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Using hexapole quantum state-selection of OCS (v2=0,1,2|JlM) and high-resolution slice imaging of quantum state-selected CO(J), the state-to-state cross section OCS (v2=0,1,2|JlM)+hν→CO(J)+S(1D2) was measured for bending states up to v2=2. The population density of the state-selected OCS (v2=0,1,2|JlM) in the molecular beam was obtained by resonantly enhanced multiphoton ionization of OCS and comparison with room temperature bulk gas. A strong increase of the cross section with increasing bending state is observed for CO(J) in the high J region, J=60–67. Integrating over all J states the authors find σ(v2=0):σ(v2=1):σ(v2=2)=1.0:7.0:15.0. A quantitative comparison is made with the dependence of the transition dipole moment function on the bending angle. [ABSTRACT FROM AUTHOR]

Using hexapole quantum state-selection of OCS (v2=0,1,2|JlM) and high-resolution slice imaging of quantum state-selected CO(J), the state-to-state cross section OCS (v2=0,1,2|JlM)+hν→CO(J)+S(1D2) was measured for bending states up to v2=2. The population density of the state-selected OCS (v2=0,1,2|JlM) in the molecular beam was obtained by resonantly enhanced multiphoton ionization of OCS and comparison with room temperature bulk gas. A strong increase of the cross section with increasing bending state is observed for CO(J) in the high J region, J=60–67. Integrating over all J states the authors find σ(v2=0):σ(v2=1):σ(v2=2)=1.0:7.0:15.0. A quantitative comparison is made with the dependence of the transition dipole moment function on the bending angle. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of CBr4 at 267 nm has been studied using time of flight (TOF) mass spectrometry and ion velocity imaging techniques. The photochemical products are detected with resonance enhanced multiphoton ionization (REMPI) as well as single-photon vacuum ultraviolet ionization at 118 nm. REMPI at 266.65 and 266.71 nm was used to detect the ground Br(2P3/2) and spin-orbit excited Br(2P1/2) atoms, respectively. The translational energy and angular distributions are consistent with direct dissociation from an excited triplet state and indirect dissociation from high vibrational levels on the singlet ground state surface. Br2+ ions are also observed in the TOF spectra with a focused 267 nm laser. The counter fragment, CBr2+, is observed when this photolysis laser is unfocused, and photons at 118 nm are used to ionize the radical products. The translational energy distributions of the CBr2+ and Br2+ products can be momentum matched, which indicates that molecular Br2 elimination is one of the primary dissociation channels. [ABSTRACT FROM AUTHOR]

The ab initio/Rice–Ramsperger–Kassel–Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of benzene at various wavelengths upon absorption of one or two UV photons followed by internal conversion into the ground electronic state. Reaction pathways leading to various decomposition products have been mapped out at the G2M level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for C6H5+H, C6H4+H2, C4H4+C2H2, C4H2+C2H4, C3H3+C3H3, C5H3+CH3, and C4H3+C2H3 have been calculated subsequently using both numerical integration of kinetic master equations and the steady-state approach. The results show that upon absorption of a 248 nm photon dissociation is too slow to be observable in molecular beam experiments. In photodissociation at 193 nm, the dominant dissociation channel is H atom elimination (99.6%) and the minor reaction channel is H2 elimination, with the branching ratio of only 0.4%. The calculated lifetime of benzene at 193 nm is about 11 μs, in excellent agreement with the experimental value of 10 μs. At 157 nm, the H loss remains the dominant channel but its branching ratio decreases to 97.5%, while that for H2 elimination increases to 2.1%. The other channels leading to C3H3+C3H3, C5H3+CH3, C4H4+C2H2, and C4H3+C2H3 play insignificant role but might be observed. For photodissociation upon absorption of two UV photons occurring through the neutral “hot” benzene mechanism excluding dissociative ionization, we predict that the C6H5+H channel should be less dominant, while the contribution of C6H4+H2 and the C3H3+C3H3, CH3+C5H3, and C4H3+C2H3 radical channels should significantly increase. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ultraviolet photodissociation of NHD[sub 2] excited to the fourth overtone state of the NH stretching mode (5ν[sub NH]) and NH[sub 2]D excited to that of the ND stretching mode (5ν[sub ND]) has been investigated by using a crossed laser and molecular beams method. Branching ratio between the NH and ND bond dissociations has been determined by utilizing a (2+1) resonance enhanced multiphoton ionization scheme of H and D atoms. For the photolysis of NHD[sub 2] in the 5ν[sub NH] state, the NH dissociation cross section is 5.1±1.4 times as large as the ND dissociation cross section per bond. On the other hand, for the photolysis of NH[sub 2]D in the 5ν[sub ND] state, the ratio of the NH dissociation cross section per bond to the ND dissociation cross section decreases to 0.68±0.16. In comparison with the branching ratios for the photolysis of vibrationally unexcited NH[sub 2]D and NHD[sub 2] [Koda and Back, Can. J. Chem. 55, 1380 (1977)], the present results indicate that the excitation of the NH stretching mode enhances the NH dissociation with ca. two times larger NH/ND branching ratio, whereas the excitation of the ND stretching mode results in the preferential ND dissociation with ca. 3–4 times larger ND/NH branching ratio than that for the vibrational ground states. The mechanism of the bond-selective enhancement has been discussed in terms of the energetics and dynamics of wave packet. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of CH3I and CD3I have been examined by using multiphoton ionization to probe the CH3, CD3, I(≡5 2P3/2) and I*(≡I 5 2P1/2) photoproducts. The parent compounds were cooled in a supersonic expansion, collimated into a molecular beam, and dissociated at 266 nm. For the CD3I dissociation, the ratio of CD3(v=0)/(v=2) was estimated to be about 1.1, with multiple determinations ranging from 0.47–2.1. The quantum number v here denotes the nascent excitation of the ν2 ‘‘umbrella’’ mode. Measurements of the CD3(v=1) and (v=3) vibronic bands indicated that the (v=1)/(v=3) ratio is greater than unity, with some measurements suggesting values as large as 10. A value for the CH3(v=0)/(v=2) ratio from dissociation of CH3I could not be estimated, although it was clearly larger than that for CD3. The CH3(v=0) and CD3(v=0) products from this dissociation are fit by 120±30 K and 105±30 K rotational distributions, respectively. The dissociation mechanism produces alignment in the molecular frame such that there is a strong preference for K=0 (rotation perpendicular to the top axis). Assuming that the relative velocity vector lies along the CH3 C3 axis, the velocity and rotation vectors tend to be perpendicular. It is likely that K=0 molecular frame alignment is produced in photodissociation through both the I and I* channels. [ABSTRACT FROM AUTHOR]