Vibrational constants and binding energies for the A 2 Π and X 2 Σ states of Sr + Krfrom photodissociation spectroscopyDimitris Prekas, Bao-Hua Feng, and Michalis Velegrakis Citation: The Journal of Chemical Physics 108, 2712 (1998); doi: 10.1063/1.475663 View online: http://dx.doi.org/10.1063/1.475663 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/108/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical-optical double resonance spectroscopy of the quasi-linear S2 state of CHF and CDF. II. Predissociationand mode-specific dynamics J. Chem. Phys. 135, 104316 (2011); 10.1063/1.3633772 [1 + 1] photodissociation of CS 2 + ( X 2 Π g ) via the vibrationally mediated B 2 Σ u + state: Multichannelsexhibiting and mode specific dynamics J. Chem. Phys. 134, 114309 (2011); 10.1063/1.3567071 Dynamics of vibrationally mediated photodissociation of CH 3 CFCl 2 J. Chem. Phys. 115, 6418 (2001); 10.1063/1.1404392 Photodissociation spectroscopy of the Ca + –N 2 complex J. Chem. Phys. 108, 2725 (1998); 10.1063/1.475700 Distributions of rovibrational states of secondary product NO X 2 Π from photodissociation of nitric acid at 193nm J. Chem. Phys. 107, 7223 (1997); 10.1063/1.474963

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Vibrational constants and binding energies for the A 2P and X 2Sstates of Sr 1Kr from photodissociation spectroscopy

Dimitris Prekas, Bao-Hua Feng,a) and Michalis Velegrakisb)

Foundation for Research and Technology-Hellas, Institute of Electronic Structure and Laser,711 10 Heraklion, Greece

~Received 29 October 1997; accepted 14 November 1997!

Two vibrational progressions corresponding to transitions from theX 2S1/2 electronic ground stateto the excitedA 2P1/2 andA 2P3/2 spin–orbit states for the Sr1Kr complex in the energy regionbetween 22 000 and 23 750 cm21 are investigated by laser photofragmentation spectroscopy. Fromisotope-resolved measurements the absolute numbering of the vibrational numbers is obtained andthus the vibrational constants and the binding energies of the involved electronic states. The resultsare compared with similar systems and explained in the framework of an electrostatic potentialmodel. © 1998 American Institute of Physics.@S0021-9606~98!51907-1#

I. INTRODUCTION

The electronic states of alkaline earth ions–noble gasvan der Waals complexes have attracted considerable interestfor both experimental1–8 and theoretical9–13 studies in thelast five years. As the electronic structure of these complexesis relatively simple they can serve as prototypes for modelinga variety of physical and chemical processes such as elec-tronic energy transfer, electrical discharges, laser media,metal–ligand interactions and solvation effects.

In particular the study of the spectroscopy of such com-plexes aim at improving our understanding of the nature ofthe interaction forces that are responsible for the molecularbonding. The development of novel cluster ion sources basedon the laser ablation of metal targets, in combination with thetechnique of photofragmentation spectroscopy of mass se-lected ions offer a direct way for studying the absorptionspectra of any desired ionic complex.

We have recently applied photofragmentation spectros-copy of mass selected Sr1Ar ~Ref. 5! and Sr1Ne ~Ref. 6!complexes in order to study theA 2P and X 2S electronicstates. We have been able to extract vibrational constants andthus binding energies for the involved states and we haveshown that the main contribution to the molecular bindingenergy arises from ion-induced dipole forces, which arestrongly influenced by the electric dipole polarizability of thenoble gas atom. In order to systematically investigate suchinteractions, in this paper we present the results obtained byemploying photofragmentation spectroscopy for the Sr1Krcomplex. The vibrational constants and the binding energiesobtained are then compared to similar alkaline earth ions-noble gas systems and discussed in the light of an electro-static model.

II. EXPERIMENT

The molecular beam apparatus used to carry out the ex-periments consists from three differentially pumped cham-

bers and is equipped with a reflectron time-of-flight~TOF!mass spectrometer. This system is described in detailelsewhere5,6 and therefore only a brief description is given inthis paper. The Sr1Krn clusters are formed in a laser vapor-ization source, where a pulsed infrared Nd:YAG laser pro-duces plasma from a pure strontium target. The plasmaplume is mixed with an expanding Kr gas pulse provided bya home built nozzle. The adiabatic expansion results in cool-ing of the nascent clusters, thus forming a cluster beam,which contains neutral clusters as well as ionic species. Afterpassing the skimmer, the ions are accelerated by a pulsedtwo field Wiley–McLaren setup.14 The spatial focus of thisdevice is located;1 m downstream from the last grid, at theposition of a two plate mass selector, which rejects the un-desired ions and allows the transmission of the selected onestowards the reflectron assembly. Finally, the reflected ionsare focused into a microchannel plate detector~MCP!, whereTOF mass spectra can be recorded with a computer con-trolled digital storage oscilloscope.

In order to obtain the fragmentation spectrum of oneselected mass, an excimer-pumped pulsed-dye laser~in col-linear arrangement with the ion beam! fires when the ionsunder investigation are located in the mass gate. The photo-fragments are separated in the TOF spectrum from the parentmasses as due to the conservation of energy the fragmentshave lower kinetic energy, penetrate less deep into the reflec-tron, and arrive earlier at the MCP detector. This collinearion-laser beam arrangement introduces a Doppler shift whichis of the order of 1 cm21, but as this is comparable to thebandwidth of the used laser no further care has to be taken.

The photofragmentation spectrum is recorded by tuningthe dye laser frequency and normalizing the intensity of thephotofragments by the intensity of the parents and the laserintensity. In practise, for each data point in the spectrum, thisprocedure is performed several hundred times with the dis-sociation laser on and the same times with the dissociationlaseroff to obtain reliable statistics and to subtract any pos-sible background arising from collisions of the parent ionswith the grids of the reflectron assembly.

a!Also at Institute of Physics, Chinese Academy of Science, Beijing 100080,China.

b!Author to whom correspondence should be addressed.

JOURNAL OF CHEMICAL PHYSICS VOLUME 108, NUMBER 7 15 FEBRUARY 1998

27120021-9606/98/108(7)/2712/6/$15.00 © 1998 American Institute of Physics

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III. RESULTS AND ANALYSIS

Figure 1 shows the photofragmentation spectrum of theSr1Kr complex observed in the region between 22 000 and23 750 cm21. The spectrum corresponds to the natural iso-tope distribution of the species. Two vibrational progressionsare clearly visible in the spectrum, accompanied from lessintense satellite peaks positioned at the red side of the mainlines. Since the spectrum is very similar to the analogoussystems Sr1Ar ~Ref. 5! and Sr1Ne ~Ref. 6!, we attribute themain progressions to the transitions from the molecularX 2S1/2(v950) ground electronic state@which correlateswith the atomic Sr1(5s 2S1/2) state#, to the two excitedspin–orbitA 2P1/2(v8) and A 2P3/2(v8) states@which cor-relate with the atomic Sr1(5p 2P1/2) and Sr1(5p 2P3/2) lev-els#. Similarly the satellite peaks are due to hot-band excita-tions from thev9518 vibrational level of theX 2S1/2 groundelectronic state to theA 2P1/2,3/2(v8) states. Both progres-sions are~i! red-shifted with respect to the ionic transitionfrequencies of Sr1, 23 715.2 cm21 for 5p 2P1/2 and24 516,6 cm21 for 5p 2P3/2, ~Ref. 15! and ~ii ! the lines areblue degraded. We thus conclude that the binding energy ofthe excitedA 2P states is greater than those of the groundstate and the equilibrium distance of the excited states areshorter than those of the ground state.

The observed differences between thev8←v951 andthe v8←v950 transitions, allow the calculation of the en-ergy separation (DG1/29 ) between the two lowest vibrationallevels of the electronic ground state of the molecule. Theaverage over all the observed values isDG1/29 551 cm21. Theassignment of the absolute vibrational quantum numbers re-lies on measurements of the isotopic shifts of the observedlines ~see Ref. 5!. The complex Sr1Kr has many isotopesranging from 166 to 174 amu. The limited resolving powerof our mass gate ism/Dm;120, preventing us from resolv-ing single isotopes. Therefore, we select two region of iso-topes, namely the mass-ranges 168–171 amu and 172–174amu for the isotope measurements. The results are displayedin Fig. 2 ~solid symbols! together with different assignments~solid lines!. The observed lines of the spectrum in Fig. 1 and

the corresponding transition frequencies are given in Table I.Assuming that the interaction energy of the system can

be represented by a Morse potential, then the spectroscopicconstants are obtained by fitting the formula

E~v8!5n001ve8~v811/2!2vexe8~v811/2!2

2~ve8/22vexe8/4!, ~1!

to the sequence of the transition frequencies of Table I~seeFig. 3!. n00 denotes thev850←v950 transition frequency,ve8 the vibrational frequency, andvexe8 the anharmonicity.The obtained values are listed in Table II for both excitedA 2P states. The given values correspond to the av-

FIG. 1. The photofragmentation spectrum of Sr1Kr arising from A 2P←X 2S1/2 transitions. The horizontal axis corresponds to the dissociationdye laser frequency and the vertical axis represents the intensity of the Sr1

fragments normalized by the Sr1Kr intensity and the dye laser intensity.

FIG. 2. Measured shifts for several vibrational lines~solid points! of theA 2P←X 2S1/2 transitions for the Sr1Kr due to selection of two regions ofisotopes with mass ranges~168–171! amu and~172–174! amu. The solidlines indicate the expected shifts for different assignmentsv08 the first ob-served line of each vibrational progression of Fig. 1.

TABLE I. Positions~in cm21! and assignment of the observed vibrationalbands for the Sr1Kr A 2P(v8)←X 2S(v9) system.

2P1/22P1/2

v8 v950 v851 DG1/29 v950 v851 DG1/29 Ds.o.

0 ••• ••• ••• ••• ••• ••• •••1 22 151.53 ••• ••• ••• ••• ••• •••2 22 251.14 22 199.8 51.34 22 802.2 ••• ••• 551.13 22 345.63 22 293.76 51.87 22 899.5 ••• ••• 553.94 22 439.1 22 388.38 50.71 22 994.03 22 943.72 50.31 554.95 22 529.63 22 479.63 50.00 23 085.63 23 035.44 50.19 556.06 22 619.01 22 568.4 50.61 23 175.64 23 124.48 51.16 556.67 22 706.3 22 654.74 51.56 23 264.12 23 212.9 51.22 557.88 22 791.3 22 740.02 51.28 23 349.3 23 298.81 50.49 5589 22 875.15 22 823.97 51.18 23 434.61 23 382.6 52.01 559.510 ••• ••• ••• 23 516.2 23 464.47 51.73 •••

2713J. Chem. Phys., Vol. 108, No. 7, 15 February 1998 Prekas, Feng, and Velegrakis

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erage from the main progressions as well as from the hotbands. In the later case the termDG1/29 has to be subtractedfrom the right hand side of Eq.~1!.

On the basis of the vibrational numbering assignment,the molecular spin–orbit splitting constantDmolecule5Ds.o.(v50) for the two 2P states can be determined, since theobserved splitting~see Table I! as a function ofv can berepresented forv<11 by the formDs.o.(v)5551 cm211v.The atomic (2P3/22

2P3/2) spin–orbit splitting constantDatom

of Sr1 is 801.4 cm21 and thus the ratioDatom/Dmolecule

50.687 which is very close to the ratio of 2/3 expected formolecules belonging to Hund’s case~a!.16,17

As in previous studies of the systems Sr1Ar and Sr1Ne,the dissociation energies of the involved states for the Sr1Krcomplex can be obtained by two different procedures,namely the Morse potential~MP! approach and theLeRoy–Bernstein18 ~LB! analysis. For a Morse potential thedissociation energyD0 is given as

D05De2ve/21vexe/45ve2/4vexe2ve/21vexe/4,

~2!

where De represents the well depth of the potential. Theresults for the excited states areD0(2P1/2)52377 cm21 andD0(2P3/2)52450 cm21, respectively. The LB procedure isbased on the assumption that the long-range interaction po-tential behaves asV(R);1/R4. This reflects the electrostaticion-induced dipole force which is dominant at larger inter-atomic separations. In this case the difference in vibrationalenergies defined as

DG~v !5@E~v11!2E~v21!#/2, ~3!

follows, for the last vibrational levels, the behavior

DG~v !4/3}D02E~v !. ~4!

Plotting DG(v)4/3 versusE(v), a linear dependency isexpected~LB-plot!. In Fig. 4 we show such a plot of theobserved spacings~symbols! and the straight lines which fitthe last three data points. The intercepts with thex axis areequal ton001D0 , and thus we obtain the values of the dis-sociation energies for the two excited states to beD0(2P1/2)53503 cm21 andD0(2P3/2)53637 cm21.

Since only the lower vibrational levels are accessible inthe experiment, the dissociation energies obtained by boththe MP and the LB methods, result from long vibrationalextrapolations, and therefore, are not very accurate. It hashowever been pointed out5,19 that the average of the twomethods constitutes a reliable value for the dissociation en-ergy. Accordingly, in Table II we list besides the values ob-tained from the MP and LB analyses the averaged onesD0(2P1/2)52940 cm21 and D0(2P3/2)53044 cm21 whichwe consider as the most reliable.

Using the determinedD08-values of the excited electronicstates we can calculate the dissociation energyD09 of theground state using the formula

D095n001D082natom, ~5!

wherenatomis the2P←2S transition frequency of the Sr1 ion~see Table II!. We obtain an average value ofD0951205 cm21. Using this value, the observed hot band sepa-

FIG. 3. Least-squares fit of Eq.~1! to the observed vibrational line positionsin order to obtain the band origin frequencyn00 for the A 2P1/2←X 2S1/2

~circles! andA 2P3/2←X 2S1/2 transitions~squares!. The solid and the opensymbols refer to transitions from the ground statev950 andv951, respec-tively.

TABLE II. Spectroscopic constants and dissociation energies for the excitedA 2P and the groundX 2S states of Sr1Kr complex. MP and LB refer toMorse potential and LeRoy–Bernstein analysis, respectively. All values arein cm21.

A 2P1/2 A 2P3/2 X 2S1/2

n00 22 05365 22 60465ve 99.560.5 10060.5 5261vexe 1.0260.05 1.060.05 ;0.6DG1/2 5161Ds.o.(0)a 55161D0 ~MP! 2377 2450 626a

D0 ~LB! 3503 3637 1782a

D0b 29406560 30446600 12056580

natomc 23715.2 24516.6

aMolecular spin–orbit splitting forv850.bRepresents the average of the MP- and LB-values.cData from Ref. 15.

FIG. 4. LeRoy–Bernstein plot of the observed vibrational progressions. Thesolid symbols are the experimental values and the straight lines are fitted tothe last three data points of each transition as these are closest to the disso-ciation limit, and therefore, represent best the spacing of the unobservedhigh v8-levels. The intercepts of the lines with thex axis are equal toD0

1n00 .

2714 J. Chem. Phys., Vol. 108, No. 7, 15 February 1998 Prekas, Feng, and Velegrakis

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ration DG1/29 551 cm21 and assuming that a Morse potentialdescribes the ground electronic state, we find

vexe952D092DG1/29 22@D09~D092DG1/29 !#1/2

and

ve95DG1/29 12vexe9 .

Thus, an estimate of the spectroscopic constants for theground state isve9552 cm21 and vexe950.55 cm21. Itshould be noticed that the anharmonicity value determinedby this method is not very accurate because it is the result ofthe difference of two large numbers.

We performed a Franck–Condon calculation of thev8←v950 vibrational progression intensities to estimate thestructural changes that occur as a result of the photoexcita-tion. Morse potentials for both ground and excited stateshave been assumed, defined from the spectroscopic constantsof the corresponding states. Thus, the only parameter that hasbeen varied is the difference in equilibrium bond lengthsDRe5Re82Re9 , whereRe8 and Re9 refer to the excited andground state, respectively. We found the best fit values~seeFig. 5! DRe520.3760.02 Å for the 2P1/2 state andDRe

520.3660.02 Å for the2P3/2.

IV. DISCUSSION

The spectroscopic constants and the potential parametersof the Sr1Kr complex, deduced by the analysis presented in

the previous section, can now be compared with similar sys-tems, such as the Sr1Ne and Sr1Ar complexes studied in ourlaboratory5,6 and also with the Mg1Kr ~Refs. 1 and 7! andCa1Kr ~Refs. 3 and 8! complexes. To this end, we list inTable III the vibrational frequenciesve and the binding en-ergiesDe for the electronic ground state2S as well as for theexcited states2P and also the difference in equilibrium bondlengthsDRe5Re82Re9 deduced by Franck–Condon analyses.

In all cases, the excited state2P is more bound than theelectronic ground state2S and also, as the differenceDRe

~last column of Table III! shows, the internuclear distance inthe excited state is shorter than that of the ground state.Comparing the binding energiesDe of the Sr1-Ne/Ar/Kr sys-tems it is evident that the molecule gets stronger bound asone goes from the lighter to the heavier noble gases, whilethe binding energy of the systems Kr-Sr1/Ca1/Mg1 in-creases as one goes from the heavier to the lighter alkalineearth ions.

Assuming that the dominant interaction in this type ofmolecular ions is mainly electrostatic, in a previous paper5

we have applied an electrostatic model using a simple inter-action potential model which describes the interaction forthese ions in the region of equilibrium internuclear distanceRe , and having the form

V~R!5A/Rm2C4 /R4. ~6!

The term A/Rm represents the repulsive forces which areimportant at small internuclear distancesR, whereas the lastterm describes the ion-induced dipole forces. The electro-static ion-induced dipole constant isC45q2a/2, wherea isthe electric dipole polarizability of the noble gas atom andqis the charge residing in the metal ion, that induces the po-larization in the noble gas atom.

Using the equilibrium conditions for the potential~i!V(Re)52De ~5binding energy!, ~ii ! V8(Re)50, and ~iii !ve5(h/2p) @V9(Re)/m#1/2 ~assuming that the molecule be-haves near the potential minimum as a harmonic oscillator!we obtain the relation

De5Fm~q2a!1/3~vem1/2!4/3, ~7!

FIG. 5. Intensity distribution of the vibrational progressionsA 2P1/2,3/2(v8)←X 2S1/2 (v950) for the system Sr1Kr. The solid pointsare the experimentally observed intensities, while the open squares are theresults from Franck–Condon calculations and assuming that both the groundand the excited states are described with Morse potentials having the vibra-tional constants given in Table II andDRe520.37 Å for the2P1/2 state andDRe520.36 Å for the2P3/2 state.

TABLE III. Vibrational frequencies and potential parameters for some al-kaline earth–noble gas systems.m is the reduced mass of the system andais the static dipole polarizability of the noble gas atom.

2S 2P

m@amu# a@Å 3# ve9 De9 ve8 De8 Re8-Re9

Sr1-Nea 16.3 0.4 20 77 106 684 21.125Sr1-Arb 27.5 1.64 50 828 121 2500 20.5Sr1-Krc 42.92 2.484 52 1231 100 304220.36Ca1-Krd 27.1 74 1445 149 4087Ca1-Kre 77 1283Mg1-Krf 18.66 112 1870 259 7052Mg1-Krg 116 1949

aReference 6.bReference 5.cThis work.dReference 3.eReference 8.fReference 1.gReference 7.

2715J. Chem. Phys., Vol. 108, No. 7, 15 February 1998 Prekas, Feng, and Velegrakis

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where

Fm5~2p/h!4/3225/3~m24!1/3/m. ~7a!

This equation allows the estimation of the binding en-ergy De by using known values for the reduced massm ofthe molecule, the dipole static polarizabilitya of the noblegas atom, and the measured vibrational frequencyve . Onlythe effective chargeq and the repulsive potential exponentm, in the right-hand side of Eq.~7!, are unknown, and shouldbe estimated.

Using theab initio results of Xantheaset al.13 for theSr1Ar potential and fitting the analytic potential of Eq.~6! tothe calculated points for both electronic states, we estimatem'8. This value ofm can be taken as representative forsuch complexes as also Kaup and Breckenridge,7,8 employ-ing similar considerations of electrostatic interactions, ob-tainedm'7 – 9 for a series of alkaline earth–noble gas ionicmolecules. For calculating binding energies from Eq.~7! inunits of cm21 when the polarizabilitya is given in Å3, thereduced massm is given in amu and the effective chargeq isgiven in units of electron chargee, we obtainF850.292using Eq.~7a!.

The effective chargeq depends mainly on the electronicstate considered and varies betweenq511e and q512e~Ref. 5!. The value ofq for each electronic state is obtainedby fitting the Eq.~7! to the measured binding energies. InFig. 6 we plot the measured binding energies~solid points!for both electronic states for the systems listed in Table III asa function of the right hand side of Eq.~7!. For the groundstate of Ca1Kr and Mg1Kr we use in the plot the averagevalue of the binding energies obtained from Refs. 1, 3, 7, and8. The almost linear dependency we observe, indicates thatthe electrostatic model describes quite well the binding en-ergies of the systems under consideration. The dashed linesin Fig. 6 represent the calculated binding energies by takingthe effective chargeq51 andq52, as indicated in the fig-ure, while the solid lines are the best-fits givingq51.4 forthe ground2S state andq51.8 for the excited2P state.

These results are justified by considering the shape ofthe ionic orbitals and their orientation with respect to theinternuclear axis in both electronic states. In the excited statethe metalp-orbital is oriented perpendicular to the internu-clear axis and the noble gas atom experiences an electrostaticattraction caused from a nearly unshielded ion core (q;2.0). However, in the ground state thes-orbital of the ionshields the core more efficiently because of the smaller ef-fective charge, resulting in a smaller interaction. This ex-plains why the excited state is more strongly bound than theground state and furthermore for which reason the equilib-rium internuclear distance of the excited state is smaller~as acloser approach is possible! than the ground-state one.

It is worth to notice that for the ground state the chargeis expected to be closer toq51.0, while our value ofq51.4 is somewhat larger. Kaup and Breckendridge7,8 simi-larly obtained larger values for the effective charge for theground electronic state of Mg1/Ca1-noble gas molecules(q'1.2– 1.5). They attributed the enhancement of thecharge exposed from thes-orbital of the metal ion to the

noble gas atom as an effect of ‘‘backpolarization’’9 of themetal electron density as the noble gas atom come closer,giving thus a stronger bond.

According to the above analysis, in the framework of theion-induced dipole electrostatic interactions the observed in-crease of the binding energy of metal ion–noble gas complexas the polarizability of the noble gas atom increases seems tobe plausible. In this manner, also the increase of the bindingenergy with decreasing ‘‘size’’ of the metal ion can be ex-plained, as in this way the noble gas atom come closer to theionic core and this results in an increased electrostatic inter-action.

ACKNOWLEDGMENT

We gratefully acknowledge support from the UltravioletLaser Facility~Contract. No. ERBFMGECT950021! operat-ing at FORTH.

1S. H. Pullins, C. T. Scurlock, J. E. Reddic, and M. A. Duncan, J. Chem.Phys.104, 7518~1996!.

2T. Buthelezi, D. Bellert, V. Lewis, and P. J. Brucat, Chem. Phys. Lett.246, 145 ~1995!.

3J. S. Pilgrim, C. S. Yeh, K. R. Berry, and M. A. Duncan, J. Chem. Phys.100, 7945~1994!.

4S. I. Panov, J. M. Williamson, and T. A. Miller, J. Chem. Phys.102, 7359~1995!.

5Ch. Luder and M. Velegrakis, J. Chem. Phys.105, 2167~1996!.

FIG. 6. Dissociation energiesDe for alkaline earth–noble gas ionic com-plexes. Thex axis represents the right-hand side of Eq.~7! and assuming arepulsive potential term;1/R8. The top part of the figure displays theresults for the excited state, while in the bottom those for the ground stateare shown. The solid points are the experimental results from Table III andthe straight lines are the predicted dissociation energies from Eq.~7!. Thedashed lines are the results with effective chargeq51 andq52, while thesolid lines are the best linear fits that giveq51.4 for the ground state2S andthe q51.8 for the excited state2P.

2716 J. Chem. Phys., Vol. 108, No. 7, 15 February 1998 Prekas, Feng, and Velegrakis

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130.113.111.210 On: Sat, 20 Dec 2014 11:44:35

6Ch. Luder, D. Prekas, A. Vourliotaki, and M. Velegrakis, Chem. Phys.Lett. 267, 149 ~1997!.

7J. G. Kaup and W. H. Breckenridge, J. Chem. Phys.107, 2180~1997!.8J. G. Kaup and W. H. Breckenridge, J. Chem. Phys.107, 4451~1997!.9H. Partridge, C. W. Bauschlicher, Jr., and S. R. Langhoff, J. Phys. Chem.96, 5350~1992!.

10C. W. Bauschlicher, Jr., and H. Partridge, Chem. Phys. Lett.239, 241~1995!.

11C. W. Bauschlicher Jr., H. Partridge, and S. R. Langhoff, inAdvances inMetal and Semiconductor Clusters, Vol II, edited by M. A. Duncan~JAI,Greenwich, 1994!.

12W. H. Breckenridge, C. Jouvet, and B. Soep, inAdvances in metal and

semiconductor clusters, Vol III, edited by M. A. Duncan~JAI, Greenwich,1995!.

13S. S. Xantheas, G. S. Fanourgakis, S. C. Farantos, and M. Velegrakis, J.Chem. Phys.108, 46 ~1998!.

14W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum.26, 1150~1955!.15A. A. Radzig and B. M. Smirnov,Reference Data on Atoms, Molecules

and Ions~Springer-Verlag, Berlin, Heidelberg, 1985!.16R. S. Mulliken, Rev. Mod. Phys.2, 60 ~1930!.17H. Lefebvre-Brion, R. W. Field,Perturbations in the Spectra of Diatomic

Molecules~Academic, London, 1986!.18R. J. LeRoy and R. B. Bernstein, J. Chem. Phys.52, 3896~1970!.19R. J. LeRoy, J. Chem. Phys.101, 10217~1994!.

2717J. Chem. Phys., Vol. 108, No. 7, 15 February 1998 Prekas, Feng, and Velegrakis

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