No abstract

No abstract

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 (Ã 2A1→X̃  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(Ã2A1) 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 process following the NH(c→b) transition. Contrary to previous theoretical interpretations, all the vibronic levels of the NH3 (B̃ and C̃) states produce the NH2(Ã→X̃) emission. The quantum yield for the C̃ state is lower than that of the B̃ state. The vibronic levels of the C̃ state are renumbered in accordance with their quantum yields.

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+(Ã), 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+(Ã), 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.

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.