Luminescence Delay and Determination of Nonradiative Relaxation Times
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 4F3/2 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.
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