Ultrafast Radiative Relaxation Processes in Multication Cross-Luminescence Materials

Scintillators with ultrafast response are of demand in medical imaging for time-of-flight positron emission tomography, in high-energy physics calorimetry, and in other applications. There are several fast luminescence processes in solid-state materials, which could be good candidates for fast timing applications. The development of new ultrafast silicon photomultipliers with increased vacuum-ultraviolet to ultraviolet (VUV-UV) sensitivity has renewed interest to one of those, cross-luminescence (CL), emitted in the VUV-UV region with subnanosecond decay time. Previous investigations have shown that CL is mostly situated in the VUV region in binary K and Ba compounds, but ternary fluorides with a complex valence band structure can provide ultrafast emissions in UV. According to the band structure calculations for K
₂
SiF
₆
, its complex valence band is composed of the F 2p, Si 2s, and Si 2p states. Transitions between the valence subbands and the outermost K 3p core band may extend CL from the VUV-UV to the visible region. This motivated us to perform spectroscopic study of phase-pure K
₂
SiF
₆
micropowders with a high degree of crystallinity. The stationary (10 keV) and pulsed (100 keV, δt ≈ 55 ps) electron beams were applied in the studies of relaxation processes of K
₂
SiF
₆
in the temperature range of 77-300 K. At 300 K, the dynamics of ultrafast processes was studied at the FemtoMAX beamline (MAX IV Lab) providing 10-keV X-ray pulses of 100-fs duration. As a result, we revealed multiple CL bands extending from 9 to 4 eV. A nonexponential decay in the subnanosecond time range was revealed for the CL bands at 4.5 and 5.25 eV under both pulsed electron-beam and X-ray excitations. This opens prospects for a band structure engineering approach in developing new ultrafast scintillators with intrinsic emissions. This approach is based on a targeted modification of material composition to influence the density of states in complex valence bands and to provide a favorable shift of CL to lower energies.