Ultrafast dynamics of excess electrons in molten salts: part II. Femtosecond investigations of Na-NaBr and Na-NaI melts.

ABSTRACT

Ultrafast dynamics of excess electrons in Na-NaBr and Na-NaI molten solutions at elevated temperatures (T = 953-1128 K) were investigated over an extended wavelength range. Modelling the time profiles resulted in two time constants tau 1 = (200 +/- 40) fs and tau 2 = (2.8 +/- 0.4) ps for both systems at 1073 K. All transients can be understood in terms of dynamical equilibria between polaron and Drude-type electrons as well as polaron and Drude-type electron forming bipolarons. In agreement with our earlier results for K-KCl melts the fast component is assigned to the relaxation of Drude-type electrons into polarons while the longer component, tau 2, represents the time during which Drude-type electrons recombine with polarons leading to bipolarons. In addition, the temperature dependence was studied in Na-Nal Decreasing the temperature to 953 K resulted in an increase of the time constants to tau 1 = (360 +/- 50) fs and tau 2 = (4.3 +/- 0.7) ps, respectively. At temperatures, where the ionic diffusion in Na-NaI melts becomes comparable to Na-NaBr melts, the time constants for the relaxation processes also coincide. The temperature-dependent investigations resulted in an Arrhenius activation energy of (25 +/- 5) kJ mol(-1) for Na-NaI melts in good agreement with literature data.