VUV photophysics of acetonitrile: Fragmentation, fluorescence and ionization in the 7-22Â eV region

VUV induced photodissociation of gaseous acetonitrile was studied in the 7-22 eV range using synchrotron radiation excitation, photofragment fluorescence spectroscopy and photoion mass spectrometry (PIMS). CN(A-X), CN(B-X) and CH(A-X) emissions were observed. Their relative intensities were studied by fluorescence excitation (FEX) spectroscopy. The fluorescence quantum yield for CN(B-X) emission is of the order of 1.5%, whereas that of CN(A-X) is of the order of 0.5%, relatively independent of excitation wavelength in the 9-20 eV region. The CN FEX spectra follow quite closely the photoabsorption spectrum. The total photoionization quantum yield of CH3CN was measured up to 22 eV. The structures in the yield curve were related to photoabsorption features. The total photoionization quantum yield is about 0.5 at 13.131 eV, the energy of the first excited state of the ion; it reaches unity at about 21.3 eV. Dissociative reaction channels forming neutral products are discussed in detail. The results of this study and of an associated VUV absorption spectroscopy study of acetonitrile are used to discuss aspects of the interpretations of astrophysical observations of acetonitrile in the interstellar medium, comets and planetary atmospheres.