Kenichi Tonokura

Fine‐structure branching ratios and Doppler profiles of Cl(2Pj) photofragments from photodissociation of the chlorine molecule near and in the ultraviolet region

Photofragment chlorine atoms in the 2Pj states from Cl2 at 266–500 nm are measured by a resonance‐enhanced multiphoton ionization technique. The measured branching ratio of [Cl*(2P1/2)]/[Cl(2P3/2)] increases monotonically with dissociation wavelengths from (9.5±1.8)×10−3 at 308 nm to (4.7±0.5)×10−1 at 475 nm. At 375 nm≤λ≤475 nm, Doppler profiles of the chlorine atoms indicate adiabatic formation of two Cl(2P3/2) from the 1Π(1u) state of Cl2 and Cl(2P3/2)+Cl*(2P1/2) from the B 3Π(0+u) state. On the other hand, at a shorter wavelength (308 nm), Cl*(2P1/2) is generated from the 1Π(1u) state through nonadiabatic transitions during the dissociation. The relative contribution of the 1Π(1u)←X 1Σ(0+g) and B 3Π(0+u)←X 1Σ(0+g) transitions in the photoabsorption is estimated from the obtained branching ratios at 375 nm≤λ≤475 nm.

Dynamics of the reactions of O(1D) with HCl, DCl, and Cl2

The reactions O(1D)+HCl→OH+Cl (1a) and OCl+H (1b), O(1D)+DCl→OD+Cl (2a) and OCl+D (2b), and O(1D)+Cl2→OCl+Cl (3) are studied at an average collision energy of 7.6, 7.7, and 8.8 kcal/mol for (1), (2), and (3), respectively. H, D, and Cl atoms are detected by the resonance‐enhanced multiphoton ionization technique. The average kinetic energies released to the products are estimated from Doppler profile measurements of the product atoms. The relative yields [OCl+H]/[OH+Cl] and [OCl+D]/[OD+Cl] are directly measured, and a strong isotope effect (H/D) on the relative yields is found. The fine‐structure branding ratios [Cl(2P1/2]/[Cl(2P3/2)] of the reaction products are also measured. The results suggest that nonadiabatic couplings take place at the exit channels of the reactions (1a) and (2a), while the reaction (3) is totally adiabatic.

Mechanism of the ultraviolet photodissociation of chloroethylenes determined from the Doppler profiles, spatial anisotropy, and power dependence of the photofragments

Doppler profiles of chlorine and hydrogen atomic fragments produced in the photodissociation of mono‐ and dichloroethylenes at 193 nm have been measured in a pump‐and‐probe experiment using 2+1 resonance‐enhanced multiphoton ionization. In a second experiment, the angular distributions of the Cl fragments produced from chloroethylenes at 235 and 238 nm were measured using a perfect‐focusing mass spectrometer. In a third experiment, we measured the power dependence of the relative yields of H, Cl, HCl, and HCl+ produced from vinyl chloride at 193 nm. For Cl detachment, two primary processes have been confirmed. One produces an isotropic angular distribution of photofragments, while the other produces an anisotropic distribution. For H atom detachment, an isotropic angular distribution and a Boltzmann velocity distribution were found. The ratio of yields of the Cl and H fragments was found to be 4±1 for CH2CCl2 and higher than 10 for t‐CHClCHCl and CCl2CClH. The H, Cl, and HCl yields were found to be first ...

Fine structure branching ratios and Doppler spectroscopy of chlorine atoms from the photodissociation of alkyl chlorides and chlorofluoromethanes at 157 and 193 nm

Alkyl chlorides (R=CH3, C2H5, C3H7, and C4H9), chloromethanes (CHnCl4−n), and chlorofluoromethanes (CFnCl4–n) are photodissociated at 157 and 193 nm. The chlorine atom photofragments are detected by a resonance enhanced multiphoton ionization technique. The branching ratios of the Cl photofragments [Cl*(3p2P1/2)]/[Cl(3p2P3/2)] are almost identical (0.23±0.03) for alkyl monochlorides at 157 and 193 nm and for CH2Cl2 and CHCl3 at 157 nm, while the ratios are rather small (0.10±0.02) for CH2Cl2 and CHCl3 at 193 nm. No discernible isotope effects on the branching ratios were observed when D atoms were substituted for H atoms in chloromethanes. For CCl4, CF3Cl, CF2Cl2, CFCl3 at 157 and 193 nm, the ratios are small (<0.05) irrespective of the number of Cl atoms in the parent molecules. The Doppler profiles of the chlorine photofragments have been obtained and it is found that (a) CF3Cl undergoes a perpendicular optical transition at 157 nm and (b) for halomethanes containing more than two Cl atoms, the Cl photo...

Photodissociation of hydrogen chloride at 157 and 193 nm: Angular distributions of hydrogen atoms and fine‐structure branching ratios of chlorine atoms in the 2Pj levels

The H, D, and Cl atoms from the photodissociation of HCl and DCl at 157 and 193 nm were detected by laser‐induced fluorescence (LIF) in the vacuum ultraviolet region. The Doppler profiles of the H and D resonance lines at 121.6 nm in the LIF spectra indicate that the absorption of HCl and DCl at 157 nm is a mixture of perpendicular and parallel transition (A 1Π, a 3Π0+←X 1Σ+), while that at 193 nm results almost solely from the A 1Π←X 1Σ+ transition. The fine‐structure branching ratios, are measured for the chlorine atom by LIF. Combined with the previous results, the branching ratios [Cl*(2P1/2)]/[Cl(2P3/2)] are 0.81±0.09 at 157 nm, 0.50±0.05 at 193 nm for HCl, and 0.23±0.05 at 193 nm for DCl. These results suggest that the nonadiabatic couplings during the breakup of HCl (DCl) in the excited states play an important role in determination of the fine‐structure branching ratios.

PHOTODISSOCIATION OF HYDROGEN CHLORIDE AND HYDROGEN BROMIDE

The Doppler profiles of the hydrogen atom photofragments from the photodissociation of HCl and HBr at 157 and 243 nm are observed to determine the symmetries of the photoexcited states by using a resonance enhanced multiphoton ionization technique. The phototransitions are perpendicular for HCl at 157 nm and 193 nm and for HBr at 157 nm and 243 nm. Isotope effects on the branching ratios of [Cl*(2P1/2)]/[Cl(2P3/2)] are observed for the photodissociation of HCl and DCl at 157 and 193 nm. Our results indicate that in the photodissociation processes of HCl (A 1Π) the dynamical curve crossings during the breakup of the molecule play important roles in the determination of the branching ratio.

The Doppler spectra of O(1D) from the photodissociation of O2, NO2, and N2O

The Doppler profiles of the O(1D) products from the photodissociation of O2 at 157.6 nm and NO2 and N2O at 205.47 nm are detected by a resonance enhanced multiphoton ionization technique. The translation energy and angular distributions are deduced for the O(1D) atoms. Present results indicate that (a) O2 (B 3Σ−u) has a short dissociative lifetime when irradiated at 157.6 nm, (b) NO(2Π) photofragment from NO2 at 205.47 nm is mostly vibrationally excited, and (c) most of the available energy is released as kinetic energy when N2O is photodissociated at 205.47 nm.

Doppler spectroscopy of hydrogen atoms from the photodissociation of saturated hydrocarbons and methyl halides at 157 nm

Hydrocarbons (CnH2n+2, n=3, 4, and 6) and methyl halides (CH3X, X=Cl, Br, I) are photodissociated at 157 nm. The hydrogen atom photofragments are detected by a resonance‐enhanced multiphoton ionization technique. The Doppler profiles of the hydrogen atoms from hydrocarbons are well represented by a Gaussian profile, while those from methyl halides by a mixture of a Gaussian and a non‐Gaussian profiles. These Doppler profiles are interpreted assuming that (a) hot ethyl photofragments from hydrocarbons result in the formation of hydrogen atoms and (b) methyl halides undergo both direct and indirect photolytic scissions of the C—H bonds at 157 nm.

Photodissociation of ICl at 235–248 nm

Photodissociation of iodine monochloride has been investigated at 235–248 nm by studying Doppler profiles of (2+1) resonance enhanced multiphoton ionization spectra of fragment chlorine atoms and two‐photon laser induced fluorescence spectra of iodine atoms. At 235.3–237.8 nm, a measured branching ratio of [Cl*(2P1/2)]/[Cl(2P3/2)] is 0.68±0.10. At 248 nm, the [I*(2P1/2)]/[I(2P3/2)] ratio is 0.71±0.27. From the measured Doppler profiles and the ab initio calculation, the photoprepared states leading to Cl and Cl* production are assigned to the 3Π0+ (and possibly 3∑−0+) and the 1Π1 states, respectively. Nonadiabatic couplings between the potential curves have been discussed.

Doppler profiles and fine-structure branching ratios of O(3Pj) from photodissociation of carbon dioxide at 157 nm

The O(2p 3Pj) (j=2, 1, and 0) fragments produced in the 157 nm photodissociation of CO2 were detected by resonance‐enhanced multiphoton ionization in a molecular beam. The Doppler profiles and fine‐structure branching ratios were measured for the oxygen‐atom photofragment in the 3Pj states. The Doppler profiles were analyzed to give an anisotropy parameter of β=2.0±0.2 and an internal energy equivalent to 3.9±0.3 vibrational quanta of CO. The fine‐structure populations were found to be 0.70±0.05, 0.16±0.03, and 0.14±0.03 (with error bars of ±σ) for j=2, 1, and 0, respectively. A mechanism is proposed in which complex on the 1B2 surface undergoes intersystem crossing to the 3B2 surface. A phase‐space model with a constraint on the impact parameter is shown to be consistent with the observed energy release. The nonstatistical fine‐structure population could be caused by long‐range interactions on the triplet surface. In a bulb experiment, O(3P) was produced by quenching of O(1D). The fine‐structure populati...