Spectroscopic Evidence for Slow Vibrational and Electronic Relaxation in Solids. The Vegard–Kaplan and Second Positive Systems of N2in Solid Rare Gases

Spectroscopic observation of nitrogen anions N(-) in solid matrices

Analysis of old and recent experiments on thermoluminescence of cryocrystals and nanoclusters of N2, Ne, Ar, and Kr containing stabilized nitrogen atoms, suggests that the so-called γ-line may correspond to the bound-bound transition (1)D-(3)P of nitrogen anions N(-) formed in solids by the association of delocalized electrons and metastable nitrogen atoms N((2)D). The recent observations of the γ-line were accompanied by simultaneous luminescence of metastable nitrogen N((2)D) atoms and exoelectron emission. The fine structure of the γ-line at 793 nm has been experimentally observed and investigated for the first time.

Radiation effects in solid nitrogen and nitrogen-containing matrices: fingerprints of N₄⁺ species

The radiation effects and relaxation processes in solid N2 and N2-doped Ne matrices, preirradiated by an electron beam, have been studied in the temperature range of 5-40 and 5-15 K, respectively. The study was performed using luminescence methods: cathodoluminescence CL and developed by our group nonstationary luminescence NsL, as well as optical and current activation spectroscopy methods: spectrally resolved thermally stimulated luminescence TSL and exoelectron emission TSEE. An appreciable accumulation of N radicals, N(+), N2(+) ions, and trapped electrons is found in nitrogen-containing Ne matrices. Neutralization reactions were shown to dominate relaxation scenario in the low-temperature range, while at higher temperatures diffusion-controlled reactions of neutral species contribute. It was conceived that in α-phase of solid N2, the dimerization reaction (N2(+) + N2 → N4(+)) proceeds: "hole self-trapping". Tetranitrogen cation N4(+) manifests itself by the dissociative recombination reaction with electron: N4(+) + e(-) → N2*(a'(1)Σ(u)(-)) + N2 → N2 + N2 + hν. In line with this assumption, we observed a growth of the a'(1)Σ(u)(-) → X(1)Σ(g)(+) transition intensity with an exposure time in CL spectra and the emergence of this emission in the course of electron detrapping on sample heating in the TSL and NsL experiments.

Optical and electron spin resonance studies of xenon-nitrogen-helium condensates containing nitrogen and oxygen atoms

We present the first observations of excimer XeO* molecules in molecular nitrogen films surrounding xenon cores of nanoclusters. Multishell nanoclusters form upon the fast cooling of a helium jet containing small admixtures of nitrogen and xenon by cold helium vapor (T = 1.5 K). Such nanoclusters injected into superfluid helium aggregate into porous impurity-helium condensates. Passage of helium gas with admixtures through a radio frequency discharge allows the storage of high densities of radicals stabilized in impurity-helium condensates. Intense recombination of the radicals occurs during destruction of such condensates and generates excited species observable because of optical emission. Rich spectra of xenon-oxygen complexes have been detected upon destruction of xenon-nitrogen-helium condensates. A xenon environment quenches metastable N((2)D) atoms but has a much weaker effect on the luminescence of N((2)P) atoms. Electron spin resonance spectra of N((4)S) atoms trapped in xenon-nitrogen-helium condensates have been studied. High local concentrations of nitrogen atoms (up to 10(21) cm(-3)) stabilized in xenon-nitrogen nanoclusters have been revealed.

Vibrational relaxation of trapped molecules in solid matrices: OH(A 2Sigma+; v = 1)/Ar

The vibrational relaxation of OH(A (2)Sigma(+);v=1) embedded in solid Ar has been studied over 4-80 K. The interaction model is based on OH undergoing local motions in a cage formed by a face-centered cubic stacking where the first shell atoms surround the guest and connect it to the heat bath through 12 ten-atom chains. The motions confined to the cage are the local translation and libration-rotation of OH and internal vibrations in OH...Ar, their energies being close to or a few times the energies of nearby first shell and chain atoms. The cage dynamics are studied by solving the equations of motion for the interaction between OH and first shell atoms, while energy propagation to the bulk phase through lattice chains is treated in the Langevin dynamics. Calculated energy transfer data are used in semiclassical procedure to obtain rate constants. In the early stage of interaction, OH transfers its energy to libration-rotation intramolecularily and then to the vibrations of the first shell and chain atoms on the time scale of several picoseconds. Libration-to-rotational transitions dispense the vibrational energy in small packages comparable to the lattice frequencies for ready flow. Energy propagation from the chains to the heat bath takes place on a long time scale of 10 ns or longer. Over the solid argon temperature range, the rate constant is on the order of 10(6) s(-1) and varies weakly with temperature.

Host-assisted intramolecular vibrational relaxation at low temperatures: OH in an argon cage

The vibrational relaxation of hydroxyl radicals in the A (2)Sigma(+) (v=1) state has been studied using the semiclassical perturbation treatment at cryogenic temperatures. The radical is considered to be trapped in a closest packed cage composed of the 12 nearest argon atoms and undergoes local translation and hindered rotation around the cage center. The primary relaxation pathway is towards local translation, followed by energy transfer to rotation through hindered-to-free rotational transitions. Free-to-free rotational transitions are found to be unimportant. All pathways are accompanied by the propagation of energy to argon phonon modes. The deexcitation probability of OH(v=1) is 1.3 x 10(-7) and the rate constant is 4.7 x 10(5) s(-1) between 4 and 10 K. The negligible temperature dependence is attributed to the presence of intermolecular attraction (>>kT) in the guest-host encounter, which counteracts the T(2) dependence resulting from local translation. Calculated relaxation time scales are much shorter than those of homonuclear molecules, suggesting the importance of the hindered and free motions of OH and strong guest-host interactions.

Vibrational Relaxation of Molecular Ions at Low Temperatures: O2-(v=1) + Ar

The vibrational relaxation of oxygen molecular ions trapped in an argon cage in the temperature range 10-85 K has been studied using semiclassical procedures. The collision model is based on the trapped molecule undergoing the restricted motions (local translation and hindered rotation) in a cage formed by its 12 nearest argon neighbors in a face-centered cubic arrangement. At 85 K in the liquid argon temperature range, the relaxation rate constant of O2(-) (v=1) is 1130 s(-1). The rate constant decreases to 270 s(-1) at 50 K and to 3.90 s(-1) at 10 K in the solid argon temperature range. In the range 10-85 K, the rate constant closely follows the temperature dependence k proportional to T2.7. Energy transfer pathways for the trapped molecular ion are vibration to local translation, argon phonon modes, and rotation (both hindered and free).

Vibrational relaxation of oxygen in an argon cage

The vibrational relaxation of oxygen embedded in an argon cage through vibrational to local translation, rotation, and argon phonon modes has been studied using semiclassical procedures. The collision model is based on the trapped molecule undergoing the restricted motions (local translation and hindered rotation) in a cage formed by its twelve nearest argon neighbors in a face-centered-cubic structure. At 85 K in the liquid argon temperature range, the deexcitation probability of O(2)(v=1) is 5.8 x 10(-12) and the relaxation rate constant with the collision frequency from local translation is 23 s(-1). The rate constant decreases to 5.1 s(-1) at 50 K and to 0.016 s(-1) at 10 K in the solid argon temperature range. Transfer of the vibrational energy to local translation, rotation (both hindered and free), and argon phonon modes is the relaxation pathway for the trapped oxygen molecule.