Kosuke Shobatake

Methylene singlet--triplet energy splitting by molecular beam photodissociation of ketene

The singlet–triplet splitting in methylene has been determined from the measurements of fragment velocities from ketene photodissociation at 351 and 308 nm in a molecular beam. The splitting is found to be 8.5±0.8 kcal/mol. This agrees with many experimental results, but not with the value of 19.5 kcal/mol derived from recent photodetachment experiments on CH−2.

Direct absorption spectra of jet-cooled benzene in 130-260 nm

The direct absorption spectrum of benzene in a free jet has been measured in the 130–260 nm region (S1, S2, and S3 states, Rydberg series, and the first ionization limit) using synchrotron radiation as a light source. The absolute molar extinction coefficients (e) of benzene in jets have been determined by scaling measured free‐jet values to the known value in the vapor phase for a broadband at 200.1 nm in the S2 state. The vibrational temperature for ν16 mode was estimated to be 185 K. The maximum value of e of the S1 absorption system was found to be 1400 l mol−1 cm−1 (spectral bandwidth=0.065 nm). A shoulder observed at 205.45 nm in the S2 absorption system is assigned to the S2 origin, induced by pseudo‐Jahn–Teller distortion.

Unimolecular decomposition of the long‐lived complex formed in the reaction F+C4H8

The reactions of fluorine atoms with four butene isomers have been studied by the method of crossed molecular beams. Angular distributions and velocity spectra of products formed by substitution of F for CH3 and H have been measured. Relative rates for CH3 and H substitution are in qualitative agreement with RRKM predictions for competitive rates of dissociation of a fluorobutyl complex. Quantitative agreement is possible with minor adjustments in the exothermicities or barriers to dissociation. The kinetic energy distributions of the products, on the other hand, are not in accord with those predicted by statistical theories of unimolecular reactions. The results suggest that the assumption of randomization of internal energy breaks down at least for complex configurations close to the transition state where coupling between separating fragments is still strong enough to allow appreciable nonrandom mixing of translational energy with rotations and vibrations of the larger fragment.

A photoelectron spectroscopic study of (3+1) resonant multiphoton ionization of NO and NH3

In the present work, we have measured energy spectra and angular distributions of photoelectrons emitted by three‐photon resonant four‐photon ionization of NO and NH3 in the gas phase. The following conclusions have been obtained: (1) Ionization of NO through the Rydberg F and H (H′) states at v′=0 and 1 gives rise to the ground‐state ion with v=0 and 1, respectively, by a Δv=0 transition; (2) Ionization of NH3 through the Rydberg C′ states with v′=0–5 gives rise to the ground‐state ion with v=0–5, respectively, by Δv=0 transitions; (3) Photoelectron angular distributions obtained for the (3+1) processes may well be interpreted in terms of cosine‐square distributions. This fact strongly indicates that the ionization step takes place by one‐photon direct ionization from the three‐photon resonant states.

Emission spectra of SiH(A 2Δ→X 2Π) and SiCl2(Ã 1B1→X̃ 1A1) in the VUV photolyses of silane and chlorinated silanes

Vacuum UV photolyses of silane and chlorinated silanes were investigated by using rare gas resonance lamps. Strong emissions, from the A 2Δ→X 2Π transition of SiH and the 1P0→1D2 transition of Si were observed in the photolysis of SiH4 by Ar and Kr resonance lamps. It was suggested that the threshold energies for the appearance of both emissions were lower than the values obtained by the electron impact of SiH4 reported by Perrin et al. A new broad unstructured emission band in the region 300–400 nm was observed in the photolysis of SiH2Cl2 and SiHCl3 by Ar, Kr, and Xe lamps. The band was attributed to the A 1B1→X 1A1 transition of SiCl2 radicals from the measurement of the appearance energy of the emission by using synchrotron orbital radiation.

Reactions of F atoms and aromatic and heterocyclic molecules: Energy distribution in the reaction complex

This paper reports studies of the reactions of fluorine atoms and several aromatic and heterocyclic molecules. As in earlier work we have used the method of crossed molecular beams to create long‐lived reaction intermediates and to deduce from the angular distribution of the product molecules information about the energy distribution in the reaction complex. The data reported in this paper complement those collected earlier. They show that when a light group is emitted from a reaction complex randomization of the internal energy of the complex is not more rapid than reaction. When a heavy atom is emitted from the reaction complex the limited data available indicate that almost all vibrational degrees of freedom of the complex participate in the reaction. This latter observation provides an interesting connection between vibration‐translation energy exchange and the mechanism of chemical reaction.

CCl2(Ã 1B1) radical formation in VUV photolyses of CCl4 and CBrCl3

In vacuum ultraviolet photolyses of CCl4 and CBrCl3 a diffuse emission band was observed in the region of 410–750 nm by Ar i resonance and H Lyman‐α line irradiation. The band was attributed to a CCl2(A 1B1 → X 1A1) transition from the measurements of the appearance energies of the emitters produced from photodissociative excitation of both CCl4 and CBrCl3 using synchrotron radiation. The fluorescence decay of the CCl2(A → X) transition showed a superposition of two lifetime components of 2.17±0.26 and 4.0±0.12 μs at pressures from 10 to 140 mTorr. The pressure dependence of the amplitudes for the two lifetimes suggests the occurrence of collision‐induced intersystem crossing between 1B1 and 3B1 states of CCl2 radicals. The absorption cross section of CBrCl3 was measured for the first time in the 106–200 nm wavelength region and tentative assignments of Rydberg transitions are presented.

Anisotropic velocity distribution of desorbing product in carbon monoxide oxidation on palladium (110)

Significant anisotropy was found in the velocity distributions of desorbing product CO2 from a Pd(110) surface. The velocity distributions were determined by a cross‐correlation time‐of‐flight technique combined with angle‐resolved thermal desorption. Heating the coadlayer of CO and oxygen produces five peaks in the CO2 formation spectrum; P1– (around 420 K), P2– (∼370 K), P3– (∼300 K), P4– (∼230 K), and P5–CO2 (∼170 K). The translational temperature of each CO2 is much higher than the corresponding surface temperature, and increases in the sequence of P1– <P2– <P3– <P4– <P5–CO2. It decreases rapidly with an increase in the desorption angle perpendicular to the surface trough and more slowly parallel to it. This anisotropy is correlated to the reaction site symmetry.

The mechanism for photofragmentation of H2S revealed by multiphoton ionization photoelectron spectroscopy

Multiphoton ionization photoelectron measurements for H2S were carried out at several laser wavelengths in the 422–475 nm region, in order to obtain a direct evidence for the mechanism of ionic fragmentation which takes place by resonantly enhanced multiphoton ionization (REMPI). An ion current spectrum of multiphoton ioniztion was also measured for H2S in this wavelength region, indicating that the ionization takes place via three‐photon resonant Rydberg states. From photoelectron spectra obtained here, it has been found that the main peaks are attributed to the H2S+ ion in the ground state with v = 0. Other photoelectron bands due to v = 1 have also been observed together with some satellite bands. It should be mentioned that no photoelectron bands above 1.3 eV have been found. These experimental evidences directly support the parent ion fragmentation mechanism that the formation of HS+ and S+ ions mainly results from the ground state H2S+ ion with v = 0 and v = 1, respectively, by additional photon abs...

Laboratory angular dependence and the recoil‐energy spectrum of the products of the reaction F+C6D6→ D+C6D5F

The reaction of fluorine atoms with perdeuterobenzene has been studied by the method of crossed molecular beams. In this reaction a long‐lived intermediate complex is formed. Analysis of the kinetic energy distribution of the products and their angular distribution in space cannot be satisfactorily accounted for by the available statistical theories of unimolecular decomposition. The results of this study can be used to infer that randomization of the internal energy of a molecule is not necessarily rapid relative to chemical reaction.