This paper presents the first experimental investigation under collisionless conditions of the competing photodissociation channels of methylamine excited in the first ultraviolet absorption band. Measurement of the nascent photofragments' velocity distributions and preliminary measurements of some photofragments' angular distributions evidence four significant dissociation channels at 222 nm: N-H, C-N, and C-H bond fission and H2 elimination. The data, taken on photofragments from both methylamine and methylamine-d2, elucidate the mechanism for each competing reaction. Measurement of the emission spectrum of methylamine excited at 222 nm gives complementary information, evidencing a progression in the amino wag (or inversion) and combination bands with one quantum in the methyl (umbrella) deformation or with two quanta in the amino torsion vibration. The emission spectrum reflects the forces in the Franck-Condon region which move the molecule toward a ciscoid geometry. The photofragment kinetic energy distributions measured for CH3ND2 show that hydrogen elimination occurs via a four-center transition state to produce HD and partitions considerable energy to relative product translation. The reaction coordinates for N-H and C-N fission are analyzed in comparison to that for ammonia dissociation from the A state and with reference to ab initio calculations of cuts along the excited state potential energy surface of methylamine which show these reactions traverse a small barrier in the excited state from a Rydbergkalence avoided crossing and then encounter a conical intersection in the exit channel. The measured kinetic energy distribution of the C-N bond fission photofragments indicates that the NH2 (NDz) product is formed in the A 2A1 state; the C-N fission reactive trajectories thus remain on the upper adiabat as they traverse the conical intersection. The mechanism for C-H bond fission is less clear; most of the kinetic energy distribution indicates the reaction evolves on a potential energy surface with no barrier to the reverse reaction, consistent with dissociation along the excited state surface or upon internal conversion to the ground state, but some of the distribution reflects more substantial partitioning to relative translation, indicating that some molecules may dissociate via a repulsive triplet surface. In general, the photofragment angular distributions were anisotropic, but the measured p -0.4 f 0.4 for C-N bond fission indicates dissociation is not instantaneous on the time scale of molecular rotation. We end with analyzing why in methylamine three other primary dissociation channels effectively compete with N-H fission while in CH30H and CH3SH primarily 0-H and S-H fission, respectively, dominate.

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The dissociation of nitric acid upon nb,O →* NO 2 excitation at 193 nm has been studied in a crossed laser-molecular beam apparatus. The primary reaction channels are OHϩNO 2 and OϩHONO. We measure the branching ratio between these two competing processes and determine ͑OHϩNO 2 ͒/͑OϩHONO͒ϭ0.50Ϯ0.05. Our experiments provide evidence of a minor OϩHONO pathway, which we assign to O(3 P) and HONO in its lowest triplet state. The dominant pathway correlates to O(1 D)ϩHONO(X 1 AЈ). The translational energy distributions reveal two distinct pathways for the OHϩNO 2 channel. One pathway produces stable NO 2 fragments in the 1 2 B 2 electronic state. The second pathway produces unstable NO 2 fragments which undergo secondary dissociation to NOϩO. We examine the influence of nonadiabaticity along the OHϩNO 2 reaction coordinate in order to explain the significant branching to this other channel. Finally, we introduce a new method for generating correlation diagrams for systems with electronic transitions localized on one moiety, in which we restrict the changes allowed in remote molecular orbitals along the reaction coordinate. Analysis of previously measured XϩNO 2 photofragment pathways in nitromethane and methyl nitrate provides further support for using a restricted correlation diagram to predict the adiabatic and nonadiabatic product channels.

We report the emission spectra of dissociating vinyl, allyl, and propargyl chloride upon photoexcitation at 199 nm. To provide a better understanding of the mixed electronic character in the Franck-Condon region of the excited states accessed, we also present ab initio calculations at the configuration interaction level for these three molecules. These experimental and theoretical results indicate large differences in the contribution of nσ* C-Cl and πσ* C-Cl character to these predominantly ππ* CdC/CtC transitions. We present arguments based on the symmetry of the pertinent molecular orbitals to explain this observed variation. We conclude by considering how the differing electronic character of the excited state in these molecules may influence the branching ratios to C-Cl fission and HCl elimination products observed in the photodissociation of vinyl, allyl, and propargyl chloride.

This paper describes the photolysis of acrylic acid (H2CCHCOOH) monomers upon π → π* excitation at 193 nm. The photofragment velocity distribution measurements indicate that only primary C−C and C−O bond fissions are major photodissociation pathways; molecular decarboxylation and decarbonylation reactions do not occur to a significant extent. There are two different primary C−C bond fission channels resulting in the production of HOCO radicals in the ground and first electronically excited states. We also determine an upper limit on the C−C bond strength of about 100 kcal/mol; this agrees with the value we calculate from literature heats of formation but is considerably less than that assumed by previous workers.

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The paper describes a general method for determining absolute branching ratios in mass spectrometric experiments. The method overcomes the chronic obstacle that the daughter ion fragmentation pattern of radical products is usually unknown. We report the absolute product branching ratio for competing primary C-Cl and C-C bond fission in chloroacetone after excitation on the 1 nπ* absorption band. To determine this branching ratio, acetyl chloride is used to calibrate the relative detection efficiency of acetyl radicals at the CH 2 CO + daughter ion relative to Cl atoms at 35 Cl + . We also calculate the daughter ion production probability for CH 2 CO + formed from internally excited CH 3 CO radicals.

The experiments presented here investigate the competing photodissociation pathways for allyl chloride upon excitation of the nominally ππ*(C=C) transition at 193 nm. The measured photofragment velocity distributions evidence C–Cl bond fission and HCl elimination. The recoil kinetic energy distribution for the HCl products is bimodal, indicating two primary processes for HCl elimination. The experimental measurements show C–Cl bond fission dominates, giving an absolute branching ratio of HCl:C–Cl=0.12±0.03 when the parent molecule is expanded through a nozzle at 200 °C. The branching ratio depends on the nozzle temperature; at 475 °C, the absolute branching ratio measured is HCl:C–Cl=0.24±0.03. We analyze the experimental results along with supporting ab initio calculations and earlier photodissociation studies of vinyl chloride in order to examine the potential influence of nonadiabaticity along the C–Cl fission reaction coordinate and its dependence on molecular conformation.