Site-selective ionization of ethanol dimer under the tunable synchrotron VUV radiation and its subsequent fragmentation

Site-selective ionization of ethanol dimer and the subsequent fragmentation were studied by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. With photoionization efficiency spectra measurements and theoretical calculations, the detailed mechanisms of the ionization-dissociation processes of ethanol dimer under VUV irradiation were explored. In 9.49-10.89 eV photon energy range, it was found that the ejection of the highest occupied molecular orbital (HOMO) electron from hydrogen bond donor induces a rapid barrierless proton-transfer process followed by two competitive dissociation channels, generating (C2H5OH)center dot H+ and CH2O center dot(C2H5OH)H+, respectively. The latter comes from a carbon-carbon bond cleavage in the donor. While the photon energy is 10.9-11.58 eV, the electron of HOMO-1 of the hydrogen bond acceptor, is removed. Besides the dissociation channel to produce C2H5OH and C2H5OH+, a new channel to generate (C2H5OH)center dot CH2OH+ is opened, where the cleavage of the carbon-carbon bond occurs in the acceptor. When the photon energy increases to 11.58 eV, the electron from HOMO-2 is ejected. (C) 2013 AIP Publishing LLC.