Pseudorotational local mode participation in OH and OD(A 2Σ+) vibrational relaxation in a Ne lattice

Abstract

Radiationless transitions within the A ²Σ⁺ state of OH and OD in a Ne lattice at 4.2 °K are observed. The radicals are substitutional impurities undergoing slightly perturbed free rotation [D. S. Tinti, J. Chem. Phys. 48, 1459 (1968)]. The local phonon relaxation time is ?10 nsec. Emission spectra show an intermediate strong coupling (or resonant mixing) occurring between rotational levels and lattice phonon levels. The v′=0 lifetimes are 582±10 and 565±10 nsec on OH and OD, respectively, with fluorescence quantum yields ≳0.90. The Lorentz–Lorenz dielectric model overestimates the vacuum to solid radiative rate increase by a factor of two. At high radical concentrations Förster transfer from OD(v′=0) to OH(v′=0) occurs. The vibrational relaxation rates are fast (4×10⁴–4×10⁵ sec⁻¹), with higher rates in OH despite the larger vibrational energy gaps in OH. The rates are extremely sensitive to lattice defects and/or additional impurities causing small (?0.3 Å) red or blue shifts from the principal site spectra. The principal accepting modes appear to be pseudorotational localized modes. Defects appear to enhance relaxation by creating additional localized modes.