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.