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.

Laser induced fluorescence spectra of expansion-cooled NOz/Ne samples (1 and 2 K) are reported for transitions that originate from the lowest rovibronic levels and terminate on levels near Do. At I K, nearly all transitions originate from N"=O. With the present resolution of 0.02 cm -1, the I K spectra are resolved rather well. The high density of transitions is due to couplings between rovibronic levels with different Nand K quantum numbers and with electronic characters that borrow oscillator strength from bright B 2 vibronic species of the mixed 2 A 1;2 B 2 electronic system. Just above reaction threshold, such rovibronic species comprise the manifold of levels sampled by optically prepared wave packets. However, at higher energies we argue that the density of B z vibronic species is a more relevant parameter to describe the nature of unimolecular reactions. Nuances of the optical excitation process are discussed.

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.

Theoretical stabilization and scattering studies of resonances in the addition reaction H+CO = HCO J. Chem. Phys. 94, 4192 (1991); 10.1063/1.460652 Photoinitiated H+CO2→OH+CO reactions: OH distributions and threebody interactions in CO2H2S complexesTime resolved studies of the title reaction have been carried out by photodissociating the HI moiety within weakly bound CO 2 -HI complexes. The HOCOt intermediate decomposes via a unimolecular decomposition mechanism, and the emerging hydroxyl radicals are monitored with subpicosecond temporal resolution by using laser-induced fluorescence. The measured rates are in good agreement with several theoretical predictions: Rice, Ramsperger, Kassel, and Marcus (RRKM) calculations; classical trajectory simulations on the best available potential energy surface; and recent quantum scattering calculations.

The unimolecular decomposition of expansion-cooled NO3 has been investigated in the threshold regime of the NO+O2 channel. Photoexcitation in the region 16 780–17 090 cm−1 (596–585 nm) prepares ensembles of molecular eigenstates, each of which is a mixture of the B 2E′ bright state and lower electronic states. The X 2A2′ ground state is believed to be the probable terminus of 2E′ radiationless decay, though participation of A 2E″ is also possible. For these photon energies, unimolecular decomposition occurs exclusively via the NO+O2 channel, and NO yield spectra and state distributions have been obtained. The yield spectra are independent of the rotational state monitored, as expected for a large reverse barrier. The state distributions are insensitive to the photolysis photon energy and can be rationalized in terms of dynamical bias. The NO yield goes to zero rapidly above the O+NO2 threshold (17 090±20 cm−1). Because of tunneling, the NO+O2 channel does not have a precise threshold; the value 16 780 cm−1 is the smallest photon energy that yielded signals under the present conditions. Very small decomposition rates were obtained via time-domain measurements in which reactive quenching of long-lived NO3 fluorescence was observed. The rates varied from 1×104 at 16 780 cm−1 to 6×107 s−1 at 16 880 cm−1, and their collision free nature was confirmed experimentally. These data were fitted by using a one-dimensional tunneling model for motion along the reaction coordinate combined with the threshold Rice–Ramsperger–Kassel–Marcus (RRKM) rate. The top of the NO+O2 barrier is estimated to lie at 16 900±15 cm−1. Translational energy measurements of specific NO (X 2ΠΩ,v,J) levels showed that O2 is highly excited, with a population inversion extending to energies above the a 1Δg threshold, in agreement with previous work. It is possible that the main O2 product is X 3∑g−, though some participation of a 1Δg cannot be ruled out. Within the experimental uncertainty, b 1∑+g is not produced.