Spectra and Photochemistry of Trifluoronitrosomethane Adsorbed on Alkali Halide Films

Trifluoronitrosomethane (CF3NO), which is readily photolyzed in the gas phase when exposed to light in the 600−700 nm portion of the spectrum, does not undergo significant photolysis when irradiated while adsorbed on sublimated films of alkali halides (NaCl, NaBr, KCl, KBr) at temperatures of 10−100 K. This quenching does not occur when the molecule is irradiated while adsorbed on argon films (thickness > 0.1 μm) or in multilayer coverage. Comparison of the quantum efficiencies for photolysis indicates that vibrational relaxation induced by surface phonons is primarily responsible for the alkali halide quenching, but lateral diffusion of the photofragments and/or excited-state molecules may also play a role in the kinetic scheme. The photochemistry was followed using infrared spectroscopy, providing the first vibrational characterization of adsorbed sub-monolayer CF3NO. A photodimer of CF3NO was also observed and vibrationally characterized on the argon films and on the overlayers. The NO stretching frequency for the dimer is significantly lower than had been previously reported for the N-nitritoamine photodimer {(CF3)2NONO} and may indicate the existence of a second photodimer species. Adsorption potentials were calculated for various sites on the four alkali halide films. The lowest energy orientation in each case had the C−N bond parallel to the surface, aligned in the [110] direction. The vibrational spectra and thermal desorption curves are consistent with the calculated energies.