Interaction of formic acid with solid water.

The interaction of formic acid (HCOOH) with solid water, deposited on tungsten at 80 K, was investigated. We have prepared and annealed formic acid (FA)/water interfaces (FA layers on thin films of solid water and H₂O adlayers on thin FA films). Metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy (He I and II) were utilized to study the electron emission from the 10a^′ to 6a^′ molecular orbitals (MOs) of FA, and the 1b₁, 3a₁, and 1b₂ MOs of H₂O. These spectra were compared with results of density-functional theory calculations on FA-H₂O complexes reported in Ref. <CITEREF>14</CITEREF> [A. Allouche, J. Chem. Phys. 122, 234703(2005), (preceding paper)]. Temperature programmed desorption was applied for information on the desorption kinetics. Initially, FA is adsorbed on top of the water film. The FA spectra are distorted with respect to those from FA monomers; it is concluded that a strong interaction exists between the adsorbates. Even though partial solvation of FA species takes place during annealing, FA remains in the top layer up to the desorption of the water film. When H₂O molecules are offered to FA films at 80 K, no water network is formed during the initial stage of water exposure; H₂O molecules interact individually via H bonds with the formic acid network. Experiment and theory agree that no water-induced deprotonation of the formic acid molecules takes place. [ABSTRACT FROM AUTHOR]