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

The relaxation behavior of the third excited (Σ) 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±15 ps and 90±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±10 ${\mathrm{cm}}^{\mathrm{-}1}$. It is argued that this radiationless electronic deexcitation between the Σ 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 ${\mathrm{cm}}^{\mathrm{-}1}$ in the ${\mathrm{Ga}}^0$(1) ground-state spectrum. In contrast, configurational relaxation within the Σ 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.