Picosecond pulse radiolysis of rare gases: Evidence for the time evolution of the subexcitation spectrum

The production of the 2p electronic excited states of the pure rare gases neon, argon, krypton, and xenon has been studied by using picosecond pulse radiolysis techniques. At pressures below about 5 Torr, the emission from these excited states shows a pressure-dependent growth-decay pattern. Kinetic analysis of these patterns reveals formation rate constants that are all greater than 1012 dm3 mol-1 s-1 (1.66 x 10-9 cm3 s-1). These observations preclude atom-atom collisional processes and support an excitation mechanism involving collisional energy loss from electrons with energies greater than the lowest excitation potential of the rare gas. It is thus concluded that the secondary electron spectrum in pure rare gases at pressures below a few Torr takes a few nanoseconds to degrade to energies below that of the lowest excited state. This time scale therefore (by definition) corresponds to the time needed to form subexcitation electrons. In 1 atm of rare gas the time taken would be of the order of 20-30 ps