ABSTRACT:The branching ratios for the spin-forbidden photodissociation channels of 12 C 16 O in the vacuum ultraviolet (VUV) photon energy region from 102 500 (12.709 eV) to 106 300 cm −1 (13.180 eV) have been investigated using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to three 1 Σ + and six 1 Π Rydberg-type states, including the progression of W(3sσ) 1 Π(v′ = 0, 1, and 2) vibrational levels of CO, have been identified and investigated. The branching ratios for the product channels C( 3 P) + O( 3 P), C( 1 D) + O( 3 P), and C( 3 P) + O( 1 D) of these predissociative states are found to depend on the electronic, vibrational, and rotational states of CO being excited. Rotation and e/f-symmetry dependences of the branching ratios into the spin-forbidden channels have been confirmed for several of the 1 Π states, which can be explained using the heterogeneous interaction with the repulsive D′ 1 Σ + state. The percentage of the photodissociation into the spin-forbidden channels is found to increase with increasing the rotational quantum number for the K(4pσ) 1 Σ + (v′ = 0) state. This has been rationalized using a 1 Σ + to 1 Π to 3 Π coupling scheme, where the final 3 Π state is a repulsive valence state correlating to the spin-forbidden channel.

Ultraviolet (UV) photodissociation dynamics of jet-cooled benzyl radical via the 4(2)B(2) electronically excited state is studied in the photolysis wavelength region of 228 to 270 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, H-atom photofragment yield (PFY) spectra are obtained using ethylbenzene and benzyl chloride as the precursors of benzyl radical, and they have a broad peak centered around 254 nm and are in a good agreement with the previous UV absorption spectra of benzyl. The H + C(7)H(6) product translational energy distributions, P(E(T))s, are derived from the H-atom TOF spectra. The P(E(T)) distributions peak near 5.5 kcal mol(-1), and the fraction of average translational energy in the total excess energy, , is ∼0.3. The P(E(T))s indicate the production of fulvenallene + H, which was suggested by recent theoretical studies. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The H/D product ratios from isotope labeling studies using C(6)H(5)CD(2) and C(6)D(5)CH(2) are reasonably close to the statistical H/D ratios, suggesting that the H/D atoms are scrambled in the photodissociation of benzyl. The dissociation mechanism is consistent with internal conversion of the electronically excited benzyl followed by unimolecular decomposition of the hot benzyl radical on the ground state.

The first direct branching ratio measurement of the three lowest energy dissociation channels of CO that produce C( 3 P) + O( 3 P), C( 1 D) + O( 3 P), and C( 3 P) + O( 1 D) is reported. Rotational resolved carbon ion yield spectra for two bands (W(3sσ ) 1 (v = 3) at 108 012.6 cm −1 and 1 (v = 2) at 109 017 cm −1 ) and two bands ((4sσ ) 1 + (v = 4) at 109 452 cm −1 and (4pσ ) 1 + (v = 3) at 109 485 cm −1 ) of CO were obtained. Our measurements show that the branching ratio in this energy region is strongly dependent on the electronic and vibrational energy but it is independent or just weakly dependent on the parity and rotational energy levels. To our knowledge, this is the first time that the triplet channel producing O( 1 D) has been experimentally observed and this is also the first time that a direct measurement of the branching ratio for the different channels in the predissociation of CO in this energy region has been made.

The branching ratios for the N(4 S) + N(2 D), N(4 S) + N(2 P), and N(2 D) + N(2 D) channels are measured for the photodissociation of X v J N ; 0, g 2 1 () S  =  + in the vacuum ultraviolet (VUV) region of 100,808-122,159 cm −1 using theVUV-VUV pump-probe approach combined with velocity-map-imaging-photoion detection. No evidence of forming the ground-state N(4 S) + N(4 S) products is found. No potential barrier is observed for the N (2 D) + N(2 D) channel, but the N(4 S) + N(2 P) channel has a small potential barrier of ≈740 cm −1. The branching ratios are found to depend on the symmetry of predissociative N 2 states instead of the total VUV excitation energy, indicating that N 2 photodissociation is nonstatistical. When the branching ratios for N(4 S) + N(2 D) and N(4 S) + N (2 P) products are plotted as a function of the VUV excitation energy for the valence N 2 1 Π u and 1 u S + states, oscillations in these ratios are observed demonstrating how these channels are competing with each other. These data can be used to select both the velocity and internal states of the atomic products by picking the quantum state that is excited. High-level ab initio potential energy curves of the excited N 2 states are calculated to provide insight into the mechanisms for the observed branching ratios. The calculations predict that the formation of both N(4 S) + N(2 D) and N(4 S) + N(2 P) channels involves potential energy barriers, in agreement with experimental observations. A discussion of the application of the present results to astronomy, planetary sciences, and comets is given.

Direct branching ratio measurements of the three lowest dissociation channels of (12)C(16)O that produce C((3)P) + O((3)P), C((1)D) + O((3)P), and C((3)P) + O((1)D) are reported in the vacuum ultraviolet region from 108,000 cm(-1) (92.59 nm) to 110,500 cm(-1) (90.50 nm) using the time-slice velocity-map ion imaging and nonlinear resonant four-wave mixing techniques. Rotationally, resolved carbon ion yield spectra for both (1)Σ(+) and (1)Π bands of CO in this region have been obtained. Our measurements using this technique show that the branching ratio in this energy region, especially the relative percentages of the two spin-forbidden channels, is strongly dependent on the particular electronic and vibrational energy levels of CO that are excited.