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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.

Identification of the metal-containing products of reactions of the neutral transition metal atom Zr(4d25s2, 3F) with ethylene and propylene is accomplished using one-photon ionization at 157 nm and time-of-flight mass spectrometry. The reactions proceed in a fast flow reactor at 298 K with He/N2 buffer gas at 0.6 Torr. Mass spectra of the products of both Zr + C2H4 and Zr + C3H6 indicate that H2 elimination occurs as the primary reaction step. The efficiency of the Zr + C2H4 reaction shows that there is no barrier larger than about 2 kcal/mol above reactants along the entire reaction path. This corroborates an earlier theoretical prediction by Blomberg and Siegbahn of facile H2 elimination by ground-state Zr. For the secondary reactions ZrC2H2 + C2H4 and ZrC3H4 + C3H6 and for the reactions ZrO + C2H4 and ZrO + C3H6, mass spectra again indicate that H2 elimination occurs. Rate constant measurements using photoionization detection show that the presence of the C2H2 and C3H4 ligands enhances the reaction efficiency over that of the bare Zr atom, while ZrO reacts at essentially the same rate as Zr.

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.

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