Femtosecond pump-probe measurements of non-radiative relaxation in LiAlO₂:V³⁺

We report on time-resolved studies of non-radiative relaxation of V³⁺ ions in LiAlO₂ by means of a two-beam, pump-probe saturation experiment performed with the 150 fs pulsed output of a Ti-sapphire laser. Exciting into the vibronically broadened ³A₂ → ³T₁ transition at 800 nm, a ³T₁ relaxation time of 199 ps has been measured at 4 K. This value decreases to 82 ps at room temperature, representing a reduction in the lifetime of a factor of 2.5 due to internal-conversion processes. The relative probabilities for non-radiative, phonon-assisted barrier hopping and quantum mechanical tunnelling through the potential barrier to the ³A₂ ground state have been obtained using Mott's expression, yielding best-fit parameters of W₀ = (5.2 ± 1) x 10⁹ Hz and W₁ = (7.5 ± 1) x 10¹⁰ Hz for a potential barrier of E_nr = 530 ± 50 cm^-1.