Inhomogeneous broadening of the dynamically split Kramers spectral line and up-conversion in the pair and quartet centers in CaF 2:Nd 3+

Abstract

The site-selective and time-resolved fluorescence laser spectroscopy and kinetic measurements with high spectral and nanosecond temporal resolution was applied to analyze the high-energy wing of the M and N absorption bands of the 4I 9/2(1)→ 4G 5/2(1) crystal-field (CF) transition in a CaF 2:Nd 3+ (0.6 wt%) crystal at 4.2 K. It was found that at helium temperatures the dynamically split spectral line assigned as the 4I 9/2(1)→ 4G 5/2(1) (CF) transition of coherently coupled Nd 3+ ions in the pair M- and quartet N-centers of CaF 2:Nd 3+ (0.6 wt%) is inhomogeneously broadened. It consists of the pair M- and quartet N-centers with at least 0.1 A variation of the positions of the fluorescence-excitation spectral lines registered at the 4F 3/2(1)→ 4I 9/2(1) CF transition. Small fluorescence-lifetimes variation of the 4F 3/2 and 4D 3/2 levels from the small variation of the distances R between Nd 3+ ions in the pair is found. At least 2.7% variation of the value of the Nd–Nd distance R in the pair M-center was determined from the lifetime variation of the 4F 3/2 manifold with the assumption of a dipole–dipole interaction between the ions in the pair. The energy transfer up-conversion process responsible for the UV fluorescence observed when pumping the 4I 9/2(1)→ 4G 5/2(1) transition has been determined.