Baruch Raz

Electronic Energy Transfer Phenomena in Rare Gases

In this paper we present the results of an experimental study of electronic energy transfer in xenon‐argon, krypton‐argon, and xenon‐krypton gaseous mixtures excited by a pulsed electric discharge. Spectroscopic evidence for electronic energy transfer is based on the decrease in the intensity of the vacuum ultraviolet emission of the excited diatomic homonuclear rare gas molecules in the presence of small amounts (10–1000 ppm) of a foreign rare gas atom, while the visible emission spectrum of the host gas is parctically unmodified under these conditions. The relative contributions of two energy transfer mechanisms involving atom‐atom and molecule‐atom energy transfer were established by a kinetic analysis of the dependence of the energy transfer efficiency on the host pressure. We have determined the cross sections for energy transfer from the lowest metastable Ar and Kr excited states, and from the lowest excited state of Ar2* and Kr2* to ground state Xe, and from metastable excited Ar and from Ar2* to g...

Photoemission from doped solid rare gases

In this paper we report the results of an experimental study of the photoelectric yield of doped solid rare gases in the extreme ultraviolet (h/ω = 8−30 eV) spanning the range of impurity excitations, exciton states, and interband transitions. Results were obtained for Xe in Kr, Xe in Ar, Kr in Ar, and benzene in Ar, Kr, and Xe. For dilute atomic and molecular impurities in solid rare gases three intrinsic photoemission mechanisms are exhibited: (a) direct excitation from the impurity state above the impurity threshold, (b) electronic energy transfer from the host exciton states to the impurity states resulting in exciton induced impurity photoemission, and (c) direct photoemission from the host matrix at energies above the matrix threshold. The photoemission thresholds from impurity states via processes (a) or (b) result in detailed information regarding electron affinities of solid rare gases which are in good agreement with recent data for the pure solids. A detailed study of exciton induced photoemiss...

Energy of the quasi-free electron state in liquid and solid rare gases

Abstract Estimates of the energy of the bottom of the conduction band in liquid and solid Ar, Kr and Xe are obtained from spectroscopic data for Wannier impurity states in the liquids and solids and for X-ray exciton states in the solids.

Extravalence molecular excitations in inert matrices

In this paper we present the results of an experimental study of extravalence excitations of molecular impurities in solid rare gases and in molecular host matrices. We report the absorption spectra of methyl iodide, benzene, ethylene, and acetylene in solid Ne, Ar, Kr, Xe, N2, and CF4 in the spectral region 2000–1150 A. Spectroscopic evidence has been obtained for the observation of high (n ≥ 2) Wannier impurity states originating from a molecular positive ion. These molecular Wannier series yield spectroscopic information concerning the energetics of molecular photoionization in a dense medium. The large blue spectral shifts of the lowest extravalence molecular excitations in dense media can be rationalized in terms of central cell corrections to the n=1 Wannier state, which are determined by the exciton binding energy.

Medium perturbations of atomic extravalence excitations

In this paper we present the results of an experimental study of the lowest 1S0→3P1 extravalence electronic excitation of atomic xenon in dense supercritical and subcritical fluid argon over the density range 0.1–1.4 g⋅cm−3, spanning the temperature region 80–300°K. Solvent perturbations were characterized in terms of the spectral shift, the linewidth and the first and the second moments of the absorption band. These energetic parameters exhibit a weak temperature sensitivity and a strong density dependence. The experimental data were analyzed in terms of the semiclassical theory of line broadening extended to account for guest–host and host–host correlations and incorporating realistic pair potentials in the ground and in the excited electronic states. The semiclassical theory results in manageable expressions for the first and for the second moments of the absorption band which are expressed in terms of a difference guest–host interaction potential together with the solute–solvent and the solvent–solven...

Electronic energy transfer in rare gas mixtures

In this paper we present the results of an experimental study of electronic energy transfer in xenon‐argon and xenon‐krypton gaseous mixtures excited by an α source. Spectroscopic evidence is presented for molecule‐atom energy transfer from Ar*2(1,3Σu) to Xe(1P1) and from Kr*2(1,3Σu) to Xe(3P1). These two electronic energy transfer processes are characterized by large ∼ 10−14 − 10−13 cm2 cross sections. Information has been obtained for collision‐induced energy conversion processes of the Xe energy acceptor involving 1P1 → 3P1 conversion and the formation of Xe*2(1,3Σu) at moderately low (≤ 200 ppm) Xe concentrations. A new emission band, observed in the xenon‐krypton system, peaking at 1530A, is tentatively assigned to a heteronuclear (KrXe)* diatomic molecule.

Temperature dependence of rare gas molecular emission in the vacuum ultraviolet

Abstract The emission spectra of Ar * 2 and of Xe * 2 ) 1 Σ u , 3 Σ u ) in the gas phase and in liquid rare gases exhibit a temperature dependence of the band width, which provides information concerning the potential of these excited rare gas molecules.

Emission spectra of solid rare‐gas alloys

In this paper we present the results of an experimental study of the α‐excited vacuum ultraviolet emission spectra of xenon‐argon, xenon‐krypton, krypton‐argon, xenon‐neon, krypton‐neon, and argon‐neon solid alloys in the temperature range 6–40°K. Three mechanisms for electronic energy transfer to single impurity states and to impurity pairs were considered: (1) energy transfer from vibrationally relaxed homonuclear diatomic molecule of the host to Xe/Ar and to Xe/Kr impurity states; (2) energy transfer from the host vibrationally excited homonuclear diatomic molecule to Kr/Ar impurity states; and (3) energy transfer via impurity ionization or the formation of metastable states in neon alloys. The single impurity emission bands originating from the lowest Wannier states exhibit large red Stokes shifts reletive to the corresponding absorption bands, being close to the corresponding gas‐phase transitions, and thus manifest the effect of medium relaxation around single impurity states. A similar effect is ex...

Experimental evidence for trapped exciton states in liquid rare gases

In connexion with studies of the electronic structure of disordered systems, we enquire whether there exist exciton states in simple liquids. We report the results of a vacuum ultraviolet spectroscopic study of liquid argon and of liquid krypton doped with xenon. Experimental evidence was obtained for Wannier-Mott type impurity states in liquids which have no parentage in the excited states of the isolated atoms constituting the dense fluid. The absorption spectra of the doped liquid rare gases were monitored in the region 160 to 120 nm. The following experimental results are reported: (a) In the Xe/Ar liquid two absorption bands corresponding to the 1S0 → 3P1 and to the 1S0 → 1P1 transitions (or alternatively to the n = 1 Wannier states) were identified at 141 nm (8.80eV)† and at 123nm (10.1 eV). An additional line was observed at 127 nm (9.76eV). (b) In the Xe/Kr liquid three absorption bands were observed at 144.5 nm (8.59 eV), 125.5 nm (9.89 eV) and 129 nm (9.6 eV). (c) The absorption spectra of the doped liquids were compared with the spectra of 1 cm thick doped solid rare-gas crystals. From these results we conclude that: (a) The 127 nm (9.76 eV) band in the Xe/Ar liquid system and the 129 nm (9.61 eV) band in the Xe/Kr liquid system cannot be attributed to a perturbed ‘atomic’ state and are assigned to the n = 2 Wannier state in the liquid. (b) Line broadening of exciton states in the liquid can be accounted for by a simple scattering model. (c) Preliminary information on band gaps in liquid rare gases were obtained from the spectroscopic data. (d) The effect of liquid-solid phase transition on the line broadening of exciton states is consistent with electron mobility data in these systems.

Solvent perturbations of extravalence excitations of atomic Xe by rare gases at high pressures

In this paper we present the results of an experimental study of the lowest doublet excitations (1S0→3P1 and 1S0→1P1) of atomic xenon in dense supercritical fluid neon and helium and the 1S0→1P1 xenon excitation in dense supercritical and subcritical fluid argon. The spectral shift, the linewidth, the first moment, and the second moment of the absorption bands exhibit a strong density and a weak temperature dependence. The semiclassical statistical theory of line shapes was applied for the calculations of the first moment, the second moment, and the entire line shape. By fitting to the experimental moments over a broad density range, the excited state guest–host interaction potential parameters were derived. An analysis of the density dependence of the splitting and of the intensity ratio of the Xe doublet perturbed by high pressure rare gases was provided.