Ion-beam excitation of liquid argon

The scintillation light of liquid argon has been recorded wavelength and time resolved with very good statistics in a wavelength interval ranging from 118 nm through 970 nm. Three different ion beams, protons, sulfur ions and gold ions, were used to excite liquid argon. Only minor differences were observed in the wavelength-spectra obtained with the different incident particles. Light emission in the wavelength range of the third excimer continuum was found to be strongly suppressed in the liquid phase. In time-resolved measurements, the time structure of the scintillation light can be directly attributed to wavelength in our studies, as no wavelength shifter has been used. These measurements confirm that the singlet-to-triplet intensity ratio in the second excimer continuum range is a useful parameter for particle discrimination, which can also be employed in wavelength-integrated measurements as long as the sensitivity of the detector system does not rise steeply for wavelengths longer than 190 nm. Using our values for the singlet-to-triplet ratio down to low energies deposited a discrimination threshold between incident protons and sulfur ions as low as ∼2.5 keV seems possible, which represents the principle limit for the discrimination of these two species in liquid argon

Hydrated electron dynamics: from clusters to bulk

The electronic relaxation dynamics of size-selected (H2O)n–/(D2O)n[25 n 50] clusters have been studied with time-resolved photoelectron imaging. The excess electron () was excited through the transition with an ultrafast laser pulse, with subsequent evolution of the excited state monitored with photodetachment and photoelectron imaging. All clusters exhibited p-state population decay with concomitant s-state repopulation (internal conversion) on time scales ranging from 180 to 130 femtoseconds for (H2O)n– and 400 to 225 femtoseconds for (D2O)n–; the lifetimes decrease with increasing cluster sizes. Our results support the "nonadiabatic relaxation" mechanism for the bulk hydrated electron (), which invokes a 50-femtosecond internal conversion lifetime

Observation of large water-cluster anions with surface-bound excess electrons

Anionic water clusters have long been studied to infer properties of the bulk hydrated electron. We used photoelectron imaging to characterize a class of (H2O)n– and (D2O)n– cluster anions (n 200 molecules) with vertical binding energies that are significantly lower than those previously recorded. The data are consistent with a structure in which the excess electron is bound to the surface of the cluster. This result implies that the excess electron in previously observed water-cluster anions, with higher vertical binding energies, was internally solvated. Thus, the properties of those clusters could be extrapolated to those of the bulk hydrated electron

Surface and interior anion solvation in water clusters

We describe the cluster size dependence of the vertical electrostatic stabilization energies ESTAB(n) of mass-selected I-(H2O)n (n = 7-60) clusters in terms of classical electrostatic solvation models for interior solvation, solvation inside the cluster surface and solvation on the cluster surface. Surface anion solvation in clusters of polar and polarizable molecules yields higher vertical electron detachment energies than those for internal anion solvation. The experimental results for ESTAB(n) fit somewhat better the model of anion surface solvation and extrapolate well to a value of ESTAB(∞) for solvation inside the surface