Photodetachment, photodissociation, and photochemistry of surface molecules of icy solids containing NH3 and pure H2O ices

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Abstract
Photodetachment and photodissociation of surface molecules of pure and mixed ices containing NH3 and H2O at 90∼130 K have been investigated by using a mass spectrometer situated in a ultrahigh vacuum apparatus. NH3 molecules were excited to the lowest singlet state by an ArF laser at 193 nm (hν=6.4 eV). The detachment of NH3 molecules from pure NH3 solids was observed at a low laser fluence of 0.1 mJ/cm2. Two photons of 248 nm laser (2hν=10 eV) also detached H2O molecules from pure H2O ices. Mixed solids of NH3 and H2O released both molecules from the surface upon irradiation with 193 nm laser. The translational energies of the detached molecules were of the order of the intermolecular interactions of the constituent molecules (0.03∼0.2 eV) and the energy distributions were not changed by the increase of laser fluence. A detachment mechanism explaining all of the observations is proposed. The force acting on the detachment is ascribed to ‘‘electronic exchange repulsion between an excited molecule and a ground state molecule.’’ At a higher laser fluence (≳500 mJ/cm2), the detachment of ions was observed at m/e=17 and 18 with comparable intensities in a mixed solid of NH3 and H2O. Translational energy distributions of the ions peaked at ∼1 eV for both species. Dissociation of atomic hydrogen from the surface molecules was observed in the solid containing NH3. A fraction of the atomic hydrogen posseses translation energy (Et=17 kcal/mol=0.74 eV) which is similar to that observed in the photolysis of free NH3 molecles in a molecular beam. A very low energy component (Et=0.6 kcal/mol=0.026 eV) appeared and exhibited an intensity change depending upon the deposition and annealing conditions. Molecular hydrogen was also released with a translational temperature very close to the surface temperature. The photoproducts N2H4, N2H2, and NH2OH were detected in the mixed solid system of NH3 and H2O. Photochemical bimolecular condensation is proposed as a mechanism of the reaction forming a hydrogen molecule and a condensed molecule. The present study affords an experimental foundation of the photoejection model of physically adsorbed molecules on interstellar grains presented by Watson and Salpeter (Ref. 2).