A photoion–photoelectron coincidence study of Kr and Xe dimers

Abstract

The photoion–photoelectron coincidence (PIPECO) spectra for Kr+2 and Xe+2 in the wavelength regions of 825–970 and 900–1030 Å, respectively, have been measured at different nozzle temperatures and stagnation pressures (P0). The ionization energies (IE) for Kr2 and Xe2 to Kr+2[I(1/2)u] and Xe+2[I(1/2)u] determined by the PIPECO spectra are in excellent agreement with the results of previous photoionization experiments. The PIPECO measurements for Kr+2 and Xe+2 also provide lower limits for the IEs of Kr2 and Xe2 to Kr+2[II(1/2)u] and Xe+2[II(1/2)u]. The PIPECO spectra for Kr+2 and Xe+2 display strong autoionization structures similar to those resolved in the corresponding photoionization efficiency spectra, indicating that a significant fraction of autoionizing electrons are slow electrons with near zero kinetic energies. The extreme weakness of the Kr+2[II(1/2)u] and Xe+2[II(1/2)u] PIPECO bands observed at low P0 support the conclusion that excited Kr+2[II(1/2)u] and Xe+2[II(1/2)u] ions are dissociative with dissociation lifetimes shorter than 68 and 85 μs, respectively. These values are in accord with the calculated radiative lifetimes for the II(1/2)u→I(1/2)g transitions in Kr+2 and Xe+2. The intensities for the II(1/2)u PIPECO bands relative to the I(1/2)u and I(3/2)g PIPECO bands for Kr+2 and Xe+2 are found to increase as P0 is increased, an observation attributed to the formation of Kr+2 and Xe+2 from fragmentation of excited Kr+n and Xe+n ions produced in the photoionization of Krn and Xen, n≥3. The fragmentation of excited Kr+n and Xe+n , n≥3, ions have the effect of lowering the appearance energies for the Kr+2[II(1/2)u] and Xe+2[II(1/2)u] PIPECO bands.