R. Schriever

Spectroscopy and photodissociation of molecular chlorine in argon matrices

A structureless band observed at 180 nm in the absorption and excitation spectra of Cl2/Ar matrices is tentatively assigned as the spin forbidden 3Σ+u ← X 1Σ+g transition of molecular chlorine. Having an absorption cross section of approximately 10−18 cm2 in the solid, a twofold order of magnitude increase in the transition probability is observed relative to the gas phase. Wavelength specific measurements of the photodissociation of molecular chlorine in crystalline argon samples showed that a dominant threshold exists in the 130 nm band at 9.2 eV corresponding to absorption into the bound 1 1Σ+u state. The maximum quantum yield for permanent dissociation in the 130 nm band was found to be 0.3. Luminescence evidence indicates that this dissociation does not involve a charge‐transfer mechanism but a crossing from the bound 1 1Σ+u state to a repulsive potential on which an impulsive cage escape occurs. Photoexcitation in the 180 nm band also results in the permanent dissociation of chlorine as well as the ...

Cage effect for the abstraction of H from H2O in Ar matrices

A red wing in the absorption of H2O in the first continuum which extends beyond 200 nm is induced by the matrix. A threshold energy of 6.95 eV is found for the quantum efficiency of permanent dissociation of H2O into OH and H. This corresponds to an additional barrier of 1.8 eV due to the matrix cage. The barrier is attributed to the sum of repulsive H–Ar pair potentials in an interstitial site of D3 symmetry, which has to be surmounted by the H atom on the way from the initial substitutional H2O site to the final stable Oh site of the abstracted H atom.

Threshold and cage effect for dissociation of H2O and D2O in Ar and Kr matrices

A threshold near 6.8 eV is observed for the dissociation of H2O (and D2O) via the repulsive A(1B1) potential in rare gas matrices. The threshold varies within 0.2 eV for exchange of Kr with Ar matrix, upon deuteration and with temperature. The observations are consistent with a prompt dissociation across matrix induced barriers of about 1.8 eV caused by the repulsive hydrogen–rare gas potentials. The increase of dissociation efficiency above threshold and its variation with matrix and isotope substitution seen in the experiments is rationalized by the mass dependence of energy exchange between fragment and matrix and the dependence of the accessible exit cone on kinetic energy. A strong increase in the dissociation efficiency with temperature is observed which is different for Ar and Kr.

Photodissociation of water in rare gas matrices : cage effect and local heating of the lattice

The H fragment produced in the photodissociation of water (H2O→H+OH) is lighter than the lattice atoms and a local heating occurs over long time scales and large spatial regions. Therefore, use of a continuum model for the lattice is justified. The local heating is promoted by a decreasing heat conductivity with increasing temperature. Solutions of a nonlinear heat conductivity equation show that the heat release inside the cage leads to a temperature increase in its nearest surroundings up to the melting point in argon. Melting enables the light fragment to overcome the cage barrier. In the case of krypton, the thermal effect is less pronounced, while in Xe, there is none. The above results are in qualitative agreement with experimental data.

Rydberg series of charge‐transfer excitations: Cl and H in rare gas crystals

Rydberg series observed in the excitation spectra of Cl and H atoms isolated in the rare gas (Rg) crystals Kr and Xe are associated with the charge transfer species Cl−Rg+ and H−Rg+. The progressions originate from hole states of Rg+ which converge to the top of the rare gas valence band in an analogous manner to the convergence of conventional exciton states to the bottom of the conduction band. A model based on the effective mass approximation and a quantum defect concept predicted such progressions in Xe and Kr crystals and it is supported by the present results.

Electronic and vibrational relaxation in Rydberg and valence states of NO in Ne matrices

New time and energy resolved data on vibrationally relaxed and unrelaxed emissions from the valence a 4 Π(v=0), B 2 Π(v=0,3,4,5), and the Rydberg A 2 Σ+ (v=0,1,2) states of NO in Ne matrices are reported. Rydberg ↔ valence and valence ↔ valence nonradiative transitions are identified. The Rydberg → valence transitions are seen to occur after lattice relaxation accommodating the Rydberg orbital. The branching ratios for intramolecular relaxation and the measured lifetimes are described in terms of a model which combines the intramolecular spin–orbit matrix elements and Franck–Condon factors with the spectroscopically determined phonon Franck–Condon factors. For the levels B 2 Π(v=5,6), a Forster–Dexter‐type energy transfer between NO molecules is also invoked in the description of the relaxation cascade.

Charge transfer excitations of doped rare gas matrices

Structured progressions, assocd. with the Cl-Rg+ and H-Rg+ charge transfer species, are obsd. for Cl and H atoms isolated in Kr and Xe (Rg) crystals and are assigned as transitions to Rydberg states. The Rydberg levels correspond to delocalized pos. hole states in the rare gas solid (Rg+) orbiting a neg. center (Cl- or H-). For OH in Ar and Kr matrixes, transitions to static charge transfer states are identified. For both of these rather extreme situations, simple model are used successfully to predict the transition energies. For the former, an exciton model based on the effective mass approxn. and utilizing quantum defect nos. is used, while in the latter, a Coulombic potential screened by the medium is used. [on SciFinder (R)]

Formation and electronic energies of the matrix-isolated hydrogen-noble gas molecule

Abstract The photolysis of hydrogen-containing molecules in noble-gas (Ng) matrices results in hydrogen atoms caged in octahedral interstitial lattice sites. The excited states give rise to characteristic absorption bands and relax most probably into a Ng 2 H charge-transfer state. A simple model is proposed for calculating the potential of this state and also for the ground state. The results can be compared with new measurements of the photolytically induced fluorescence bands at 5 to 6 eV of H-doped solid noble gases.

Cage effect on the photodissociation of small molecules in rare gas matrices

Photodissociation of H2O (D2O) and Cl2 trapped in rare gas matrices has been investigated in relation to the cage effect. Barrier energies for the exit of the fragments from the matrix cage have been determined, together with the excess kinetic energy dependence of the photodissociation quantum efficiency. Simple analytical models as well as classical Molecular Dynamics simulations are used to discuss the results.