Ikuo Tokue

Photoexcitation of dimethyl sulfide and dimethyl disulfide in the vacuum ultraviolet region: Rydberg states and photofragment emissions

Abstract Photoabsorption cross sections and fluorescence excitation spectra of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) vapors have been studied in the 110–220 nm region using synchrotron radiation. For DMS, a new Rydberg series originating from the ns orbital is identified. A number of broad bands from DMDS are assigned as Rydberg transitions. Emissions from DMS and DMDS are assigned as the CH3( A - X ) band. For DMDS, another emission which is attributable to the S2(B-X) band appears in the excitation below 125 nm. Photodissociation processes forming the excited fragments are discussed.

Theoretical studies of absorption cross sections for the C̃ B12-X̃ A11 system of sulfur dioxide and isotope effects

The C (1)B(2)-X (1)A(1) photoexcitation of SO(2) was studied to investigate excited-state dynamics and the effects of the initial vibrational state. Ultraviolet photoabsorption cross sections (sigmas) of seven isotopologues ((32)S (16)O(2), (33)S (16)O(2), (34)S (16)O(2), (36)S (16)O(2), (32)S(16)O(17)O, (32)S(16)O(18)O, (34)S(16)O(18)O) were computed using the wave packet propagation technique based on the three-dimensional potential energy surfaces of the X and C states, which were calculated using the ab initio molecular orbital configuration interaction method. Numerous wave packet simulations were carried out under the adiabatic approximation and used to calculate the sigmas of the seven isotopologues at 298 K; we concluded that the absorption spectrum of SO(2) can be reliably modeled within the adiabatic framework based on the analysis of the time evolution of the wave packet. The calculated sigmas are in reasonable agreement with the recent experiment in the 190-228 nm region, and the isotope shifts of the peaks for (33)S (16)O(2) and (34)S (16)O(2) relative to the corresponding peaks for (32)S (16)O(2) are in good agreement with the observed data. Relative to the sigma of (32)S (16)O(2), isotopic substitution shows a significant increment for those of (34)S (16)O(2) and (36)S (16)O(2) in the 190-228 nm region. This trend is consistent with the observed data.

Vibrational distributions of the à 2Πu state of CO+2 and CS+2 produced by electron impact on jet‐cooled CO2 and CS2

Fluorescence spectra of the A2Πu–X2Πg system of CO+2 and CS+2 ions, following electron impact on supersonic jet targets of CO2 and CS2, have been analyzed to determine vibrational populations. The vibrational distributions of the A2Πu state of CO+2 and CS+2 were found to be independent of the impact energy in the 30–300 eV range. The distribution of the CO+2(A) state is very similar to the result obtained by He i photoelectron spectroscopy (PES), which is a Franck–Condon‐type vertical ionization, whereas that of the CS+2(A) state is significantly different from the PES data. This non‐Franck–Condon behavior for formation of CS+2(A) in the electron impact is discussed.

Formation of NH(A 3Π, c1Π) by the electron impact dissociation of ammonia

Abstract Emission spectra of the NH(A 3Π1−X3Σ) and NH(c 1Π−a 1Δ) systems are observed by the electron impact of ammonia from the threshold energies up to 120 eV. The formation of NH(A) and NH(c) by the dissociative excitation of ammonia was investigated. The ratio of the vibrational populations, P(v′ = 1)/P(v′ = 0) for NH(A 3Π) is 0.4–0.6, depending on the impact energy. The rotational energy distribution of the v′ = 0 level of the c 1Π state can be approximated by the effective Boltzmann temperature of 1690 ± 100 K. The rotational excitations of the v′ = 0 and v′ = 1 levels of the A3Π state are much higher and their rotational populations deviate from Boltzmann distributions, showing slight dependence on the impact energy. The ratio of the formation rate of NH(A) to that of NH(c) is estimated from the threshold to 92 eV.

Electron-impact-induced light emission from CXCl3 (X=H, F, Cl, Br)

Abstract Emission spectra in the 180–600 nm region produced by electron impact on CHCl3, CFCl3, CCl4 and CBrCl3 have been studied. A broad band at 258 nm observed from all compounds is assigned to the D′ 2g-A′ 3Πu transition of Cl2, which is found to be produced by single collisions of parent molecules with electrons. Continuous emissions in the 420–580 nm region from CCl4 and CBrCl3 are assigned to the CCl2(A 1B1-X 1A1) transition. Radiative lifetimes of several fragments are obtained. Dissociation processes leading to electronically excited fragments are discussed.

Photoexcitation and photofragment fluorescence studies of methanethiol in vacuum ultraviolet

Abstract Photoabsorption cross sections and fluorescence excitation spectra of methanethiol vapor have been measured in the 130–200 nm wavelength region using synchrotron radiation. Many sharp structures observed in the 130–180 nm region are classified into three Rydberg series. The fluorescence which starts to appear at 175.2±2.0 nm is attributed to the CH 3 S(A 2 A 1 -X 2 E) band. The photodissociation processes are discussed in accord with the CH 3 S(A-X) fluorescence observed.

Nonadiabatic calculations of ultraviolet absorption cross section of sulfur monoxide: Isotopic effects on the photodissociation reaction

Ultraviolet absorption cross sections of the main and substituted sulfur monoxide (SO) isotopologues were calculated using R-Matrix expansion technique. Energies, transition dipole moments, and nonadiabatic coupling matrix elements were calculated at MRCI/AV6Z level. The calculated absorption cross section of (32)S(16)O was compared with experimental spectrum; the spectral feature and the absolute value of photoabsorption cross sections are in good agreement. Our calculation predicts a long lived photoexcited SO* species which causes large non-mass dependent isotopic effects depending on the excitation energy in the ultraviolet region.

Vacuum ultraviolet absorption spectra of thiirane and thietane

Photoabsorption cross sections of thiirane and thietane vapors have been studied in the 110–240 nm region using synchrotron radiation. A number of peaks from thiirane and thietane are arranged into four and five Rydberg series, respectively, converging to the first ionization potential. Many Rydberg peaks from both molecules are observed to possess vibronic structures. The vibrational progressions from thiirane are assigned to the CH2 wagging (ν4) and CS symmetry stretching (ν5) modes. For thietane, the CH2 wagging (ν5) and CS symmetry stretching (ν7) modes of the excited states are found to be active. Molecular geometries and vibrational frequencies for the excited states are discussed on the basis of ab initio calculations.

Photoexcitation of CH3NCO, CH3NCS and CH3SCN in the vacuum ultraviolet: Rydberg states and photofragment emission

Abstract Photoabsorption cross sections and fluorescence excitation spectra of CH 3 NCO, CH 3 NCS and CH 3 SCN vapor were measured in the vacuum ultraviolet using synchrotron radiation. Many sharp structures observed from CH 3 NCO and CH 3 SCN in the 120–180 nm region are classified into three Rydberg series and their vibrational progressions, whereas for CH 3 NCS six broad bands exhibit no fine structure. The emission which starts to appear at 172.8 ± 1.0 nm excitation of CH 3 NCO is attributed to the NCO(A 2 Σ + -X 2 Π) band. The emissions from CH 3 NCS and CH 3 SCN are assigned to the A 2 Π-X 2 Π and B 2 Σ + -X 2 Π bands of NCS; the CN(B 2 Σ + -X 2 Σ + ) band is also observed at 125 nm excitation of CH 3 SCN. The photodissociation processes are discussed in accord with the emission observed.

Electron-impact dissociation of HNCO: Formation of NH(A3Π, c1Π) and NCO(A2Σ+, B2Π)

Abstract Emission spectra of the NH(A 3 Π-X 3 Σ − ), NH(c 1 Π-a 1 Δ), NCO(A 2 Σ + -X 2 Π) and NCO(B 2 Π-X 2 Π) systems were observed by electron impact on HNCO up to 40 eV. The NH(c 1 Π) is shown to be formed directly, with the onset of 9.1 ± 0.6 eV. The onsets of NCO(A 2 Σ + ) and NCO(B 2 Π) formation are found to be 8.3 ± 0.6 and 9.2 ± 0.6 eV, respectively. The observed internal energy distributions for NH are compared with those predicted statistically. The precursor states forming NH(A), NH(c), NCO(A) and NCO(B) are discussed with the aid of a schematic correlation diagram based on a semi-empirical calculation (CNDO/S).