The high-n Rydberg time-of-flight (HRTOF) technique has been used to obtain translational energy distributions of hydrogen atoms deriving from weakly-bound (HI)2 clusters photoexcited at 266 nm. A number of distinct features were observed and were used to establish much of the photophysics and photochemistry. Though the geometric structure of (HIh has not been determined experimentally, equilibrium geometries have been estimated by using several semiempirical theoretical methods, all of which predict an approximately 90" L-shaped structure with one hydrogen localized between the two iodine atoms (the interior hydrogen) and the other pointing outward (the exterior hydrogen). Zero-point amplitudes are expected to be large. The photolytic removal of the exterior hydrogen yields I-HI and I*-HI radical-molecule clusters whose properties can be described, at least qualitatively, by using the formalism put forth by Hutson and co-workers, who carried out detailed calculations for the analogous C1-k HCl system. Photodissociation of the HI moiety whose hydrogen is interior can also yield radical-molecule clusters, as well as initiate intracluster reactive and/or inelastic scattering processes. Photoproducts that contain the HI chromophore such as HI(vj), I-HI, and I*-HI can also be efficiently photoexcited, yielding hydrogen atoms having signatures that reflect their parentages. Peaks in the translational energy distribution corresponding to photodissociation of HI in v = 1 and 2 are identified and confirmed by H -D substitution. Furthermore, v = 0 rotational levels having 7 5 j 5 13 are just barely resolved. The most likely source of intemally excited HI is believed to be inelastic scattering in which the internal hydrogen strikes the adjacent HI. This is deduced from the theoretical work of Aker and Valentini, who employed the method of quasiclassical trajectories with a potential surface developed by Last and Baer and modified by Clary. These calculations suggest that the likely L-shaped geometry of (HI);? is compatible with inelastic scattering via a failed reaction mechanism, whereas the hydrogen exchange reaction has low probability since it favors near-linear H-IH approaches. Low-energy shoulders offset slightly from the monomer peaks are most likely due to inelastic and elastic scattering of the internal hydrogen as it leaves. The photolysis of I*-HI clusters can be identified by an inelastic process in which I* is deactivated, thereby yielding hydrogen atoms having translational energies in excess of the highest monomer peak by slightly less than the iodine spin-orbit splitting. Such a peak in the TOF spectrum is observed. It is inevitable that some 12 is formed with large interatom separation via the photolytic removal of hydrogen, which can occur by either H2 formation (hydrogen abstraction) or sequential photolysis.

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Build-up times for the OH product of photoinduced reactions in C0,-HI complexes were measured for the photolysis wavelength range 235263 nm by using the subpicosecond resolution pump-probe method pioneered by Zewail and co-workers. There is reasonable accord with.theory and recent results from crossed-molecularbeams experiments. To illustrate the squeezed-atom effect, complementary measurements of hydrogen atom translational-energy distributions were carried out by using the high resolution high-n Rydberg time-of-flight (HRTOF) method of Welge and co-workers. Prototypical cases of Ar-HBr and Ar-HI complexes are reported.The former illustrates the squeezed-atom effect or caging of the exiting hydrogen. The latter is dominated by the formation of radical-molecule complexes. Extensions to other radical-molecule complexes are discussed.

Translational energy distributions of the products of the 193 and 157 nm photodissociation of chloroethylenesCenter-of-mass translational energy distributions of the dominant primary products resulting from 193.3 nm excitation of jet-cooled CH 3 0H, CH 3 0D, and CD 3 0H were obtained by using the high-n Rydberg time-of-flight (HRTOF) technique. The appearance threshold in the HRTOF spectrum yields a bond dissociation energy, D o (CH 3 0-H), of 105::±: 1 kcal mol-I, in agreement with recent literature values. Translational energy release spectra from the three isotopomers exhibit progressions of 950::±:lOO cm-I , which are attributed to excitation in the v3 O-CH3 stretch of the methoxy product. The progressions peak at v = 1, with population out to at least v = 5. This differs from the results of a recent wave packet dynamics study on a calculated excited state potential energy surface [Marston et al., J. Chern. Phys. 98, 4718 (1993)], which predicted no O-CH3 stretch excitation in the methoxy fragment following photolysis of ground state methanol. The spatial anisotropy of the fragments ({3--0.7) implies a dissociation time ~ 1 ps. The impulsive model for rotational excitation is compared to the unresolved rotational contour of the vibrational peaks in the translational energy release spectra and is found to underestimate the extent of rotational excitation, though the model correctly predicts the increase in contour width observed for the O-deuterated species. The unresolved rotational contours are fit empirically. The inferred vibrational energy distributions are discussed in terms of a simple Franck-Condon model for the pseudotriatomic, Me-O-H. Implications of the vibrational and rotational photofragment distributions for the full 1 I A 1/ surface are discussed.Moreover, other spectroscopic studies have aSSigned this ab-J.

The near IR and visible vibrational absorption spectra of CHF3 were recorded up to wave numbers of 17 500 cm−1 providing complete frequency coverage, together with paper I, from the low frequency fundamentals to the N=6  CH stretching–bending overtone multiplet. All strong bands in the high overtone spectra could be predicted and assigned by means of the tridiagonal Fermi resonance Hamiltonian, including a few combinations with intense CF3 stretching vibrations already observed for the low overtones. Improved vibrational Fermi resonance constants are presented on the basis of a fit to 35 assigned bands. An analysis of the rotational fine structure of the 2ν4 (E) overtone component and several Fermi resonance component bands result in values for αb and αs, which allow us to determine Be. In the high overtone bands no rotational fine structure is observed. The bands can be understood by introducing additional homogeneous rovibrational structures of phenomenological widths Γ≊1 to 10 cm−1. The results are discussed in relation to the separation of time scales for mode selective vibrational redistribution and further evolution. The overtone band strengths are reported and analyzed approximately with the empirical local Mecke dipole function.