Picosecond Studies of Transient Absorption Induced by BandGap Excitation of CsI and CsI:Tl at Room Temperature

Abstract-We report picosecond time-resolved measurements of optical absorption induced by a sub-picosecond pulse of light producing two-photon bandgap excitation of Csl and CsI:Tl at room temperature. The transient spectrum of undoped Csl reveals for the first time strong infrared absorption rising through the 0.8-eV limit of present measurements. We suggest that this infrared band is due to transitions of the bound electron in the off-center self-trapped exciton (STE), implying that there should be a band deeper in the infrared associated with the known on-center STE in Csl. Previously reported visible and ultraviolet transient absorption bands at 1.7, 2.5, and 3.4 eV are confirmed in these measurements as attributable to hole excitations of STE. In 0.3% thallium doped Csl, infrared absorption possibly attributable to STEs is observed for approximately the first 5 ps after excitation at room temperature, but decays quickly. The absorption bands of Tl⁰ (electron trapped at Ti⁺ activator) and of self-trapped holes are the main species seen at longer times after excitation, during which most of a scintillation pulse would occur. This is in accord with a recently published report of nanosecond induced absorption in CsI:Tl.