Vibrational Predissociation Dynamics of p-Difluorobenzene-N2 Complexes. Comparison with p-Difluorobenzene-Ar

Abstract

The SI vibrational predissociation (VP) dynamics and physical characteristics of the p-difluorobenzene-N2(~DFB-Nz) van der Waals complex are reported. The geometry of the complex is roposed to be similar to pDFB ring, and, by analogy to the benzene-NZ complex, with the NZ nearly freely rotating parallel to the aromatic molecular plane. Upper limits to the SI and So van der Waals binding energies of Do' I 240 cm-I and DO" I 213 cm-', respectively, were obtained. Only two of the nine observed SO pDFB ring modes (~6 " the symmetric ring stretch and v< the out-of-plane ring puckering mode) appear to be perturbed by complexation, and these only slightly. In SI, none of the observed ring levels appears significantly perturbed, but surprisingly, spectroscopic evidence concerning Y ( could not be obtained. In general, YS (or Y16a in Wilson notation) is the most perturbed vibration in aromatic-rare gas van der Waals complexes. Vibrational predissociation from four initial SI ring levels lying within the first 800 cm-I of the pDFB-NZ vibrational manifold was characterized using single vibronic level fluorescence spectroscopy. State-to-state dissociation from each level produces the pDFB product molecule in only a few of many accessible SI vibrational levels.Evidence of intramolecular vibration'al redistribution (IVR) within the pDFB-Nz complex is observed for one level. The dissociation is treated by preliminary modeling based on a serial mechanism (involving IVR within the complex followed by VP) that is related to that developed by Kelley and Bernstein [J. Phys. Chem. 1986, 90, 51641 for s-tetrazine-Ar VP. The modeling accounts for the final state selectivity and most but not all of the observed VP channels. The experimental and modeling results are compared with those of the pDFB-Ar complex whose vibrational level structure differs only modestly and in a known way from that of pDFB-N*.

The partitioning of energy amongst vibration, rotation, and translation during the dissociation of p-difluorobenzene–Ar neutral and cation complexes

“…͑ϳ25% of complexes are ionized and subsequently dissociate-see Sec. III A.͒ For 29 2 , dissociation does not occur within S 1 on the experimental time scale [2][3][4]23 and the pDFB-Ar complex is ionized. The ionization process leads to the complex being produced in a number of vibrational states whose population distribution can be determined from photoelectron spectra.…”

“…31 For the images reported here there is no anisotropy in the two-dimensional distribution, which is consistent with previous measurements of the dissociation lifetimes for the complex, which show them to be long relative to the rotational period. [2][3][4]23 This allows the experimental distribution to be collapsed to a onedimensional distribution of intensity versus image radius ͑a so-called radial plot͒ and this distribution processed using the inverse Abel transform. 18 The intensities in the transformed distribution are corrected to provide the desired distribution of intensity versus total translational energy released as discussed in Ref.…”

“…This was first reported by Parmenter and co-workers using dispersed fluorescence spectroscopy. [2][3][4] They assigned the nascent population to the 6 1 ͑51%͒ and 0 0 ͑49%͒ levels. 6 1 and 30 1 cannot be distinguished in dispersed fluorescence and the authors assigned the population to 6 1 on the basis that the vibrational changes generally found for vibrational predissociation in pDFB-Ar suggest 6 1 to be a more important channel than 30 1 .…”

“…31 Second, Parmenter and co-workers observed in their studies of dissociation within S 1 that only a few of the vibrational modes were populated and that these were largely the same for all the initial states excited. [2][3][4] On this basis very few of the ten possible destination levels will be accessed. Third, the vibrational frequencies are similar in D 0 and S 1 .…”

“…Most relevant to the present work are the studies of p-difluorobenzene complexes by Parmenter's group. [2][3][4] Recently, mass analyzed threshold ionization ͑MATI͒ has been shown to provide an alternative method for observing the nascent vibrational population. 5,6 There were early hopes that VP of van der Waals complexes would provide an alternative view of vibrational energy transfer during collisions, with predissociation being a ''half collision'' process.…”