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

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194316-4 Nizkorodov et al. J. Chem. Phys. 122, 194316 2005FIG. 4. Survey action spectrum. The probe laser is tuned to the 2 − 3/2 8 rotational state of OHv=0, while the pump laser is scanned in frequency. The timedelay between the IR pump and UV photolysis laser is sufficiently long to allow all vibrationally exited complexes to predissociate. Apart from a few easilyidentifiable H 2 O monomer lines, all the structure in the spectrum is due to Ar–H 2 O and H 2 O 2 . The bands labeled with asterisks almost certainly belong toAr–H 2 O but require further studies for definitive assignment.4. Indeed, some of these bands display partially resolvedrotational structure consistent with the Ar–H 2 O binary complex.A significant number of bands in the spectrum remaineven if Ar is completely replaced by He in the carrier gasmixture. However, their signal intensities are reduced becauseof much smaller clustering efficiency in pure He jets.Since He–H 2 O clustering is expected to be negligible underroom temperature stagnation conditions, these can be assignedto overtone spectra of H 2 O n complexes Table I.Tothe best of our knowledge, this represents the first such overtonespectra for neutral H 2 O clusters, specifically made possibleby the enhanced sensitivity of vibrationally mediatedphotodissociation methods. Indeed, at least one of thesebands at 7193 cm −1 even exhibits partially resolved rotationalstructure characteristic of H 2 O dimer, to which wenext direct our attention.C. First overtone „v OH =2… spectra of H 2 O dimerVibrational assignment of the OH-stretching bands ofH 2 O 2 has historically proven to be a challenging task, evenat the fundamental level. Indeed, the four OH-stretching fundamentalshave been reassigned several times and only moreTABLE I. Positions and assignments of the observed overtone bands ofAr–H 2 O and H 2 O n . Positions are accurate to within 0.2 cm −1 . WhereasAr–H 2 O assignments are relatively certain, H 2 O 2 assignments should beconsidered speculative and a source of stimulation for further theoreticalefforts.Positions cm −1 Carrier Band shape Assignment7193 a H 2 O 2 02 + a or 1 f 1 b7218.45 b Ar–H 2 O 02 − 0 00 ←1 01 7230.05 c Ar–H 2 O 02 − 0 00 ←1 01 7240 b H 2 O 2 d Most likely 2 f 0 b7249.8 b H 2 O 2 d Most likely 2 f 0 b7263.7 a Ar–H 2 O 02 − 1 01 ←0 00 7275.0 b Ar–H 2 O 02 − 1 01 ←0 00 7282 a H 2 O 2 da Approximate band center.b Q-branch position.c Band origin from fitting.d Poorly defined band shape.or less definitively understood from recent cluster sizeselectivespectroscopic work of Huisken et al. 21 Table IIsummarizes the presently accepted assignments at the v OH=1 level. With two quanta of OH-stretching excitation, theovertone spectral region is certain to be significantly morecomplex; for example, there are as many as ten differentpossibilities to distribute them among the four OH bonds inH 2 O 2 .Fortunately, theory predicts only a few of these combinationstates to be efficiently produced from the ground stateof H 2 O 2 via direct overtone pumping. Harmonicallycoupled anharmonic oscillator HCAO calculations byKjaergaard and co-workers 24,25 predict that the strongest OHovertone transitions in H 2 O 2 should be 02 − a , 2 f 0 b ,02 + a , and 1 f 1 b listed in the order of decreasing transitionstrengths, where a, f, and b refer to proton acceptor, freeproton donor, and bound proton donor OH stretches, respectively.Calculations by Chaban and Gerber done at CC-VSCFlevel 22 predict a somewhat different order of intensities:02 − a , 1 f 1 b , 2 f 0 b , 02 + a , but both studies agree that thesefour transitions should dominate the v OH =2 spectrum ofH 2 O 2 . The strongest v OH =2 bands in the cyclic water trimerspectrum are predicted to be of the type 2 f 0 b by calculationof Ref. 24. Chaban and Gerber predict that transitionsof 1 f 1 b and 0 f 2 b types should be just as strong.Figure 5 shows simulated low-resolution spectra of H 2 O 2TABLE II. Currently accepted gas-phase positions of H 2 O 2 stretching fundamentals.The positions are taken from Ref. 10 with 01 a + reassigned to0 f 1 b based on the results of Ref. 21. The 01 a + transition has only beenobserved in Ar matrices Ref. 26, where it is quite weak. Calculationssuggest that it should occur at around 3650 cm −1 in gas phase.Mode K←K Position cm −1 01 a−0 lower ←1 lower 3738.41 upper ←0 upper and 1 lower ←0 lower 37532 upper ←1 lower 37771 f 0 b 0 upper ←1 lower and 1 lower ←0 upper 3731.7+01 a Not observed3633 a0 f 1 b 0 upper ←1 lower and 1 lower ←0 upper 3601a Ar matrix.Downloaded 26 May 2005 to 128.200.198.26. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
194316-5 Overtone spectroscopy of H 2 O clusters J. Chem. Phys. 122, 194316 2005FIG. 5. Predicted band positions and intensities of v OH =2 bands of H 2 O 2 .a CC-VSCF calculation by Chaban and Gerber Ref. 22. b HCAO calculationby Schofield and Kjaergaard Ref. 25. For better representation ofintegrated band intensities, the transitions are convoluted over a Gaussianwith half-width at half-maximum HWHM=7 cm −1 .based on the prediction of Refs. 22, 24, and 25. Despite apromising concurrence in general theoretical predictions forthe overtone intensities, these studies fail to agree on themore detailed relative frequency ordering of the H 2 O 2bands, making spectroscopic assignment of the observedH 2 O n overtones quite difficult. The most significant disagreementappears to exist for the relative frequencies of the1 f 1 b and 2 f 0 b overtone bands in both the dimer andtrimer species.The analysis of the H 2 O n band centered at 7193 cm −1may help shed some light on this issue. At higher sensitivityand lower IR pump powers to avoid saturation, this bandclearly reveals a partially resolved rotational structure seeFig. 6, with a characteristic spacing between adjacent linesof roughly 0.4 cm −1 . Though not fully resolved, this isnevertheless consistent with an a-type band for H 2 O 2 ,which is known to have a near-prolate symmetric top structurewith B0.2 cm −1 , 2,10,13,16 and is clearly inconsistentwith any cluster larger than dimer. Simulations of this bandprofile using the fundamental spectroscopic constants forH 2 O 2 quickly reveals it to be composed of at least twooverlapping a-type transitions. Most relevantly to the abovediscussion, the profile cannot be satisfactorily modeled withoverlapping b- orc-type transitions, since such bands wouldbe dominated by prominent Q-branch features not observedin the experimental spectrum.This lack of strong Q-branch transitions rules out assignmentto the overtone vibration of the hydrogen bond accep-FIG. 6. A slow scan over the H 2 O 2 band at 7193 cm −1 . This band has apartially resolved rotational structure characteristic of a parallel transition inH 2 O 2 complex. The band is simulated as a superposition of three K=0←0 subbands with origins at 7192.5 cm −1 B 2 − subband; odd J: evenJ=3:6, 7191.3 cm −1 E subband; no alternation, 7193.3 cm −1 A 2 − subband;odd J: even J=6:3 with a linewidth of 0.25 cm −1 . The band origins are notuniquely determined by the simulation, though a B0.2 cm −1 rotationalconstant for water dimer is clearly consistent with the observed structure.tor, 02 − a , since this transition moment would be predominantlyalong the b axis of H 2 O 2 . In support of this, Huangand Miller 10 only observe b-type transitions for the corresponding01 − a fundamental in H 2 O dimer see Table II.Indeed, H 2 O dimer exhibits considerable perpendicularstructure in the 01 −a fundamental region due to i K=0 lower ←1 lower , ii K=1 upper ←0 upper /K=1 lower ←0 lower , andiii K=2 upper ←1 lower subbands, which are observed−18 cm −1 ,−3cm −1 , and −22 cm −1 away from the 01 − a bandorigin, respectively. By way of contrast, we do not observeadditional H 2 O 2 features in this spectral region out to atleast ±40 cm −1 away from the 7193 cm −1 band. In summary,both the 7193 cm −1 band shape as well as lack of H 2 O 2transitions in the vicinity make assignment to 02 − a unlikely.In light of the a-type rotational contour, a more plausibleassignment for the 7193 cm −1 band is the 1 f 1 b vibrationcentered on the proton donor unit of H 2 O 2 . This vibrationalmotion promotes a dipole transition moment along the a axisof H 2 O 2 , which is more consistent with the observed bandprofile. In addition, there is a strong similarity between theband observed here and the a-type transition profile fromHuang and Miller, 10 which has been assigned to 0 f 1 b , 21i.e., one quantum of the bound OH stretch. Furthermore, the1 f 1 b vibration is predicted to be the second strongest OHstretchingovertone in H 2 O 2 by Chaban and Gerber 22 andexecutes a motion that correlates with the strong 02 − overtonein H 2 O monomer. Also, the 1 f 1 b vibrational predictionof 7110 cm −1 by Chaban and Gerber see Fig. 5 is inrelatively good agreement with experiment. We note that the02 + a acceptor overtone is yet another possibility for achievingsuch a strong a-type transition moment, with predictedband origins 7170 cm −1 Ref. 22 and 7200 cm −1 Ref. 25in even closer agreement with experiment; however, the predictedovertone intensities are relatively weak in both sets oftheoretical calculations. 22,25 In this regard, however, it isworth remembering the “action” nature of these spectra, intensitiesof which rely on vibrationally mediated UV photolysisof the resulting predissociated complex. For example,excitation of the symmetric 02 + a could well predissociatepreferentially into H 2 Oinv OH =1, which, in turn, photodissociatesefficiently at 193 nm to yield OH. We will return tothis point later in the discussion but stress the importance ofvibrational overtone dynamics in interpreting the spectra.Clearly a predictive understanding of overtone frequencies,intensities, and dynamics in hydrogen-bonded systems remainsa challenging area for future progress, which thepresent work hopes to further stimulate.With a suggested assignment of the 7193 cm −1 bandstructure to either 1 f 1 b or 02 + a , we next address whetherone can reproduce the observed rotational profile withknown spectroscopic constants of H 2 O 2 . Any a-type transitionin H 2 O 2 can, in principle, exhibit complicated finestructure because of the presence of five uncoolable nuclearspin symmetry species, specifically three K=1←1 lower subbandsA − 2 ,E − ,B − 2 ; three K=0←0 upper subbands A − 2 ,E − ,B − 2 ;and three K=0←0 lower subbands A + 1 ,E + ,B + 1 . Note that thespecified subband symmetries do not include rotational symmetriesunlike the labels shown in Fig. 2. For example, allof these a-type subbands appear in close proximity to eachDownloaded 26 May 2005 to 128.200.198.26. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
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