N. Schwentner

Rydberg states of NO trapped in rare gas matrices

Several new progressions consisting of broadbands have been observed in the absorption spectra of NO trapped in rare gas matrices. They are identified as the blue shifted transitions to the n=3 (A 2Σ+,C 2Π,D 2Σ+) and n=4 (E 2Σ+,K 2Π,M 2Σ+) Rydberg states of NO in Ne matrices and to the n=3 Rydberg states in Ar, Kr, and Xe matrices. Linewidths (50 to 180 meV) and matrix shifts (0.3 to 1.2 eV) decrease from neon to xenon matrices. For each matrix, the shifts decrease according to the symmetry of the excited state orbital, in the sequence sσ–pσ–pπ. An attempt to apply the quantum defect formula with values, for the Rydberg constant and the ionization potential of the matrix isolated molecule taken from literature, fails to describe the vertical transition energies of n=3 and n=4 states in neon matrices. On the other hand, using the adiabatic energies of 3sσ and 4sσ states to generate a series shows that the vertical transitions of n≥5 states lie in the ionization continuum. This could explain the nonobservat...

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...

Rydberg fluorescence of NO trapped in rare gas matrices

Fluorescence spectra from the Rydberg A 2Σ+ (v=0) level of NO trapped in Ne, Ar, Kr, and Xe matrices have been obtained for two different sites. The main site is characterized by broad A(0,v‘) bands (FWHM≂80 meV), blue gas‐to‐matrix shifts of 70 to 300 meV, and by absorption–emission Stokes shifts of 300 meV in Xe to 800 meV in Ne matrices. The line shapes and Stokes shifts are treated within a configuration coordinate model by a moment analysis. A linear and quadratic coupling is invoked with relaxation energies ranging for the excited state from 160 meV in Xe to 540 meV in Ne and for the ground state from 150 meV in Xe to 280 meV in Ne and an increase of the cage radius of 3% in Xe to 15% in Ne. The ‘‘red’’ site fluorescence shows emission bands with matrix shifts of −140 meV in Xe to 80 meV in Ne and absorption–emission Stokes shifts of 210 meV in Xe to 830 meV in Ne. Red site ‘‘hot’’ A (v=1) fluorescence is also observed. Red sites are attributed to local disorder around the molecule in Ar, Kr, and Xe...

A solid state rare gas halide laser: XeF in crystalline argon

Abstract Optically pumped laser action has been achieved over the XeF (B→X) and (C→A) transitions in crystalline argon. Conversion efficiencies as high as 30% are observed. Gain measurements indicate that the laser should be tunable over the broad C→A band of ≈80 nm.

A system for time and energy resolved VUV luminescence spectroscopy using synchrotron radiation for excitation

Abstract Synchrotron radiation of an electron storage ring is an excellent excitation source for luminescence studies because it provides an intense continuum of photon energies and the light is pulsed with a high repetition rate (DORIS: 1 MHz−500 MHz) and with short pulse lengths (DORIS: 130 psec fwhm). At the Deutsches Elektronen Synchrotron in Hamburg a system has been developed for the storage ring DORIS with the following properties: 1. 1) monochromatic excitation in a photon energy range of 5 eV up to 50 eV ( E ΔE ≈ 500) ; 2. 2) energy resolution of luminescence in an energy range of 2 eV up to 20 eV ( E ΔE ≈ 50−100) ; 3. 3) time resolution of luminescence using photomultipliers and channel plate detectors. Rise and decay times down to 50 psec can be determined; 4. 4) gaseous samples, solid samples and doped matrices can be investigated at room temperature down to 4 K.

Spectroscopy and photodissociation of chlorine monomers and clusters in argon matrices

Steady‐state and time‐resolved emission spectroscopy of the A–X system of Cl2 is used to distinguish molecular chlorine isolated as monomers and clusters in solid argon samples. The lifetime of the metastable A’ 3Π2u state is measured to be an order of magnitude less, when chlorine is present as clusters, than when it is truly isolated. Photodissociation of molecular chlorine clusters was found to be insignificant as monitored by the emission of Ar2Cl at 260 nm. Measurement of the dissociation threshold of molecular chlorine in the 9 eV region as a function of temperature showed little variation. Using spectroscopic data it is concluded that dissociation is occurring by an impulsive mechanism involving curve‐crossing from the initially populated Ar+(Cl2)− charge transfer state to repulsive potentials correlating with ground state atomic chlorine and not via a harpooning mechanism. A simple microscopic model, drawn from experimental data and pairwise addition of ArCl potential terms, is constructed to desc...

Spectroscopy and photodissociation of molecular chlorine in argon matrices

A structureless band observed at 180 nm in the absorption and excitation spectra of Cl2/Ar matrices is tentatively assigned as the spin forbidden 3Σ+u ← X 1Σ+g transition of molecular chlorine. Having an absorption cross section of approximately 10−18 cm2 in the solid, a twofold order of magnitude increase in the transition probability is observed relative to the gas phase. Wavelength specific measurements of the photodissociation of molecular chlorine in crystalline argon samples showed that a dominant threshold exists in the 130 nm band at 9.2 eV corresponding to absorption into the bound 1 1Σ+u state. The maximum quantum yield for permanent dissociation in the 130 nm band was found to be 0.3. Luminescence evidence indicates that this dissociation does not involve a charge‐transfer mechanism but a crossing from the bound 1 1Σ+u state to a repulsive potential on which an impulsive cage escape occurs. Photoexcitation in the 180 nm band also results in the permanent dissociation of chlorine as well as the ...

Photodynamics in superfluid helium: Femtosecond laser-induced ionization, charge recombination, and preparation of molecular Rydberg states

Femtosecond pulses (790 nm) are used for nonresonant laser excitation of superfluid liquid helium to prepare ionic and neutral excited states at energies above 18 eV. Measurements of laser-induced fluorescence and photocurrent enable a detailed description of the primary photoprocesses. A controllable excitation regime unique to femtosecond pulses is realized at laser intensities below the dielectric breakdown threshold, I<5×1013 W/cm2. A steady state of the long-lived triplet excimers He2*(3a) (lowest Rydberg state) is established; the concentration decays between laser pulses through diffusion-controlled bimolecular annihilation to ∼1012 cm−3 at a laser repetition rate of 500 Hz. The triplet population is amplified with each pulse in a sequence that involves: (1) ionization of the Rydberg electron of He2* via complete Coulomb barrier suppression; (2) cascade electron impact ionization of the ground-state He atoms by the ponderomotively accelerated quasifree electrons in liquid He; (3) localization and t...

Cage effect for the abstraction of H from H2O in Ar matrices

A red wing in the absorption of H2O in the first continuum which extends beyond 200 nm is induced by the matrix. A threshold energy of 6.95 eV is found for the quantum efficiency of permanent dissociation of H2O into OH and H. This corresponds to an additional barrier of 1.8 eV due to the matrix cage. The barrier is attributed to the sum of repulsive H–Ar pair potentials in an interstitial site of D3 symmetry, which has to be surmounted by the H atom on the way from the initial substitutional H2O site to the final stable Oh site of the abstracted H atom.

Selectivity in dry etching of Si(100) with XeF2 and VUV light

Abstract Replica of a mask were etched in Si(100) wafers with a sub-micrometer lateral resolution by VUV irradiation using XeF2. A high selectivity is achieved if the spontaneous reaction of XeF2 is suppressed by a buffer gas and if the unselective light induced reaction is avoided. The strength of unselective etching is strongly wavelength dependent and follows the XeF2 gas phase absorption coefficient. Fragments from the XeF2 gas phase dissociation reach the Si surface and thus cause unselective etching. Optimal dry etching is achieved around 120 nm because the selectivity is high and also the quantum efficiency is very large. The absolute quantum efficiency at 120 nm corresponds to 10 etched Si atoms per incident photon.