Masako Suto

Photodissociation of NH3 at 106–200 nm

Cross sections and quantum yields of emissions from the photofragments of NH3 were measured in the 106–200 nm region using synchrotron radiation as a light source. The emission systems of NH2 (A 2A1→X  2B1), NH(c 1Π→a 1Δ, b 1Σ+), and NH(b 1Σ+→X 3Σ−) were observed from the NH3 photodissociation. The dependences of emission intensities on the NH3 and/or He (buffer gas) pressures were studied. The NH(c→a,b) and NH(b→X) emissions were observed when the NH2(A2A1) species was quenched by 10 Torr of He. The NH(b→X) and NH(c→a) emissions start to appear at 182.3±1.5 and 127.9±0.9 nm, respectively. From these emission thresholds the threshold energy for photodissociation of NH3 into NH(X 3Σ−)+H2(X 1Σ+) is determined to be 4.17±0.06 eV, and the upper limit for the heat of formation of NH is 3.81±0.06 eV. For excitation wavelengths longer than 128 nm the NH(b 1Σ+) species is produced by a primary photodissociation process, and at shorter excitation wavelengths the NH(b 1Σ+) may be partly produced by a cascading p...

Quantitative photoabsorption and fluorescence study of H2O and D2O at 50-190 nm

Abstract The photoabsorption cross sections and the fluorescence quantum yields of H 2 O and D 2 O were measured in the 50-190 nm region using synchrotron radiation as a light source. The oscillator streng4ths for the Rydberg states of H 2 O and D 2 O were determined from the absorption cross sections measured. The processes for the production of fluorescence from the excited species of H*( n > 2), D*( n > 2), OH*(A) and OD*(A)are discussed. The upper limit for the dissociation energy of D (D-OD) was determined, from the threshold of the OD(A-X) fluorescence,to be 5.14±0.01 eV. The upper limit for the cross section of visible fluorescence from the excited H 2 O + ions was determined to be 2x10 -19 cm 2 . A comparison between the photoexcitation spectra of H 2 O and D 2 O is made.

Quantitative photoexcitation and fluorescence studies of C2H2 in vacuum ultraviolet

The photoabsorption and fluorescence cross sections of C2H2 were measured in the 105–155 nm region using synchrotron radiation as a light source. The quantum yield for producing the C2H* fluorescence from photodissociation of C2H2 was measured in the 106–136.5 nm (threshold) region. The absorption spectrum shows sharp Rydberg states, but the quantum yield is a smooth function of excitation wavelength. The quantum yield data were used to derive a quasidiatomic repulsive potential curve for dissociation of C2H2 into C2H*+H. The quenching of the C2H* fluorescence by C2H2, N2, and Ar was studied. The products of radiative lifetime quenching rate constant increase with increasing excitation wavelengths, varying from 10−16 cm3 at 110 nm to 5.5×10−16 cm3 at 134 nm for N2 and Ar as quenchers. The data of lifetimes, quenchings, and fluorescence spectra point to a conclusion that the upper state of the C2H* fluorescence is well bound, and the lower state is a repulsive or weak‐bound state.

Quantitative photoabsorption and fluorescence spectroscopy of H2S and D2S at 49–240 nm

Photoabsorption and fluorescence cross sections of H2S and D2S were measured in the 49–240 nm region using synchrotron radiation as a light source. Fluorescence from photoexcitation of H2S appears at 49–97 nm but not in the longer wavelength region. Fluorescence spectra were dispersed, and used to identify the emitters to be H2S+(A), SH+(A), and H(n>2). The fluorescence quantum yield is about 6%. Photoexcitation of D2S at 49–96 nm produces fluorescence with a quantum yield of about 5%. The emitters are identified from the fluorescence spectra to be D2S+(A), SD+(A), and D(n>2). The Franck–Condon factors for the SH+ and SD+ (A–X) transitions were determined. The SD(A–X) fluorescence was observed from photoexcitation of D2S at 100–151 nm, for which the fluorescence cross section and quantum yield were measured.

Photoexcitation processes of CH3OH: Rydberg states and photofragment fluorescence

Abstract Photoabsorption and fluorescence cross sections of methanol vapor were mearured using synchrotron radiation. Weak structures observed in the 110–140 nm region are classified into three Rydberg series. Quasidiatomic repulsive potential curves for the states dissociating into CH 3 + OH(A 2 Σ + ) are obtained from the measured fluorescence cross section. The photodissociation processes are discussed in accord with the fluorescence observed. The fluorescence quantum yield ( 3 OH is one order of magnitude smaller than that of H 2 O, indicating a correlation that the fluorescence quantum yield decreases with increasing number of molecular orbitals.

Emission spectra of CF3 radicals. V. Photodissociation of CF3H, CF3Cl, and CF3Br by vacuum ultraviolet

The absorption cross sections of CF3H, CF3Cl, and CF3Br and the fluorescence cross sections for their photofragments are measured in the 106–155 nm region using synchrotron radiation as a light source. The threshold wavelengths for producing the CF3 UV fluorescence from photodissociation of CF3X (X=H, Cl, and Br) are 113.0, 122.2, and 132.4 nm, respectively. The threshold energy is nearly equal to the calculated sum of the CF*3 excited state energy and the dissociation energy for each molecule. The excitation spectra for the UV and visible fluorescence are different, and the threshold wavelength for the CF3 visible fluorescence is slightly longer than that for the UV fluorescence. These results strongly support previous conclusions that the upper states of the CF3 UV and visible fluorescence are different, and that the upper state energy of the UV fluorescence is higher than that of the visible one. The processes for photodissociation of CF3X into various fragments are discussed.

OH(A2Σ+ → X2∏) yield from photodissociation of H2O2 at 106–193 nm

Abstract The absorption cross section of H 2 O 2 and the (A 2 Σ + → X 2 ∏) fluorescence cross section of the resulting OH photofragments were measured in the 106–193 nm region using synchrotron radiation. The OH(A → X) emission has a significant cross section at excitation wavelengths shorter than 172.2 nm. Absorption structure observed in the 137–165 nm region is tentatively assigned.

Quantitative photoabsorption and fluorescence spectroscopy of benzene, naphthalene, and some derivatives at 106-295 nm

Abstract Photoabsorption and fluorescence cross sections of benzene, (o-, m-, p-) xylenes, naphthalene, 1-methylnaphthalene, and 2-ethylnaphthalene in the gas phase have been measured at 106–295 nm using synchrotron radiation as a light source. Fluorescences are observed from the photoexcitation of benzene and xylenes at 230–280 nm and from naphthalene and its derivatives at 190–295 nm. The absolute fluorescence cross section is determined by calibration with respect to the emission intensity of the NO(A-X) system, for which the fluorescence quantum yield is equal to 1. To cross-check the current calibration method, the quantum yield of the SO2(C-X) system at 220–230 nm was measured since it is about equal to 1. The current quantum-yield data are compared with previously published values measured by different methods.

Quantitative photoexcitation study of SiH4 in vacuum ultraviolet

The photoabsorption and fluorescence cross sections of SiH4 were measured in the 106–160 nm region using synchrotron radiation as a light source. Two states with long vibrational progressions in the 106–123 nm region were observed and assigned to the 5s and 6s Rydberg series converging to the first ionization state (2t2)−1. The wavelength thresholds for producing fluorescences from the SiH ( A 2Δ→X 2Π) and Si (4s 1P0→3P2 1D2) transitions were observed at 131.5 and 120.0 nm, respectively. The quantum yields for both fluorescence systems were measured. The photodissociation processes for producing the excited fragments of SiH* ( A ) and Si* are discussed.

Quantitative photoabsorption and fluorescence study of HCl in vacuum ultraviolet

The photoabsorption and fluorescence cross sections of HCl were measured in the 106–185 nm region. Sharp absortion bands appear at wavelengths shorter than 135 nm, and fluorescence occurs at several excited states. The fluorescence cross sections are generally quite small, indicating that the excited states are strongly predissociative. The molecular processes for producing the VUV and UV fluorescences are investigated, and the Rydberg characteristics of the strong absorption bands are discussed.