixed orientation of the molecular axis with respect to the

FIG. 7. MeasuredFLN functions for processPBNO1 at hn519.5 eV~dots!, computedFLN functions at the 19.75 eV corresponding energy~full line!, computed

FLN functions at 19.75 eV assuming a recoil angle of 30°~dashed line!.

FIG. 8. The value ofF22 as a function of photon energy;~a! using only theCoulomb phase to model the dipole matrix elements as indicated in Eq.~9!and normalized using the factors'(MCSCI)/s'(Model); ~b! from theMCSCI calculation.

8238 J. Chem. Phys., Vol. 120, No. 17, 1 May 2004 Lebech et al.

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linear polarization, exhibit rather smooth structures, althougha significant evolution of the electron scattering anisotropieswith photon energy is observed. In particular, the evolutionof theF22(ue) function characteristic of the azimuthal depen-dence of the MFPAD when the molecule is oriented perpen-dicular to the polarization of the light, is negative close to theionization threshold and becomes positive at higher energy.

This feature is identified as a trend for a photoionizationtransition between an initial neutral state ofS symmetry anda final ionic state ofP symmetry. The comparison of themeasured MFPADs with the reported MCSCI calculationssuggests that a significant bending of the N2O1(B 2P) mo-lecular ion takes place prior to dissociation. The inclusion ofthe ion recoil effects in the general form of theI (x,ue ,fe)distribution improves significantly the agreement betweenthe experimental results and the MCSCI calculations at thesensitive level of the MFPADs, and leads to estimate a bend-ing angle of theB state of the order of 70°. Finally, it isshown that a simple model considering the coherent super-position of twoss anddd partial waves and the associatedCoulomb phases in the description of the perpendicular com-ponent of aS to P photoionization transition, accounts forsome of the main charactistics of the MFPADs and asymme-try parameters, and their energy dependence. This study ofDPI of N2O involving the N2O1(B 2P) ionic state will becompleted in a forthcoming publication devoted the role ofdissociative autoionization of Rydberg states converging tothe N2O1(C 2S1) state, induced by circularly polarized syn-chrotron radiation.

ACKNOWLEDGMENTS

We are very grateful to G. Chambaud for fruitful discus-sions, and to L. Nahon and C. Alcaraz, beamline scientistsfor beamlines SU5 and SA63 at Super-ACO. We also thankB. Pilette at LURE and A. Abadia and co-workers at theLCAM for their technical assistance, as well the machinedepartment staff for operating Super-ACO~LURE!. The sup-port of the National Science Foundation~USA! throughGrant. No. INT-0089831, and of the Center National de laRecherche Scientifique~France! is gratefully acknowledged.This work is also in part supported by the Welch Foundationunder Grant A-1020 and by the Texas A&M SupercomputingFacility.

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FIG. 9. A comparison ofbe and F22(ue590°)/s' evaluated using theMCSCI method and the model discussed in the text. The experimentalbe

data are from Ref. 21 with additional points at low photon energy from thepresent experiment.

FIG. 10. The value of F21 as a function of photon energy;~a!using only the Coulomb phase to model the dipole matrix elements asdiscussed in the text and normalized using the factorA@s'(MCSCI)s i(MCSCI)#/@s'(Model)s i(Model)#; ~b! from the MCSCIcalculation.

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