Kari Vaskonen

Thermal mobility of atomic hydrogen in solid argon and krypton matrices

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...

193 NM PHOTODYNAMICS OF NO IN RARE GAS MATRICES : FLUORESCENCE, THERMOLUMINESCENCE, AND PHOTODISSOCIATION

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 4S 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.

PHOTOGENERATION OF ATOMIC HYDROGEN IN RARE GAS MATRICES

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...

Trapping of laser-vaporized alkali metal atoms in rare-gas matrices

Abstract Alkali metal atoms prepared by laser ablation of solid Li and Na are trapped in Ar, Kr, and Xe matrices and studied by electron paramagnetic resonance spectroscopy (EPR) at 15 K. Evidence for tight trapping sites, not observed for atoms generated by conventional Knudsen oven techniques, is presented. The novel tight trapping sites are characterized by a large increase in the isotropic hyperfine coupling constant and a simultaneous decrease in the isotropic g -value. Based on the EPR data, it is suggested that the observed tight trapping corresponds to single substitution of lattice atoms in Ar, Kr, and Xe matrices.

Molecular orbital study of conformational isomers and rotational barriers of methyl substituted hydroquinone cation radicals

Abstract The torsional potential energy curve of the hydroxyl group of hydroquinone and tetramethyl-hydroquinone cation radicals were explored with various ab initio methods. The minimum and the torsional transition state geometries and energies were computed by using high accuracy density functional methods yielding the rotation barrier height and the energy difference between the cis- and trans-isomers. The obtained minimum energy geometry for the hydroquinone cation radical indicates that the CO bond has shortened when compared to the neutral species. We attribute this to the increased double-bond character of this bond. The energy minima were located for methyl-hydroquinone, 2,3-dimethyl-hydroquinone, 2,5-dimethyl-hydroquinone, 2,6-dimethylhydroquinone and trimethyl-hydroquinone cation radicals. By assuming the Boltzman distribution among the energy levels of the cis- and trans-isomers of these compounds the ratio of expected isomers at 230K was obtained.

Host–guest charge transfer states: CN doped Kr and Xe

The host–guest charge transfer absorption of CN doped krypton and xenon matrices are identified through direct analogy with the previously assigned transitions of Cl/Kr and Cl/Xe. These intense, structured absorption bands appear with the onset at 245 nm in Kr and 360 nm in Xe. Excitation of the CN/Kr charge transfer band at 193 nm leads to emission over CN(A(2Π)→X(2Σ)) transition, indicating that an efficient curve crossing precludes the ionic state from radiating. No emissions were seen in CN/Xe when excited at 193 nm. The charge transfer absorption spectrum of CN/Kr is reproduced through an extended diatomics-in-ionic-systems treatment, using accurate ab initio pair potentials and transition dipoles as input, without further adjustment. The delocalized hole states are then analyzed in real-space, using atomic bases distributed over as many as eleven shells surrounding the CN− center. The ionic states are well described as J=1/2, 3/2 valence bands bound to CN−, with a substructure that cannot be exclusi...