Picosecond radiationless electronic relaxation of theGa0(1) andIn0(1) defects in KCl

Abstract

The relaxation behavior of the third excited (\ensuremath{\Sigma}) state of the ${\mathrm{Ga}}^{0}$(1) and ${\mathrm{In}}^{0}$(1) defects in KCl, which possess the laser-active type of structure, has been investigated with picosecond (ps) optical pulses. With a pump-probe technique based on a pair of synchronously pumped Rhodamine 6G dye lasers and a double-modulation detection scheme, a subnanosecond recovery was measured of the ground-state population of ${\mathrm{Ga}}^{0}$(1) and ${\mathrm{In}}^{0}$(1) after excitation in their third optical transition [OT(3)] at about 615 nm. The relaxation times at 7 K are 260\ifmmode\pm\else\textpm\fi{}15 ps and 90\ifmmode\pm\else\textpm\fi{}15 ps for ${\mathrm{Ga}}^{0}$(1) and ${\mathrm{In}}^{0}$(1), respectively. For the ${\mathrm{Ga}}^{0}$(1) defect an Arrhenius fit of the temperature dependence of the decay time between 50 and 150 K yields an activation energy of 60\ifmmode\pm\else\textpm\fi{}10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. It is argued that this radiationless electronic deexcitation between the \ensuremath{\Sigma} and the ground state is promoted by a low-frequency vibration of the ${M}^{0}$ atom along the 〈100〉 defect axis. A similar mode is observed by means of resonant Raman measurements at 59 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the ${\mathrm{Ga}}^{0}$(1) ground-state spectrum. In contrast, configurational relaxation within the \ensuremath{\Sigma} state probably takes place through emission of defect-perturbed lattice modes, as was earlier demonstrated for the ${F}_{A}$(${\mathrm{Li}}^{+}$) center in its lowest excited state.