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.

The emission spectroscopy of acrolein ͑C 3 H 4 O͒, acrylic acid ͑C 2 H 3 COOH͒, and acryloyl chloride ͑C 2 H 3 COCl͒ excited at 199 nm elucidates the dominant electronic character of the excited state reached by the optical transition at this wavelength. Progressions in the CvC and CvO stretching overtones and various combination bands suggest the antibonding orbital has mixed *͑CvC͒/*͑CvO͒ character. We interpret the results in conjunction with ab initio calculations at the configuration interaction singles level to identify the influence of resonance in the excited state of these conjugated molecules. The results on acrylic acid are of particular interest as excitation in this absorption band produces the HOCO intermediate of the OHϩCO→HϩCO 2 reaction that is important in combustion.

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 study uses emission spectroscopy of H,S at excitation energies near 200 nm to probe the dissociation dynamics from a conical intersection in the Franck-Condon region to the H+SH product exit channel. Photoexcitation accesses these coupled surfaces near the transition state region of the lower adiabat, a potential surface for the excited state H+SH--+HS+H reaction. Excitation wavelengths from 199-203 nm tune through the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal to the reaction coordinate and also access energies just above and at the conical intersection. We disperse the emission from the dissociating molecules at each of five excitation wavelengths in this region to probe several features of the reaction dynamics on the coupled potential energy surfaces. The resulting emission spectra cover the range of final vibrational eigenstates from 500 to 11 000 cm-' above the initial ground vibrational state for all five excitation wavelengths, and go out to 16 500 cm-' for the 199 and 201 nm excitation wavelengths. The resulting spectra, when considered in conjunction with recent scattering calculations by Heumann and Schinke on ab initio potential energy surfaces for this system, evidence a progression of emission features to low vibrational eigenstates in the SH stretch that result from coupling of the nuclear motion from the bound to the dissociative region of the potential energy surfaces. This emission, into local mode eigenstates such as OO+l, 1 l+O, 1 l+l, 21+0, 21f 1, evidences the antisymmetric dissociative motion and bending induced near the conical intersection, and dominates the spectrum at excitation wavelengths only near 200 nm. We analyze the excitation wavelength dependence of these features and also of the n0 +O progression for ns4, which reflect the exit channel dynamics. The excitation wavelength dependence shows that while the emission spectra do not reveal any dynamics unique to scattering states that access a symmetric stretch resonance in the Franck-Condon region, they do reveal the energy location of and the dynamics at the conical intersection. A reanalysis of other workers' measurements of the SH product vibrational state distribution shows that u =0 products are strongly favored at excitation wavelengths near the conical intersection. 5652