M. Dulligan

Photodissociation of HCl at 193.3 nm: Spin–orbit branching ratio

HCl was photodissociated by ultraviolet (uv) radiation at 193.3 nm. Time-of-flight spectra of the hydrogen atom fragment provided the spin–orbit state distribution of the chlorine fragment, [Cl(2P1/2)]/[Cl(2P3/2)]=0.69±0.02, in excellent agreement with recent theoretical studies. The H atom angular distribution studied by changing the uv photolysis laser polarization confirmed a dominant A 1Π←X 1Σ+ electronic transition in the photoexcitation process (β=−1.01±0.04 and β*=−0.94±0.07).

Photoinitiated H2CO unimolecular decomposition: Accessing H+HCO products via S0 and T1 pathways

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. A...

Photodissociation of methanol at 193.3 nm: Translational energy release spectra

Center‐of‐mass translational energy distributions of the dominant primary products resulting from 193.3 nm excitation of jet‐cooled CH3OH, CH3OD, and CD3OH were obtained by using the high‐n Rydberg time‐of‐flight (HRTOF) technique. The appearance threshold in the HRTOF spectrum yields a bond dissociation energy, D0(CH3O–H), of 105±1 kcal mol−1, in agreement with recent literature values. Translational energy release spectra from the three isotopomers exhibit progressions of 950±100 cm−1, which are attributed to excitation in the ν3 O–CH3 stretch of the methoxy product. The progressions peak at v=1, with population out to at least v=5. This differs from the results of a recent wave packet dynamics study on a calculated excited state potential energy surface [Marston et al., J. Chem. Phys. 98, 4718 (1993)], which predicted no O–CH3 stretch excitation in the methoxy fragment following photolysis of ground state methanol. The spatial anisotropy of the fragments (β∼−0.7) implies a dissociation time ≤1 ps. The ...

An experimental study of HF photodissociation: Spin–orbit branching ratio and infrared alignment

Single rotational levels of HF (v=3) were prepared by using overtone excitation and these molecules were then photodissociated by ultraviolet (UV) radiation at 193.3 nm. Time‐of‐flight spectra of the hydrogen atom fragment provided the spin–orbit state distribution of the fluorine fragment. Changing the UV photolysis laser polarization confirmed an A 1Π←X 1Σ+ electronic transition in the photodissociation step. Photodissociation of HF at 121.6 nm is also reported. Infrared (IR) induced alignment of the diatom was studied by monitoring the IR laser polarization dependence of the H‐atom product angular distribution. Depolarization due to hyperfine interaction was studied by using the R(0) transition. Agreement with theory is excellent.

Vibrationally resolved translational energy release spectra from the ultraviolet photodissociation of methyl mercaptan

Product translational energy release spectra resulting from 248 and 193 nm photodissociation of methyl mercaptan are obtained for the hydrogen atom channels (CH3SH+hν→CH3S+H) by using the high‐n Rydberg time‐of‐flight technique. The spectra exhibit vibrational structure that is assigned to a CH3–S stretch progression. At 248 nm, the progression extends only to v=2, while at 193 nm levels up to approximately v=17 are populated. The progression observed at 193 nm is bimodal, with the higher kinetic energy component showing greater spatial anisotropy than the lower energy component, suggesting that two different processes occurring on different time scales are responsible for the two components. The results at 248 nm are consistent with excitation to a repulsive electronic surface. For 193 nm excitation, the high kinetic energy component is consistent with direct photoexcitation to a repulsive surface and/or rapid intramolecular access to a repulsive surface. The lower kinetic energy component presumably der...

Propensities toward C2H(Ã 2Π) in acetylene photodissociation

When expansion‐cooled acetylene is excited to the ν″1+3ν″3 vibrational level (4 quanta of CH‐stretch) and then photodissociated at 248.3 nm, the dominant product channel is C2H(A 2Π). This differs markedly from one‐photon 193.3 nm photodissociation, which provides 1200 cm−1 less energy and yields C2H(X 2Σ+) as the primary product. Photodissociation at 121.6 nm yields C2H(A 2Π) exclusively.

HCO rotational excitation in the photoinitiated unimolecular decomposition of H2CO

The unimolecular decomposition of H2CO via the H + HCO radical channel has been examined by photoexcitation of the S1 2243 and 2341 vibrational bands (31500-31855 cm−1). The H-atom translational energy distributions, obtained by using the high-n Rydberg time-of-flight technique, reflect the HCO internal energy distributions and reveal rotational excitation as high as Ka = 6 for ν = 0. Such high-Ka levels are believed to be the result of contributions from the S0 ground state surface at energies where the S0 and T1 radical pathways compete.

Quenching of interconversion tunneling: The free HCl stretch first overtone of (HCl)2

Cavity ringdown laser absorption spectroscopy has been used to record spectra of (H35Cl)2 and its Cl-substituted mixed dimers at the first overtone of the free hydrogen stretch (2ν1). The dimers were produced in pulsed planar supersonic expansions. Significant quenching of interconversion tunneling (i.e., which exchanges the roles of H-bond donor and acceptor) has been observed. Thus, the H35Cl–H37Cl and H37Cl–H35Cl heterodimers are distinguished in the 2ν1 eigenstates, which is not the case for the ground and HCl-stretch fundamental eigenstates because of facile tunneling mixing.

Propensities toward C{sub 2}H({ital {tilde A}} {sup 2}{Pi}) in acetylene photodissociation

When expansion‐cooled acetylene is excited to the ν″1+3ν″3 vibrational level (4 quanta of CH‐stretch) and then photodissociated at 248.3 nm, the dominant product channel is C2H(A 2Π). This differs markedly from one‐photon 193.3 nm photodissociation, which provides 1200 cm−1 less energy and yields C2H(X 2Σ+) as the primary product. Photodissociation at 121.6 nm yields C2H(A 2Π) exclusively.

Propensities toward C2H(Ã 2Π) in acetylene photodissociation

When expansion‐cooled acetylene is excited to the ν″1+3ν″3 vibrational level (4 quanta of CH‐stretch) and then photodissociated at 248.3 nm, the dominant product channel is C2H(A 2Π). This differs markedly from one‐photon 193.3 nm photodissociation, which provides 1200 cm−1 less energy and yields C2H(X 2Σ+) as the primary product. Photodissociation at 121.6 nm yields C2H(A 2Π) exclusively.