Reactions of Charged Species in Supercritical Xenon as Studied by Pulse Radiolysis

Abstract

The results of an initial study of the pulse radiolysis of supercritical xenon are reported. In pure xenon, transients are formed that absorb broadly throughout the visible. These transients are assigned to excimer species, Xe2*, on the basis of lifetime and kinetic data. The formation of excimers by electron−ion recombination was time-resolved by pulse−probe measurements. The excimers can be quenched by adding small amounts of ethane, which then facilitates detection of other transients by absorption spectroscopy. The added ethane also accelerates the thermalization of electrons and allows measurements of fast reaction rates of thermal electrons. Electron attachment to hexafluorobenzene occurs near the maximum rate at high pressures in xenon−ethane mixtures. The C6F6 - anion formed absorbs with a maximum at 500 nm and disappears by second-order kinetics. The mobility of this anion, as measured by conductivity, indicates sizable clusters of solvent around the ion at all pressures, which are of maximum size near critical density. The rate of electron transfer from C6F6 - to benzoquinone exceeds 1 × 1011 m -1 s-1 at most pressures. The rate maximizes near 62 bar at 21.4 °C. A maximum at this pressure is predicted by diffusion. The maximum is related to the increase in cluster size around the anion, which occurs at this pressure.

The LEAF picosecond pulse radiolysis facility at Brookhaven National Laboratory

“…18 For these experiments samples are irradiated using Bremsstrahlung x rays from the electron beam stopping in the Faraday cup of the transient absorption cell holder. The experimental apparatus has been described previously.…”

Ultrafast pulse radiolysis using a terawatt laser wakefield accelerator

“…Recent picosecond pulse radiolysis experiments that take advantage of the new time window include a reevaluation of the initial radiolytic yield of the hydrated electron in water, 30 ultrafast studies of quantum confinement in semiconducting scintillators, 32 fast charge transfer mechanisms, 33 radiolytic reactions 31,34,35 and radiolysis of supercritical liquids. 36,37 For photocathode linacs, the experimental time resolution is highly dependent upon the degree of synchronization between the phase of the RF field and the laser pulse that generates the photoelectrons at the photocathode as well as the synchronization between the resulting electron pulse and the probe laser pulse. For example, to generate an 800 fs electron pulse, the time jitter between the RF and the laser must be reduced to better than 500 fs.…”

Energetics and Volume Changes in Electron Attachment to Pyrazine in Supercritical Xenon

“…Included for comparison in Figure 2 is the rate constant for attachment to C 6 F 6 at 20°C. 9 Whereas the rate increases rapidly for C 6 F 6 near 62 bar, the rate of attachment to pyrazine at 20°C is decreasing at this pressure. The differences observed for k a in supercritical ethane and xenon lead to quite different behavior of the activation volumes, ∆V a * .…”

“…The ethane prevents reactions of hot electrons from occurring and has no effect on the mobility. 9 Although ethane is known to quench excimers, 9 it is assumed that it has no effect on the electron reactions studied here. At this point, the rate of electron attachment to residual impurities, k imp [impurity], was determined as a function of Xe pressure.…”