Decay patterns of atomic hydrogen trapped in argon and krypton matrices are followed by electron paramagnetic resonance (EPR). Hydrogen atoms are generated by uv-photolysis of HBr and HCl precursor molecules. The EPR signals due to interstitially trapped hydrogen atoms in octahedral sites disappear near 16 and 24 K in Ar and Kr, respectively. Substitutionally trapped H atoms are thermally stable up to evaporation temperature of the solids. The fate of thermally released H atoms in Ar is exclusively due to geminate recombination of the parent molecule. The observed kinetics is well fitted with double exponential decay. The kinetic behavior reflects short-range dissociation and recombination dynamics in Ar. In the Kr matrix, a change from first-order to second-order kinetics is observed at higher concentrations as formation of molecular hydrogen becomes important. From bimolecular decay kinetics, a diffusion constant of 4×10−15 cm2 s−1 is deduced for H-atom diffusion in Kr at 26.9 K. The obtained activation energies, 6–7 kJ/mol in Ar and 9–14 kJ/mol in Kr, are measures of thermally activated cage dynamics and show only weak dependence on the hydrogen isotope.

193 nm excited time gated emission spectra of a NO monomer isolated in Ar, Kr, and Xe matrices are presented. In the Ar matrix a 4Π→X 2Π, B 2Π→X 2Π, and A 2Σ→X 2Π band systems are completely separable. In solid Kr, both B 2Π→X 2Π and A 2Σ→X 2Π appear promptly from the laser pulse, and in the Xe matrix only Rydberg A 2Σ→X 2Π fluorescence is observed. Prolonged photolysis at 193 nm yields electron paramagnetic resonance signals attributed to isolated S4 nitrogen atoms. This is the first observation of condensed phase photodissociation of NO. Annealing of the extensively irradiated Ar matrix produces strong a 4Π→X 2Π and B 2Π→X 2Π thermoluminescence emissions due to N(4S)+O(3P) recombination. In the Kr matrix thermoluminescence is entirely due to a 4Π→X 2Π transition. No thermoluminescence is observed in Xe. Thermoluminescence is ascribed to short-range trapping of N and O fragments, and well separated atoms do not have significant contribution to recombination.

The electronic UV absorption spectra of thermal reaction products H−Xe−Y (Y= Cl, Br, I, or CN) have been measured in solid Xe at 12 K. The spectra are obtained after the annealing of an extensively irradiated matrix doped with an HCl, HBr, HI, or HCN precursor. The spectral assignment is based on the correlation between the UV spectra and the known infrared absorptions of these compounds. An analysis of the annealing behavior of the UV absorptions due to H−Xe−Y, Y/Xe and H/Xe yields a quantitative estimate that 20−30% of the photogenerated Y is converted to H−Xe−Y. Present multireference configuration interaction (MRCI) calculations provide strong support that the spectral observations are due to the A 1Σ ← X 1Σ transitions of H−Xe−Y. The spectral width of the absorptions indicate that the transitions are from a bound ground state to a repulsive excited state.

Photodissociation of HCl and HBr upon excitation on their repulsive A 1Π states is studied in low-temperature Ar, Kr, and Xe matrices at photon energies of 5.0 and 6.4 eV. The dissociation is followed by fluorescence spectroscopy and electron paramagnetic resonance. In Ar matrix dissociation can be considered as a local event with simple first-order kinetics and 100% conversion efficiency of the precursor into isolated hydrogen atoms. In Kr matrix the conversion efficiency varies from 18% in 1:500 matrix to 100% in 1:8000 matrix. In Xe matrix the obtained H atom number density is extremely low and prevents detailed analysis of the photogeneration dynamics. The observed behavior is ascribed to long-range dissociation followed by efficient bimolecular reactive loss channels, and thus supports the previous findings by LaBrake, Ryan, and Weitz [J. Chem. Phys. 102, 4112 (1995)]. Molecular dynamics simulations based on a simplified model for dissociation are carried out. The initial 2.6 eV excess kinetic energy of the excited H atom is relaxed as local heating in Ar matrix, whereas in Kr and Xe matrices the excess energy is directed to long-range mobility with flight distances up to 40 Å.

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