Luminescence Delay and Determination of Nonradiative Relaxation Times

Abstract

An analysis is made of the kinetics of the luminescence of a three-level system in which a transition takes place from a lower metastable state (lifetime τ ₁) under the influence of excitation pulses of duration t₀ to an upper state (lifetime τ ₂ < τ ₁) from which the system relaxes back to the metastable state. It is shown that if t₀ ⪝ τ, the luminescence is delayed after the end of excitation and this delay can be used to calculate the nonradiative relaxation time τ ₂. The luminescence delay was observed experimentally when Y₃Al₅O₁₂:Nd³⁺ was excited with electrons pulses of t₀ ⪝ 3 μsec duration (radiative transitions of Nd³⁺ from the ⁴F_3/2 level due to radiative relaxation from an S level of the neodymium ions) and t₀ = 10 nsec duration (transitions from an S level due to the transfer of energy from the host lattice to Nd³⁺). It was found that the transfer time was τ ₂ = 40 nsec. The delay of the luminescence emitted from electron — hole drops in Ge at 4.2°K was used to determine the drop formation time, which was 200 nsec.