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

194316-6 Nizkorodov et al. J. Chem. Phys. 122, 194316 2005other in far-infrared spectra of acceptor-wag vibration inH 2 O 2 and D 2 O 2 . 13,49 In practice, the A + 1 and B + 1 subbandsfrom K=0 lower are weak because of low statistical weightsFig. 2. The E subbands from K=1 lower and K=0 upper arealso weak because these levels can relax all the way down toK=0 lower under supersonic jet conditions. Finally, based onv OH =1 fundamental transitions in H 2 O 2 , 10 the K=1←1bands are likely to be significantly broadened by predissociation.Therefore, the dominant a-type contributions to the rotationalstructure should come from i K=0←0 upper A − 2 andB − 2 and ii K=0←0 lower E + subbands.The K=0 upper states in the vibrationless H 2 O 2 are wellunderstood: the donor-acceptor interchange splitting betweenK=0 upper A − 2 and B − 2 states is 0.65 cm −1 . 13,16,50 Althoughthis splitting is known to increase for some intermolecularmodes of H 2 O 2 that encourage the donor-acceptorinterchange, 13 excitation of the OH-stretching states is expectedto reduce it significantly. For example, the interchangeA − 2 /B − 2 splitting is just 0.061 cm −1 in the 01 − a K=0 lower state. 10 Since H 2 O 2 retains its plane of symmetry inboth 1 f 1 b and 02 + a states, the origin difference of the K=0←0 upper A − 2 and B − 2 subbands should equal the sum of theK=0 A − 2 /B − 2 interchange splittings between lower and uppervibrational states, i.e., on the order of 0.7–0.8 cm −1 . Indeed,at our modest resolution, a0.8 cm −1 −separation of the A 2and B − 2 subband origins would nicely, albeit fortuitously, explainthe lack of intensity alternation in the spectrum especiallyevident in the R-branch region, because of cancellationat low J of the predicted B −−2 J even /J odd =6/3 vs A 2J even /J odd =3/6 nuclear spin statistical ratios.In the interest of simplicity, therefore, we have modeledthe observed transition profile as a combination of i twoK=0←0 upper A − 2 and B − 2 subbands, separated by 0.8 cm −1and ii one K=0←0 lower E + subband, with the relative locationof the K=0←0 lower and K=0←0 upper subbands treatedas an adjustable parameter. Each H 2 O 2 subband is calculatedas a near prolate symmetric top with rotational parameterstaken from Ref. 10, with the result shown in Fig. 6. Thesimulation is consistent with a 7±3 K rotational temperaturesame as for Ar–H 2 O bands discussed below and readilyreproduces several salient features of the observed band,namely, i parallel structure, ii absence of a band gap, andiii no obvious intensity alternation. However, with thepresent instrumental resolution, the simulation is not verysensitive to the transition band origins, which thus remainpoorly determined. Nevertheless, the rotational structureclearly confirms the carrier of the observed band to beH 2 O 2 that we can tentatively assign to either the 1 f 1 b or02 + a overtone vibration.Inspection of the spectrum in Fig. 4 indicates the potentialpresence of several other H 2 O n bands, the assignmentof which requires identification or suppression of the muchstronger Ar–H 2 O transitions. Since the water complexes predissociatemuch faster than the laser pulse duration see below,the Ar–H 2 O bands can be largely suppressed by recordingthe spectrum using very small IR pump–UVphotolysis delays. This procedure reveals that the bands at7240, 7250, and 7282 cm −1 can be ascribed to H 2 O n complexesthat, based on both theoretical predictions and matrixstudies, most likely correspond to 2 f 0 b transitions in waterdimer Table I. Specifically, Perchard reported a strong bandat 7236 cm −1 in argon matrix 26 and a corresponding band at7220 cm −1 in nitrogen matrix, 27 which he assigned to 2 f 0 bovertone of the proton donor unit as well. His assignmentswere recently corroborated by HCAO calculations. 25 CC-VSCF calculations of Chaban and Gerber place this bandhigher in frequency but also predict a large transitionstrength for 2 f 0 b . 22 The present gas-phase studies providesome additional information; in particular, the 7240 and7250 cm −1 bands appear perpendicular which is consistentwith 2 f 0 b vibrational motion predominantly along the caxis. The subband spacings and rotational contours are consistentwith a 2 f 0 b overtone band assignment, but basedon theoretical predictions, it could, in principle, arise fromthe acceptor 02 − a band. Further theoretical efforts in thisovertone region would be extremely useful to settle theseissues.There has been a lot of interest in the spectroscopicquantification of H 2 O 2 in the atmosphere via its overtonetransitions. 35 In view of the difficulties associated with precisetheoretical predictions of H 2 O 2 overtone frequenciesand intensities, it is challenging to assign features observedin atmospheric transmission spectra to H 2 O 2 withcertainty. 26 Present observation of a rotationally resolvedovertone of H 2 O 2 is an important step towards resolvingthis problem. Although the proximity of the 1 f 1 b band ofH 2 O 2 to free H 2 O transitions reduces its potential usefulnessfor observational work on H 2 O 2 in the atmosphere, itis the only known rotationally resolved overtone transitionrigorously assigned to H 2 O 2 . High-resolution spectra ofH 2 O 2 overtones, such as v OH =3, should be even more usefulfor the observational studies, and this is where futureefforts of spectroscopists studying H 2 O 2 should be directed.D. Overtone „v OH =2… spectra of Ar–H 2 OThe vibrationally mediated IR spectra in Fig. 4 areclearly dominated by transitions of Ar–H 2 O van der Waalsclusters, to which we now turn our attention. Indeed, a questionworth raising is why the spectra of such weakly boundvan der Waals complexes can be so prominent over the muchmore strongly bound H 2 O n species, even though the latterare likely present in much higher concentrations. The answeralmost certainly has to do with the vibrationally mediatednature of the action spectroscopy that requires the IR photonto enhance the subsequent 193 nm photodissociation of H 2 O,either in the complex or its predissociated fragments. Thisenhancement, in turn, depends very sensitively on the numberof quanta in OH stretch excitation in the H 2 O subunit, asbeautifully elucidated by Crim co-workers. 36,44,51 For anatom-polyatom species such as Ar–H 2 O, predissociation atthe first overtone level occurs on a relatively slow time scale10–100 ns, depending on the specific internal rotor quantumstate excited that, with 7 ns laser time resolution, readilypermits efficient photolysis of H 2 O in the v OH =2 manifold.Furthermore, v OH =−1 predissociation of the weakly boundAr–H 2 O complex D 0 140 cm −1 Ref. 42 most likelyDownloaded 26 May 2005 to 128.200.198.26. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp