Pure dephasing and nonradiative decay processes in the excited electronic states of NaF:Cu+

Abstract

We report the first optical dephasing study of an inorganic impurity system possessing sharp, low frequency mode structure in its ground and excited state spectra. The total dephasing times T2 and population relaxation times T1 for the 1T2g and 1Eg d9s excited states of NaF:Cu+ are determined at a series of temperatures between 1.8 and 296 K. The T2 values are determined by extracting the Lorentzian components from one- and two-photon excitation line shapes. The T1 values caused by nonradiative decay rates are obtained by detecting very low quantum yield emission from the fast-relaxing excited states and applying the formula tnr=Qtr, where t’s are radiative and nonradiative lifetimes and Q is the quantum yield. T1(1T2g)=4.6 ps and T1(1Eg )=2.0 ps at 8 K. Significantly, these are very similar to the T1 values calculated from lowest temperature Lorentzian linewidths by the relationship 1/T2=1/T′2 +1/(2T1). The T1 values stay approximately constant over the temperature range 1.8–45 K, while the linewidths rapidly increase indicating that pure dephasing dominates. Using ground and excited state information on the low frequency modes, we test optical Redfield theory and the nonperturbative harmonic theory for pure dephasing by pseudolocal phonons against data for this system which displays strongly anhamonic progressions. The nonperturbative theory fits the line broadening data to higher temperatures than optical Redfield theory for the least anharmonic excited state potential, 1T2g. Both theories underpredict the broadening with temperature of the extremely anharmonic 1Eg state. A simple anharmonic theory including scattering to overtone levels also fails to predict the observed linewidth temperature behavior, although it is demonstrated these processes should be occurring.