Adsorbed states of acetonitrile and chloroform on amorphous and crystalline ice studied with X-ray photoelectron spectroscopy

The adsorbed states of acetonitrile (CH3CN) and chloroform (CHCl3) on ultrathin films of amorphous and crystalline ice were studied with X-ray photoelectron spectroscopy (XPS). Results were in agreement with previously published temperature programmed desorption (TPD) and Fourier transform infrared reflection absorption spectroscopy (FTIRAS) studies. The N(ls) spectrum of acetonitrile on amorphous ice exhibits two states of binding energies, 402.2 and 401.4 eV, which are assigned to hydrogen bonded and physisorbed/multilayer acetonitrile, respectively. The Cl(2p) spectrum of chloroform on amorphous ice exhibits a single set of peaks, associated with the 2p(1/2) and 2p(3/2) spin states of the physisorbed molecules. There is no hydrogen bonded state, as expected for a weak hydrogen bond acceptor like CHCl3. The N(1s) spectra of acetonitrile on crystalline ice show that very little hydrogen bonded CH3CN is present. Instead, there are two states, one at 401.4 eV (the multilayer) and the other at 400.4 eV. The latter state saturates at a coverage of similar to 1 monolayer. The C(1s) spectra of acetonitrile on crystalline ice and the Cl(2p) spectra of chloroform on crystalline ice also reveal the: presence of states shifted by roughly -1 eV with respect to the multilayer. These states are attributed to the first layer of physisorbed CH3CN or CHCl3, their binding energies are shifted relative to the multilayer because of final state interactions between the adsorbed molecules and the dipolar crystalline ice surface. The crystalline surface is suggested to be dipolar because of a reconstruction that results in a decrease in the coverage of free surface OH groups. (C) 1999 Elsevier Science B.V. All rights reserved.