The H + NCO(X211) channel in the 193.3-nm photodissociation of HNCO has been examined by using high-n Rydberg hydrogen atom time-of-flight (HRTOF) spectroscopy, and the center-of-mass (cm) translational energy distribution has been obtained. The cm translational energy distribution peaks near the maximum available energy and shows considerable structure corresponding to NCO vibrational excitation. This is attributed to geometric changes in going from HNCO to the electronically excited potential surface then to products. Specifically, a strongly bent N-C-0 angle in excited HNCO accounts for the long NCO bending progression that is observed. A strongly anisotropic product angular distribution was observed, in agreement with an 'A" excited state and rapid dissociation via a repulsive surface. Do(H-NCO) is found to be 110.1 f 0.5 kcal mol-', in agreement with recent experimental and theoretical values.

The photoinitiated unimolecular decomposition of formaldehyde via the H+HCO radical channel has been examined at energies where the S0 and T1 pathways both participate. The barrierless S0 pathway has a loose transition state (which tightens somewhat with increasing energy), while the T1 pathway involves a barrier and therefore a tight transition state. The product state distributions which derive from the S0 and T1 pathways differ qualitatively, thereby providing a means of discerning the respective S0 and T1 contributions. Energies in excess of the H+HCO threshold have been examined throughout the range 1103⩽E†⩽2654 cm−1 by using two complementary experimental techniques; ion imaging and high-n Rydberg time-of-flight spectroscopy. It was found that S0 dominates at the low end of the energy range. Here, T1 participation is sporadic, presumably due to poor coupling between zeroth-order S1 levels and T1 reactive resonances. These T1 resonances have small decay widths because they lie below the T1 barrier. Alternatively, at the high end of the energy range, the T1 pathway dominates, though a modest S0 contribution is always present. The transition from S0 dominance to T1 dominance occurs over a broad energy range. The most reliable value for the T1 barrier (1920±210 cm−1) is given by the recent ab initio calculations of Yamaguchi et al. It lies near the center of the region where the transition from S0 dominance to T1 dominance takes place. Thus, the present results are consistent with the best theoretical calculations as well as the earlier study of Chuang et al., which bracketed the T1 barrier energy between 1020 and 2100 cm−1 above the H+HCO threshold. The main contribution of the present work is an experimental demonstration of the transition from S0 to T1 dominance, highlighting the sporadic nature of this competition.

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.

When expansion-cooled acetylene is excited to the ν″1+3ν″3 vibrational level (4 quanta of CH-stretch) and then photodissociated at 248.3 nm, the dominant product channel is C2H(Ã 2Π). This differs markedly from one-photon 193.3 nm photodissociation, which provides 1200 cm−1 less energy and yields C2H(X̃ 2Σ+) as the primary product. Photodissociation at 121.6 nm yields C2H(Ã 2Π) exclusively.

Photodissociation of alkyl and aryl iodides and effect of fluorination: Analysis of proposed mechanisms and vertical excitations by spin-orbit ab initio study