Vibrational overtone spectroscopy of the 4OH+OH’ combination level of HOOH v i asequential local mode–local mode excitationX. Luo, P. R. Fleming, and T. R. Rizzo

Citation: The Journal of Chemical Physics 96, 5659 (1992); doi: 10.1063/1.462665 View online: http://dx.doi.org/10.1063/1.462665 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/96/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High resolution spectra of GeH4 v=6 and 7 stretch overtones. The perturbed local mode vibrational states J. Chem. Phys. 99, 2359 (1993); 10.1063/1.465251 Infrared spectrum of tbutyl hydroperoxide excited to the 4OH vibrational overtone level J. Chem. Phys. 95, 1461 (1991); 10.1063/1.461060 Vibration–rotation spectroscopy by stimulated emission pumping in a supersonic beam: aAxis Corioliscoupled v 4=1 and v 8=1 levels of X1 A g glyoxal J. Chem. Phys. 89, 2609 (1988); 10.1063/1.455009 Unimolecular reactions near threshold: The overtone vibration initiated decomposition of HOOH (5OH) J. Chem. Phys. 84, 1508 (1986); 10.1063/1.450496 Stateresolved product detection in the overtone vibration initiated unimolecular decomposition ofHOOH(6OH) J. Chem. Phys. 81, 4501 (1984); 10.1063/1.447419

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Vibrational overtone spectroscopy of the 4VOH +VOH' combination level of HOOH via sequential local mode-local mode excitation

x. Luo, P. R. Fleming, and T. R. Rizzoa)

Department a/Chemistry, University 0/ Rochester, Rochester, New York 14627

(Received 23 October 1991; accepted 10 January 1992)

Sequential pumping of the local OH stretch vibrations in hydrogen peroxide using infrared­optical double resonance permits spectroscopic access to the 4VOH + VOH' combination level. Analysis of the rotationally resolved vibrational overtone spectra generated by this technique determines approximate rotational constants for this level and a value of 17 051.8 ± 3.4 cm-\ for the 0-0 bond dissociation energy. The linewidths of individual zeroth-order rotational transitions increase sharply with increasing K and change from smooth Lorentzian profiles to clumps of individual lines. The K dependence of the clump widths suggests that an a-axis Coriolis interaction is the primary coupling mechanism between the zeroth-order bright state and dark bath states. As a function of increasing J, each clump coalesces into a smooth Lorentzian profile. We interpret this J dependence in terms of a model that includes rotationally induced vibrational coupling among zeroth-order dark states.

I. INTRODUCTION

The local mode model successfully accounts for both the frequencies and intensities of light atom stretch overtone transitions in most polyatomic molecules. \-3 For symmetri­cal molecules with identical light atom oscillators, the most intense vibrational overtone transitions are those that depo­sit all the energy into a single oscillator, indicating that both the vibrational Hamiltonian and the dipole operator are nearly separable in a local mode basis. \,2,4

Although direct overtone excitation predominantly ac­cesses states with all the quanta in a single oscillator, double resonance overtone excitation can access local-local combi­nation levels which have the excitation energy distributed among identical oscillators. We have recently demonstrated this principle in our optical-infrared double resonance stud­ies of hydrogen peroxide HOOH. 5 Direct overtone excita­tion to the VOH = 4 level prepares a vibrational eigenstate that consists primarily of the zeroth-order state with all the quanta in one (or the other) OH stretch:

V) ~ 14) 10) 10) 10) 10) 10) ± 10) 10) 10) 10) 10) 14). (1)

Subsequent excitation of a 1 ..... 0 transition of the initially unexcited OH stretch provided access the (4,1) local-local combination level. The frequency of the 1 ..... 0 OH stretch transition of the excited molecule occurs close to the OH stretch fundamental in hydrogen peroxide,6 supporting the localized nature of this mode.

In a manner analogous to this 4 ..... 0, 1 ..... 0 optical-in­frared excitation scheme,s one can also access the (4,1) lo­cal-local combination level via a 1 ..... 0,4 ..... 0 infrared-optical double resonance excitation. Because the 1 ..... 0 OH stretch fundamental has less rotational congestion than the 4 ..... 0 OH stretch band, this latter scheme can resolve individual rotational transitions to the (4,1) local-local combination level. This report describes our spectroscopic and dynamical

.) Alfred P. Sloan Research Fellow, 1991-93.

studies of HOOH in single rotational states of the (4, I) lo­cal-local combination level.

The (4,1) combination level is below the 0-0 dissocia­tion threshold, and because our spectroscopic detection method monitors dissociation products,7.8 transitions to low rotational states of this level do not contribute to the vibra­tional overtone excitation spectra. However, the state selec­tivity of our excitation scheme permits us to determine the dissociation threshold accurately by observing the lowest en­ergy transitions that do appear in the spectra. Excitation via the (4,1) band also allows us to investigate the rotational state dependence of HOOH unimolecular dissociation dy­namics at energies extremely close to threshold.

After briefly describing our experimental approach, we present rotationally resolved vibrational overtone spectra of the (4,1) local-local combination band. Analysis of these spectra yield values for the 0-0 bond dissociation energy, the vibrational band origin, and the excited state rotational constants. Our measurement of the vibrational band origin allows us to determine the off-diagonal anharmonicity be­tween the two local OH stretch modes. We then present higher resolution spectra which show the linewidths ofindi­vidual rovibrational transitions. The J and K dependences of these linewidths provide information on the role of rota­tional motion in intramolecular vibrational energy redis­tribution.

II. EXPERIMENTAL APPROACH

Figure 1 shows an energy level diagram for the experi­ment. An infrared pulse from a Nd:YAG pumped optical parametric oscillator (OPO) promotes HOOH molecules in a static gas cell (~50 mTorr) from the ground vibrational state to v = 1 of the OH stretch (v5 ), selecting a single or small subset of rotational states from the room temperature distribution. After a 15 ns delay, a visible pulse from a Nd:Y AG laser pumped dye laser, operating on a mixture of Rhodamine 590 and Rhodamine 610 laser dyes, passes

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5660 Luo, Fleming, and Rizzo: Overtone spectroscopy of HOOH

~ ~A~'

~:_)l x'n s -_-_-_--_- - - - - - - - - - -4 -- - - - - - -

HOOH' ---.... 20H

4--

3-- 2

~(vis)

: f ,,;~; '00': 0 l' vOH=o ----------------------- -

FIG. I. Energy level schematic for the infrared-optical double resonance excitation to the (4,1) local-local combination level of HOOH. We denote ground state rotational quantum numbers with double primes and those in the UOH = I level with single primes. Rotational quantum numbers of HOOH molecules in the (4,1) combination level are left unprimed.

through a Raman shifting cell containing -4.5 atm ofH2 •

The resulting first Stokes beam (- 5-10 mJ/pulse; .1.v~0.11 em -I) excites a .1.VOH = 4 vibrational overtone transition of the initially excited HOOH molecules to put them in a state with 4 quanta in one OH stretch and one quantum in the other OH stretch. Approximately 10 ns later, an ultraviolet pulse from a frequency doubled dye laser probes the quan­tum state distribution of the OH dissociation products via laser induced fluorescence LIF. Further details of the ex­perimental approach can be found in two recent publica­tions.',8

The data in these experiments result from fixing the in­frared frequency to promote HOOH molecules to a particu­lar rotational state in VOH = 1, fixing the UV probe frequen­cy to a particular rovibrational transition in theA-Xband of the OH fragments,9 and scanning the visible overtone exci­tation frequency while collecting the total OH emission. This yields a rotationally resolved vibrational overtone exci­tation spectrum of HOOH molecules excited to the (4,1) local-local combination level. In the ensuing discussion and figures, we use double primes to designate the rotational quantum numbers of ground state molecules and primes to denote the rotational quantum numbers of molecules in the intermediate state (i.e., VOH = O. We leave the quantum numbers of molecules in the (4,1) combination level un­primed.

III. RESULTS AND DISCUSSION

A. Spectral assignments and determination of the dissociation threshold

Figure 2 displays a series of double resonance vibration­al overtone excitation spectra of HOOH which access states with K = I in the (4, I ) local-local combination level. These spectra are obtained by first exciting P P2 (J") transitions with J" = 11-13 in the VS OH stretch fundamental band6

and then scanning the overtone excitation laser frequency

HOOH (4,1) band K=1 spectra

J'=1O

1'=11

1'=12

13320 13340 13360 13380

Wavenumber (em-I)

FIG. 2. Vibrational overtone excitation spectra generated by scanning A.2 •

In each spectrum, the wavelength of the infrared photon (A., ) is set to pro­duce HOOH molecules in the UOH = I level with rotational quantum numbers K' = 1 and J' as indicated. The major peaks in each spectrum rep­resent Pand R branch transitions from the UOH = 1 level to the (4,1) combi­nation level. Because the transition is a parallel band, all transitions termi­nate on states with K = I.

over the vibrational overtone transition. The major features in each spectrum correspond to P and R branch transitions of a parallel band of a prolate symmetric top. At the values of J' and K shown in the figure, the Q branch transitions should be less than 2% of the P and R branch intensities. to Other small features in the spectrum arise from transitions which accidentally overlap the PP2 (J") transitions in the infrared excitation step and not from vibrational mixing of the excit­ed level. Note that the QPI (10) transition, which should ap­pear in the top spectrum at the frequency indicated by the arrow, is missing. Because we detect the overtone absorption by monitoring the dissociation fragments, transitions which access states below the 0-0 bond dissociation energy will not contribute to the spectrum. Assuming there is no barrier to dissociation (which is supported by our product energy partitioning studies II ), the absence of this transition indi­cates that the state with J = 9 and K = 1 in the (4,1) combi­nation level lies below the 0-0 bond dissociation energy.

In Fig. 3 we show an analogous series of double reso­nance overtone spectra which terminate on states with K = 2 in the (4,1) combination level. The primary features in these spectra correspond to QP2 (J') and QR2 (J') transi­tions, although there is another set of transitions in the J' = 12-22 spectra that result from incomplete state selec­tion in the infrared excitation step. As anticipated for levels

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Luo, Fleming, and Rizzo: Overtone spectroscopy of HOOH 5661

HOOH, (4,1) band %p) K-2 spectra

L J'=8

1 J'=IO '---

e-..... J'=12 en

I:::: ~ .s ~

J'=14

Q)

> 1'=16 ~~

q) P:::

J'=18

J'=20

J'=22

13300 13350 13400

Wavenumber (em-I)

FIG. 3. Vibrational overtone excitation spectra containing transitions to the ( 4,1 ) combination level that terminate on rotational states with K = 2.

HOOH, (4,1) band K=3 spectra

1'=4

1'=5

1'=7

13320 13340 13360 13380

Wavenumber (em-I)

FIG. 4. Vibrational overtone excitation spectra containing transitions to the ( 4,1) combination ievel that terminate on rotational states with K = 3.

with K = 2, Q branch transitions appear in the lower J spec­tra of Fig. 3 at about 10% of the P and R branch intensity. The absence of the QP2 (8) transition from the upper spec­trum indicates that the state with J = 7 and K = 2 is below the dissociation threshold.

This second series of transitions in Fig. 3 (i.e., those which result from spectral overlap in the infrared step) go to an excited torsional level 1

2 of HOOH with the same K and with J' two less than the main series. This overlap occurs because the 3.3 cm -\ difference in the torsional splittings between the ground state and intermediate vibrational level of our double resonance scheme is close to 4 times the B rotational constant. The band origin of these transitions will be shifted from the main series by the difference in the tor­sional splittings between the VOH = 1 level and the (4,1) combination level. The observed 7.06 cm - 1 shift, taken to­gether with the 8.14 cm -I torsional splitting in VOH = 1, determines the torsional splitting in the (4,1) combination level to be 1.08 cm - I. This is close to the value predicted by Diibal and Crim for the VOH = 5 level. 12

Figure 4 shows vibrational overtone spectra that access K = 3 levels of the ( 4,1) combination state. The higher value of K and the lower values of J cause the QQ3 (J') transitions to appear prominently in the spectra along with the Q P3 (J') and QR3 (J') series. The large number of additional transi­tions in the J' = 6 spectrum results from the coincidental overlap of a Q branch with the PP4 (J") transition in the infrared excitation step. Despite the complexity of this spec­trum, the ability to label the intermediate state allows us to assign the QP3 (6), QQ3 (6), and QR3 (6) transitions in the

.S en

.@ I:::: ...... ~ .....:l Q)

> 'a ro q) P:::

HOOH (4,1) band 1'=10 spectra

K=l

K=2

K=3 I

K=4

13300 13320 13340 13360 Wavenumber (em-I)

13380

FIG. 5. Vibrational overtone excitation spectra in which J' is held fixed at 10 and K' (and hence K) is varied from 1-4.

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5662 Luo, Fleming, and Rizzo: Overtone spectroscopy of HOOH

overtone spectrum. The absence of the QP3 (4) transition in the upper spectrum indicates that the level with J = 3 and K = 3 lies below the 0-0 dissociation energy.

In Fig. 5 we display a series of spectra with J' held fixed at 10 and K varied from 1 to 4. The subband origins change with K as expected for a parallel band of prolate symmetric top. \0 Although the 0.11 cm - 1 linewidth of our overtone excitation laser and the homogeneous broadening of the in­dividual rotational transitions (particularly for high K) lim­it the accuracy with which we can determine spectroscopic constants, the spectra of Figs. 2-5 provide values for Band (A-B)-(A '-B '). Together with the literature value for A'­B', we determine the values for B, A-B, and the vibrational band origin Vo listed in Table I. Using these constants and noting the absence of transitions to the J = 3, K = 3 level and the appearance of transitions to the J = 4, K = 3 level, we determine the 0-0 bond dissociation energy to be 17051.8 ± 3.4 cm - I. The accurate determination of this threshold is important for statistical calculations of product energy partitioning.

Comparison of the 4 -- 0 vibrational band origin of an HOOH molecule with VOH = 1 to the 4--0 frequency of a ground state molecule provides information on the degree of interaction between the two OH stretch oscillators. The high resolution molecular beam studies of ReiIIy and co-workers have determined the 4--0 band origin of the ground state molecule to be 13 366.23 cm - 1.13 Our spectra provide a val­ue of 13353.2 cm -I for the band origin of the 4--0 OH stretch transition in a molecule with one quantum in the other OH stretch. The 13 cm - 1 redshift determines the off­diagonal anharmonicity of the two local OH stretches, X OH ,OH

2 to be - 3.3 cm - I, since

V64•1) - V64 ,O)

X OH ,OH2 = 4

This small value of the off-diagonal anharmonicity supports the notion that the OH stretches in HOOH are very good local modes. Moreover, the similarity in the4--00H stretch band origins for excited and unexcited HOOH molecules confirms our spectral assignment of the former. The band origin we determine is ~ 720 cm - 1 (8 times the OH stretch anharmonicity) higher in frequency than a 5 -- 1 OH stretch transition frequencyl4 and cannot be mistaken with the lat­ter.

TABLE I. Spectroscopic parameters for the (4,1) combination level in cm-'.

B A-B Vo b

Do (HO-OH) Tunneling splittingC

0.8549 ± 0.0004 8.250 ± O.013a

13 353.17 ± 0.16 17051.8 ±3.4

1.1 ± 0.1

a Calculated from our measured value of [ (A-B)-(A '-B') I and the value for (A '-B') measured by Redington etal. (Ref. 6).

b Band origin for the transition from the Vs level to the (4,1) combination level.

cIn the notation of Olson, et al., (Ref. 30) this is the OG-OU tunneling splitting.

B. Rotational state dependence of the intramolecular dynamics

Thus far we have interpreted the rotationally resolved vibrational overtone spectra as if the molecule were bound in the excited state. At low resolution, the quasibound nature of the excited vibrational state is not apparent from the spec­tra. The assignability of the spectra to zeroth order J and K quantu~ numbers indicates that IVR and unimolecular dis­sociation are slow compared to the rotational period of the molecule. These processes can occur no faster than the time implied by the linewidths of individual rotational transi­tions. We now consider in more detail the information about the intramolecular dynamics of the excited molecule con­tained in these linewidths.

The linewidth (or structure) of a single rovibrational transition to the (4,1) local-local combination level is deter­mined by a combination of vibrational state mixing (or IVR) and coupling to the dissociative continuum (i.e., life­time broadening due to unimolecular dissociation). We have previously discussed in detail the spectral consequences of these two processes. 7

,8 In the frequency domain IVR mani­fests itself as a mixing of the zeroth-order bright state with nearly isoenergetic zeroth-order dark states. If this mixing distributes the bright state character among two or more eigenstates, single rotational transitions will be split into a clump of discrete lines. The terms in the Hamiltonian re­sponsible for such mixing may be purely vibrational (anhar­monic) in origin or may arise from Corio lis or centrifugal interactions which depend on the rotational quantum numbers. The J and K dependence of the clump structure associated with a single zeroth-order rotational level allows us to determine the dominant mechanism responsible for the splitting.

Coupling to the dissociative continuum imposes a finite width upon each spectral line (i.e., each component of a clump), and the magnitude of this lifetime broadening rela­tive to the clump width determines the overall appearance of a particular zeroth-order rovibrational feature in the spec­trum. If coupling to the continuum is strong compared to the coupling among zeroth-order vibrational states, each clump wiII appear as a single smooth broad line. Physically, this corresponds to unimolecular dissociation occurring faster than IVR. If the coupling to the dissociative continuum is significantly weaker than the coupling between zeroth-order vibrational states, the spectral splittings resulting from IVR will be evident.

Although our double resonance excitation scheme elim­inates spectral congestion, it is possible for residual spectral overlap to contaminate some spectra. To ensure that the structure of a spectral line is homogeneous, we compare transitions that access the same upper state by two different routes-typically a P and an R branch transition. Figure 6 displays an example of such a comparison. Spectra (a) and (b) show transitions that access the state with J = 11 and K = 4 via a Pbranch and R branch, respectively. Although these features are 39.4 cm -I apart in frequency, they exhibit very nearly identical clump structure, indicating that the structure arises from splitting of the excited level rather than inhomogeneous spectral congestion. In the few cases where

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Luo, Fleming, and Rizzo: Overtone spectroscopy of HOOH 5663

ROOR, (4,1) band

(a) J=11, K=4 via Pbraneh

(b) J=II, K=4 via R braneh

-3 -2 -1 0 1 2

Relative Wavenumber (em-I)

FIG. 6. Transitions to the rotational state with J = II and K = 4 in the (4,1) combination level via (a) a Pbranch transition and (b) an R branch transition.

HOOH, (4,1) band J=ll

K=_1 ___ *~.c-,,,,"-_O'17 em-

I

4 ~ 0 2 4

Wavenumber (em-I)

FIG. 7. Vibrational overtone transitions to rotational states with J = II and K = 1-4. The dotted lines represent Lorentzian fits to the data. The cited Iinewidths have been corrected for the 0.11 cm - I laser Iinewidth (Ref. 16) and carry an uncertainty of ± 10%.

the comparison of a P and R branch transition to the same state shows a significant difference in line shape, we discard the transitions from our analysis.

The spectra of Fig. 6 clearly reveal that the vibrational splitting (i.e., the clump width) is considerably larger than the lifetime broadening of an individual component. This is consistent with statistical adiabatic channel model (SACM) calculations of the unimolecular dissociation rates which predict that lifetime broadening is an order of magnitude smaller than our observed clump widths. IS Moreover, as de­scribed more fully below, the J dependence of the clump widths is qualitatively different than the SACM predictions for lifetime broadening. We now examine the J and K de­pendence of the clump structure to identify the physical ori­gin of the spectral splittings.

1. K dependence of the spectra/line shapes

Figures 7-9 show the K dependence of the spectral line shapes for J = 11, 13, and 15, respectively. Where included, the dotted lines represent Lorentzian fits to the data. The quoted linewidths have been corrected for the 0.11 cm - I

linewidth of our overtone excitation laserl6 and carry an uncertainty of approximately ± 10%. For each value of J, one can see a clear trend ofincreasing linewidth with increas­ing K. While low K spectra exhibit smooth Lorentzian pro­files, higher K transitions split into partially resolved clumps oflines. The K dependence of the clump widths suggests that the coupling between the bright state and zeroth-order dark states results predominantly from Coriolis interactions in­duced by a-axis rotation. 17.18 As shown in Table II, the ma-

ROOR, (4,1) band J=13

K=l je0ll,m' --------

K=2

-3 -2 -1

Relative Wavenumber (em-I)

FIG. 8. Vibrational overtone transitions to rotational states with J = 13 and K= 1-4.

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5664 Luo, Fleming, and Rizzo: Overtone spectroscopy of HOOH

HOOH, (4,1) band 1=15

K=1 ____ ±006om-' K=2

K=3

K=4

4 ~ 4 ~ 0

Relative Wavenumber (em-I)

FIG. 9. Vibrational overtone transitions to rotational states with J = 15 and K=I-4.

trix elements for a-axis Coriolis coupling are linearly pro­portional to the quantum number K, while those for b- and c-axis Coriolis coupling decrease with increasing K. 17

•18 In a

case where the density of coupled states is large, one expects the overall clump width to be proportional to the square of the coupling matrix elements; however, in a small molecule like HOOH where the states are relatively sparse, the de­pendence can be somewhere between linear and quadratic. 19

Moreover, lumpiness in the state density will cause fluctu­ations about a smooth trend. Given these considerations, the observed increase in clump widths with increasing K is clear­ly consistent with a mechanism that involves a-axis Coriolis coupling.

Centrifugal coupling can also produce K dependent linewidths; however, centrifugal terms in the Hamiltonian are typically weaker than Coriolis terms. 17 Because they de­pend more strongly upon the rotational quantum numbers, they will become important at higher values of K. Anhar­monic (Fermi) coupling will show at most a weak rotational dependence if rotational constants for the zeroth-order dark states differ from the bright state and cannot produce the

TABLE II. Rotational part of the Coriolis coupling matrix elements for a prolate symmetric rotor.

(J,K,MIP.IJ,K,M} =1iK

(J,K± I,MIPbIJ,K,M} = ± iii [J(J+ 1) -K(K 1= 1)]'12 2

(J,K± I,MIPcIJ,K,M} =~ [J(J+ I) -K(K± 1)]'12 2

strong K dependence observed in these experiments. 17 Un­fortunately, spectral congestion in the Vs fundamental band,6 which is used as an intermediate in the double reso­nance excitation, prevents us from cleanly exciting transi­tions to states with K = O. This would have allowed us to assess the contribution of anharmonic state mixing to the observed clump structure, since the a-axis Coriolis coupling matrix elements vanish for K = O. The K dependence of an­harmonic coupling matrix elements should be weak, and the linewidths of the K = 1 transitions provide an upper limit to their contribution.

2. J dependence of the spectral line shapes

In Figs. 10 and 11 we show the change in the overtone linewidths or clump structure as a function of J for K = 2 and 3, respectively. What appear as clumps of individual lines at low J gradually coalesce into smooth Lorentzian pro­files as J increases, reaching an asymptotic linewidth at about J = 15. This unusual trend in linewidths differs qual­itatively from the expected behavior for direct Coriolis cou­pling between the bright state and zeroth-order dark states. 17 As is shown in Table II, a-axis Coriolis coupling, which we know to be the dominant broadening mechanism from theK dependence of the linewidths, is independent of J.

We have observed a similar J dependence to the clump structure of single zeroth-order rotational transitions in our infrared-optical double resonance studies ofthe 5voH + Voo

combination level of HOOH.8 In that case, the VOH + Voo

combination band used for state selection is sufficiently sparse to allow the preparation of reactant molecules in

HOOH, (4,1) band K=2 spectra

J=9

J = 11

.£ til c: B

J", 13

..s ~ J '" 15 ....:I ~

.i:!; J = 17 0.21 em" ....... ~ -~ ~

J = 19 0.21 em"

J = 21

J = 23

-2 -1 0 2

Wavenumber (em-I)

FIG. 10. Vibrational overtone transitions to rotational states with K = 2 andJ=9-23.

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Luo, Fleming, and Rizzo: Overtone spectroscopy of HOOH 5665

-3

HOOH, (4,1) band K=3

-2 -1 0 1 2

Relative Wavenumber (em-I)

3

FIG. II. Vibrational overtone transitions to rotational states with K = 3 andJ= 4-15.

K = 0 levels. Because we observe extensive vibrational split­ting for the J = 1, K = 0 level, we concluded that the clump structure arises primarily from anharmonic interactions. In a manner similar to the present case, increasing J at fixed K results in the coalescence of the individual components of a clump into a smooth Lorentzian profile which initially nar­rows before reaching an asymptotic linewidth. 8 We inter­preted the SVOH + Voo data in terms of the "keyhole" model proposed by Perry and co-workers,20 and we believe that an analogous coupling scheme may be involved at the (4,1) local-local combination level. The essence of this model, il­lustrated schematically in Fig. 12, is that the primary cou-

zeroth·order bright state

dark states coupled to Is> via anhannonic

or a-axis Coriolis terms

{II, Jo • K o>}

-~

dark states coupled to II> via b- or c-axis Coriolis terms (J dependent)

pseudo molecular eigenstates

pIing mechanism between the zeroth-order bright state and some subset of zeroth-order dark states (the so-called "first tier" or "keyhole" states) is J independent. In the case of Perry's work on ethanol20 and our studies of the SVOH + VOO band in hydrogen peroxide,8 this coupling is primarily anharmonic, whereas at the (4, I) combination level of HOOH it results from a-axis Coriolis interactions. These first tier dark states in turn couple to other zeroth­order dark states by b- or c-axis Coriolis interactions which depend upon J. Because these types of Coriolis coupling do not conserve K, the density of states available to mix with the first tier states depends upon J, since there are 2J + I K levels for a given J. B- or C- axis Coriolis mixing therefore dilutes the first tier levels among the J dependent density of dark states. Since the model proposes that direct coupling to the bright state is proportional to a level's first tier character, the number of components in a spectral clump will increase with J, filling in a Lorentzian profile. The overall width of the clump is determined by the coupling matrix elements between the bright state and the first tier of dark states. Al­though this bright to first tier coupling does not depend upon J, we have previously shown that in the limit of low density of states, this model predicts an initial narrowing of the clump before it reaches its asymptotic width. 8 This narrow­ing arises from a density of states effect in the limit of low density.

Up to this point we have ignored the possibility that a J dependent unimolecular dissociation rate is responsible for the change in spectral Iineshapes we observe. SACM calcu­lations21 predict a slowly increasing unimolecular dissocia­tion rate with increasing J for the energy and angular mo­mentum range accessed by the transitions in Figs. 10 and II, although the magnitude of the predicted rates is an order magnitude of smaller than that implied by the overall clump widths. While a J dependent lifetime broadening can ac­count for the smoothening of the clump, it will not predict the narrowing of the profile with increasing J. Even if the dissociation rate were to decrease with J, this would result only in a sharpening of the components of a clump and not the narrowing of the overall clump width.

dissociative continuum

excitation spectrum

FIG. 12. Schematic of tiered coupling scheme or "Keyhole" model (Refs. 20 and 29) that results in a J dependent density of

+ zeroth-order dark states coupled to the ze-V roth-order bright state.

determined by Vs1

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5666 Luo, Fleming, and Rizzo: Overtone spectroscopy of HOOH

C. Dynamical implications

Our rotationally resolved vibrational overtone spectra of hydrogen peroxide have important implications for our understanding the intramolecular dynamics of highly vibra­tionally excited molecules. The restricted coupling of the high frequency OR stretch serves as a bottleneck which pre­vents extensive mixing to the rest of the rovibrational bath and allows our overtone spectra to be assigned and easily interpreted. Once the bright state couples to some first tier of dark states, coupling to the rest of the molecule becomes facile (as well as J dependent). This implies that other exci­tation techniques which access different parts of the poten­tial surface at the same energy may produce qualitatively different spectra. Indeed, SEP and high vibrational overtone excitation of acetylene at the same ground state energy result in radically different spectra, suggesting the possibility of excitation dependent dynamics. Overtone excitation in this molecule produces spectra that are completely assignable to zeroth-order rotational and vibrational quantum num­bers/2023 whereas SEP produces spectra so strongly per­turbed that conventional methods of spectral analysis are inapplicable.24 Coherent excitation via these two different methods would clearly lead to different intramolecular dy­namics.

The restricted coupling of the OR stretch in ROOR to the rest of the molecule could in principle be exploited for developing selective chemical bond breaking schemes, and the homogeneous linewidths we measure define the time­scale on which the chemistry must occur to maintain selec­tivity. Recent experiments ofCrim and co-workers use OR stretch overtone excitation in ROD to achieve bond selective photofragmentation25 and bimolecular reactions.26 Bar et al. have extended selective photodissociation studies of ROD to the v = 1 level using stimulated Raman pumping. 27

More recently, Bronikowski et al. have used hot R atoms in combination with OPO excitation of ROD to extend the selective bimolecular experiments to v = 1.28 Understand­ing the conditions needed to successfully execute such schemes require detailed understanding of the vibrational coupling from the state that carries oscillator strength for the excitation. This can only come from highly resolved dou­ble resonance or molecular beam spectroscopic techniques which eliminate thermal inhomogeneous spectral conges­tion.

IV. SUMMARY

Infrared-optical double resonance spectroscopy of the (4,1) local-local OR stretch combination band of ROOR permits fully rotationally resolved vibrational overtone spectra of states in the region of the unimolecular dissocia­tion threshold. These spectra allow us to determine approxi­mate rotational constants for the vibrationally excited mole­cule as well as an accurate value for the 0-0 bond dissociation energy. The linewidths of individual rotational transitions increase sharply with increasing K and change from smooth Lorentzian profiles to clumps of individual lines centered at the frequency of the zeroth-order bright state. The K dependence of the linewidths suggests that a-

axis Coriolis coupling is the primary mechanism responsible for the observed spectral splittings. As a function of increas­ing J, these clumps of lines coalesce into a smooth Lorent­zian profile. We interpret this unusual J dependence in terms of a model which includes J dependent vibrational coupling among zeroth-order dark states. The restricted coupling of the OR stretch in ROOR, as determined by the spectral linewidths, could in principle be exploited for bond selective chemical bond breaking schemes. Righly resolved spectra such as these are needed to access the viability of such schemes.

ACKNOWLEDGMENTS

We gratefully acknowledge the support of this work by the Office of Basic Energy Sciences of the Department of Energy. We also thank the Donors of the Petroleum Re­search Fund, administered by the American Chemical So­ciety, for partial support of this research.

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"X. Luo and T. R. Rizzo, J. Chern. Phys. 96,5129 (1992). 12The splitting between the torsional levels ofHOOH arises frorn tunneling

through the trans barrier for rotation about the 0-0 bond. This splitting, which represents the separation of the OG and OU torsional levels in the notation of Olson et al. [J. Mol. Spec. 127, 12 (1988) I, is 11.44 crn- I in the VOH = 0 level and decreases with increasing OH excitation. [See H.-R. Diibal and F. F. Crim, J. Chern. Phys. 83,3863 (1985).1

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