Overtone spectroscopy of H2O clusters in the vOH - UCI Aerosol ...

194316-10 Nizkorodov et al. J. Chem. Phys. 122, 194316 2005ter, peaking at N5–6, and qualitatively much more consistentwith the Ar–H 2 O dimer results. Although further experimentswill be necessary to establish this definitively, theresults plausibly suggest that vibrational predissociation ofAr–H 2 O from 02 − 0 00 has strong contributions from thenear resonant V-V pathway:Ar – H 2 O02 − 0 00 → H 2 Ov OH =1;v bend =2;J+ Ar, E released 240 cm −1 ,which would then photofragment into the high-N OH distributionsobserved in Fig. 10. The smoothness of the resultingOH state distribution would also be consistent with severaldifferent J states produced in Eq. 4, since photolysis ofsingle J states of H 2 O generally result in much more structuredOH quantum state distributions for example, see Figs.9 and 10. As a final comment, it is worth noting that the OHstate distributions resulting from the photolysis of overtoneexcited H 2 O 2 i.e., 7193 cm −1 band are remarkably similarto the above results for Ar–H 2 O. This might again suggestsubstantial bending excitation in one or more of the HOHproducts. Because of the higher binding energy of H 2 O 2 vsAr–H 2 O D 0 1700 cm −1 vs 140 cm −1 , however, predissociationinto same H 2 Ov OH =1;v bend =2;J states is now notenergetically possible. Nevertheless, several bend-excitedchannels remain energetically open such as formation ofH 2 Ov OH =0;v bend 3 and H 2 Ov OH =1;v bend 1. Basedon the requirement of vibrationally enhanced photodissociationcross section at 193 nm, the observed distributions plausiblyarise from photolysis of H 2 Ov OH =1;v bend 1,J.However, it is worth noting that since the action spectra deriveboth their sensitivity and specificity from strong vibrationallymediated skewing of the photodissociation cross sections,this need not be representative of the full distributionof predissociated H 2 O. Nevertheless, these studies makesimple predictions and highlight some interesting directionsfor further exploration with quantum state resolution in theejected H 2 O, as perhaps could be studied by IR photofragmentationrecoil spectroscopy. 56,57IV. CONCLUSION AND SUMMARYThe combination of slit jet expansions with i IR pumpvibrational excitation, ii vibrationally selective excimerphotolysis, followed by iii state-resolved laser-inducedfluorescence probing of fragments, reveals itself as a powerfulspectroscopic tool for extending traditional vibrationallymediated photodissociation methods into the overtone regionof the water clusters. Rich vibrational structure has been observedin vibrationally mediated dissociation spectra ofH 2 O/Ar mixtures under supersonically cooled conditions inthe vicinity of the first OH-stretching overtones of H 2 O. Theobserved resonances can be assigned to overtone transitionsof Ar–H 2 O and H 2 O 2 based on their spectral structure andphotodissociation dynamics and, in favorable cases, evenpermitting direct detection of resolved rotational structure.Indeed, this is the first reported gas-phase spectra of H 2 O4dimer overtone in the gas phase that reveals both agreementand disagreement with currently available theoretical models.The use of time delayed IR pump and photolysis lasersallows direct observation of predissociation dynamics ofH 2 O complexes on the 10 ns 1 s time scale, as demonstratedon v OH =2 of Ar–H 2 O clusters. For sufficiently longlived vibrational states, this method provides a novel schemefor initiating photochemical events inside size-selected andquantum-state-selected clusters. In conjunction with parallelstudies of the isolated monomer, solvent effects on thephotofragmentation dynamics of H 2 O can be directly probedby comparison with vibrationally mediated photodissociationof the same free rotor state H 2 O state in the absence of theperturbing Ar atom. Specifically, vibrationally mediated dissociationof H 2 O within Ar–H 2 O complex clearly produceshotter rotational OH distributions as well as promoting partialnonadiabatic energy transfer between 3/2,1/2 andlambda-doublet electronic levels. A simple physical modelfor this would be intracluster collisions between the recoilingOH photofragment and Ar atom. 40,41 At a more challenginglevel, however, these data reflect the detailed photofragmentationdynamics of H 2 O in the presence of a single solventatom, yet with the considerable spectroscopic simplificationof aligned, fully quantum-state-selected reagents as well asthe special intracluster advantage of well-determined impactparameter and total angular momentum.ACKNOWLEDGMENTSFinancial support by the National Science Foundationand Air Force Office of Scientific Research is gratefully acknowledged.A.E.W.K. wishes to acknowledge gratitude toJILA for Visiting Fellowship support during the time of thisresearch.1 M. Van Thiel, E. D. Becker, and G. C. Pimentel, J. Chem. Phys. 27, 4861957.2 T. R. Dyke and J. S. Muenter, J. Chem. Phys. 60, 2929 1974.3 T. R. Dyke, K. M. Mack, and J. S. Muenter, J. Chem. Phys. 66, 4981977.4 T. R. Dyke, J. Chem. Phys. 66, 492 1977.5 R. H. Page, J. 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Coudert, J. Chem. Phys. 90, 60771989.18 R. S. Fellers, C. Leforestier, L. B. Braly, M. G. Brown, and R. J. Saykally,Science 284, 945 1999.Downloaded 26 May 2005 to 128.200.198.26. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp