Charge Recombination via Electron Tunneling after Two-Photon Ionization of Dopant Chromophore in Poly(butyl methacrylate) Film at 20 K

The charge recombination of electron–cation pairs produced in a poly(butyl methacrylate) film through two-photon ionization was examined at 20 K by emission spectroscopy and absorption spectroscopy. Both the isothermal luminescence (ITL) decay and the absorbance decay of the radical cation obeyed the same kinetics at 20 K: the t−m law, I(t) ∝ t−m with m ≈ 1. The temperature dependence of the ITL decay kinetics remained the same from 20 to 200 K far below the glass-transition temperature (293 K). These findings show that the ITL decay at low temperatures can be explained in terms of a long-range electron transfer by electron tunneling. Multi-shot photoirradiation was found to cause a deviation of the ITL decay from the t−m law. Multi-shot photoirradiation probably induces a larger separation of a photoejected electron from the parent cation and/or a deepening of the trap depth.