Induced far-infrared absorption in rare-gas mixtures

Translational collision-induced absorption spectra of five binary rare gas mixtures have been recorded at various densities. The experimental procedure with special control requirements is described first. The binary feature of the absorption is shown to be valid over a large density range. For this particular situation, absorption coefficients and integrated intensity values have been expressed in terms of the product of density units. Finally, a comparison between experimental and theoretical results is discussed.     

Self-trapped excitons in rare-gas dimers and solids

The excited electron states in isolated rare-gas dimers are described by a model Hamiltonian with appropriate simplifications for the short-range terms. Then the energies of self-trapped excitons are obtained by including solid state effects. The results are compared with the available transient absorption spectroscopy measurements.     

High-intensity femtosecond laser absorption by rare-gas clusters

The energy absorption efficiency of high-intensity (～10^{16}W/cm^2) femtosecond laser pulses in a dense jet of large rare-gas clusters has been measured. Experimental results show that the energy absorption efficiency is strongly dependent on the cluster size and can be higher than 90%. The measurement of the ion energy indicates that the average ion energies of argon and xenon can be as high as 90 and 100keV, respectively. The dependence of the average energy of the ions on the cluster size is also measured. At comparatively low gas backing pressure, the average ion energies of argon and xenon increase with increasing gas backing pressure. The average ion energy of argon becomes saturated gradually with further increase of the gas backing pressure. For xenon, the average ion energy drops a little after the gas backing pressure exceeds 9 bar (3.2×10^5 atoms/cluster). The result showing the existence of a maximum average ion energy has been interpreted within the framework of the microplasma sphere model.     

Small rare-gas clusters in soft x-ray pulses

We develop a microscopic model for the interaction of small rare-gas clusters with soft x-ray radiation from a free electron laser. It is shown that, while the overall charging of the clusters is rather low, unexpectedly high atomic charge states can arise due to charge imbalances inside the cluster. These findings are explained by an increased absorption via inverse bremsstrahlung due to high intermediate charge states and by a nonhomogenous charge distribution inside the cluster.     

Nonlinear laser ablation from solid rare-gas films

Optical breakdown on Ar films is studied in an intensity range from 10⁶ to 2 × 10⁹ W/cm² for wavelengths of 228, 281 and 282.8 nm. The amount of ablated H₂O, N₂, O₂ and Ar increases quadratically with laser intensity and depends strongly on the exposure time to residual gas and on substrate temperature around 24 K. The results can be explained by a model which assumes that the dominant process causing ablation is the heating of a condensed residual gas layer by two-photon absorption.     

Absorption spectrum of naphthalene in rare-gas matrices

The absorption spectrum of naphthalene has been studied in rare-gas matrices (RGM). Molar extinction coefficients have been determined and oscillator strengths of various $\text{π}{}^1\text{-π}$ transitions deduced. Bands in the 6.5 to 7.1 eV region, some of which were previously observed and interpreted as members of an impurity exciton (Rydberg) series, have been assigned to a $\text{π}{}^1\text{-π}$ transition. The results are compared with the theoretical prediction.     

Optical absorption spectra of PtO and Pt2 in rare-gas matrices

The diatomic molecules, PtO and Pt₂, as well as atomic Pt, were isolated in Ar and Kr matrices at a temperature of ca.12K. The absorption spectra of these species were photographed in the wavelength region 2500–9000 Å. Probably three systems of PtO were observed but none of them seemed to correspond to the earlier analyzed intense systems of gaseous PtO having a ¹Σ state, designated X¹Σ, as their common lower state. The assumption that this state should be the ground state of PtO is thus rather uncertain. A single progression of sharp bands in the near-infrared spectrum was ascribed to the dimer Pt₂. In addition, some absorption lines due to Pt atoms were found in the ultraviolet region.     

Optical excitation spectra of rare-gas atoms physisorbed on metal surfaces

Numerical computations are presented for the optical excitation spectra of neutral rare-gas atoms physisorbed on various metal surfaces. At low coverage and when the damping of the surface plasmons is much greater than the effective radiative damping, the occasional disappearance of the spectral lines of the optical absorption is due to a cancellation process, which occurs between the frequency profiles arising from two nearby excited states of the adsorbed atom. The red (blue) shifted peak of the symmetric mode of the higher (lower) excited state and the blue (red) shifted peak of the antisymmetric mode of the lower (higher) excited state of the atom cancel each other out provided that the frequency profiles of their lineshapes nearly coincide. Results of numerical calculations indicate that at low coverage the peaks of excited Xe on Ti and Au and excited Kr on Mg, in addition to those of Xe on Al and Kr on Au, vanish; this is compatible with the experimental observations. Predictions have been made for the adsorption spectra as a function of the distance from the atom to the surface of excited Xe and Kr, which are physisorbed on the surfaces of W, Ni, and Rh, respectively.Issued as NRCC No. 29204     

Spectroscopic study of solid benzene and benzene isolated in rare-gas matrices

Absorption and luminescence excitation spectra of solid benzene and benzene isolated in rare-gas matrices have been studied. The absolute absorption cross sections of various electronic band systems including the Rydberg region of C₆H₆ are obtained using transmission spectra of C₆H₆-doped rare-gas films of different thickness. An interpretation of the structure in the Rydberg region is given in terms of two Rydberg series using the quantum defect method. A comparative study has been made of luminescence excitation spectra of matrix-isolated and of pure C₆H₆. The luminescence excitation spectra of matrix-isolated benzene gives information on the internal conversion and medium-induced vibrational relaxation.     

Dispersion dipole between rare-gas atoms

Accurate calculations of the dispersion dipole coefficient D ₇ and its components are described for the systems HeH and HeHe, and used to check an approximate formula for D ₇ developed in a previous paper [1]. By means of a further approximation for a new atomic property, values of D ₇ are obtained for the rare-gas diatoms, based on semi-empirical Van der Waals C ₆ coefficients, Hartree-Fock expectation values and sum-rule values for the atoms. The dispersion contributions to the dipole are compared with the Hartree-Fock contributions [35], and at the conventional collision diameters are found to be of comparable magnitude but usually of opposite sign.     

Laser-enabled Auger decay in rare-gas atoms

In rare-gas atoms, Auger decay in which an inner-valence shell ns hole is filled is not energetically allowed. However, in the presence of a strong laser field, a new laser-enabled Auger decay channel can open up to increase the double-ionization yield. This process is efficient at high laser intensities, where an ns hole can be filled within a few femtoseconds of its creation. This novel laser-enabled Auger decay process is of fundamental importance for controlling electron dynamics in atoms, molecules, and materials.     

The problem of the Mössbauer isomer shift calibration for the119Sn resonance from rare-gas matrix isolation experiments

Data of new rare-gas matrix isolation experiments are presented and discussed in connection with the problem of the Mössbauer isomer shift calibration for¹¹⁹Sn. These experiments are: (i) A Mössbauer source experiment with^119mSn in solid xenon yielding the isomer shift of Sn⁺ with the atomic configuration 4d¹⁰5s²5p; (ii) Mössbauer absorption studies of isolated Sn(II) halide molecules (SnX₂, X-F, Cl, Br, I) in argon matrices; and (iii) Mössbauer absorption experiments with isolated¹²⁵Te atoms and molecules in argon and krypton matrices.This work was supported in part by a grant from the National Science Foundation and from the Bundesministerium für Forschung und Technologie     

Attosecond resolved charging of ions in a rare-gas cluster

A scheme to probe dissipative multielectron motion in time is introduced. In this context attosecond probing enables one to obtain information which is lost at later times and cannot be retrieved by conventional methods in the energy domain due to the incoherent nature of the dynamics. As a specific example we will trace the transient charging of ions in a rare-gas cluster during a strong femtosecond vacuum-ultraviolet pulse by means of delayed attosecond pulses.     

Resonances in low-energy rare-gas atom scattering

Precise values of energies and widths of low-lying resonances in some rare-gas atom systems (Ar-Ar, Ne-Kr, Ne-Xe) are reported.     

Electronic and geometric structure of doped rare-gas clusters: surface, site and size effects studied with luminescence spectroscopy

The electronic and geometric structure of rare gas clusters doped with rare-gas atoms Rg = Xe, Kr or Ar is investigated with fluorescence excitation spectroscopy in the VUV spectral range. Several absorption bands are observed in the region of the first electronic excitations of the impurity atoms, which are related to the lowest spin-orbit split atomic ³P₁ and ¹P₁ states. Due to influence of surrounding atoms of the cluster, the atomic lines are shifted to the blue and broadened (“electronical cage effect”). From the known interaction potentials and the measured spectral shifts the coordination of the impurity atom in Ar_N, Kr_N, Ne_N and He_N could be studied in great detail. In the interior of Kr_N and Ar_N the Xe atoms are located in substitutional sites with 12 nearest neighbours and internuclear distances comparable to that of the host matrix. In Ne_N and He_N the cluster atoms (18 and 22, respectively) arrange themselves around the Xe impurity with a bondlength comparable to that of the heteronuclear dimer. The results confirm that He clusters are liquid while Ne clusters are solid for N≥ 300. Smaller Ne clusters exhibit a liquid like behaviour. When doping is strong, small Rg_m-clusters (Rg = Xe, Kr, Ar, m≤10 ²) are formed in the interior sites of the host cluster made of Ne or He. Specific electronically excited states, assigned to interface excitons are observed. Their absorption bands appear and shift towards lower energy when the cluster size m increases, according to the Frenkel exciton model. The characteristic bulk excitons appear in the spectra, only when the cluster radius exceeds the penetration depth of the interface exciton, which can be considerably larger than that in free Rg_m clusters. This effect is sensitive to electron affinities of the guest and the host cluster.     

稀有气体原子在纳米碳管中的周期性振荡

采用TLHT势和经典分子动力学方法研究了稀有气体原子进入单壁纳米碳管(SWCNT)的动力学过程，计算得出SWCNT能吸入稀有气体原子(He，Ne，Ar，Kr，Xe)的管径阈值r₀分别为6.3 ?，7.0 ?，8.6 ?，8.6 ?，8.6 ?，同时计算了对应的每种稀有气体原子能封装在不同管径的SWCNT中的最大初始动能E_k0.计算给出有趣的结果是封装在纳米碳管中的稀有气体原子在管中不停地作周期性振荡，振荡周期与原子进入管中的能量无关，振幅与原子进入管中的能量有关，即振幅随着入射能量的增加而增加.分析表明：给定合适类型的碳管，具有很小初始动能的稀有气体原子可在碳管中稳定的周期性振荡，其振荡频率可达GHz.     

Glow discharge in rare-gas and metal vapour mixture

Conclusion The results are summarized in the form of graphs which illustrate the dependence of the rate coefficients on three parameters: E/N (electric field over total neutral concentration) x_e (ionization degree) and n₁ (relative concentration of cadmium). In such a way, we have tried to point out the main feature of the collisional rates under the conditions of steady-state, weakly ionized plasma. The use of the maxwellian distribution function for an evaluation of the rate coefficients would yield the results which differ substantially from ours. This concerns, particularly, the excitations from the ground state of atom efficacy of which depends on the high energy distribution tail. Since the true distribution function from Ref. [1] was employed in our calculations, it can be inferred that the main source of the inaccuracy in the rate coefficients is essentially given by the cross-sections used. It is believed that the present results can serve for a more quantitative understanding or for obtaining insight into the microscopic phenomena occurring in the positive column of the discharge.     

Scattering of Stark-decelerated OH radicals with rare-gas atoms

We present a combined experimental and theoretical study on the rotationally inelastic scattering of OH (X²Π_3/2, J = 3/2, f) radicals with the collision partners He, Ne, Ar, Kr, Xe, and D₂ as a function of the collision energy between ～70 cm^?1 and 400 cm^?1. The OH radicals are state selected and velocity tuned prior to the collision using a Stark decelerator, and field-free parity-resolved state-to-state inelastic relative scattering cross sections are measured in a crossed molecular beam configuration. For all OH-rare gas atom systems excellent agreement is obtained with the cross sections predicted by coupled channel scattering calculations based on accurate ab initio potential energy surfaces. This series of experiments complements recent studies on the scattering of OH radicals with Xe [J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, G. Meijer, Science 313, 1617 (2006)], Ar [L. Scharfenberg, J. K?os, P.J. Dagdigian, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Chem. Chem. Phys. 12, 10660 (2010)], He, and D₂ [M. Kirste, L. Scharfenberg, J. K?os, F. Lique, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Rev. A 82, 042717 (2010)]. A comparison of the relative scattering cross sections for this set of collision partners reveals interesting trends in the scattering behavior.     

Glow discharge in rare-gas and metal vapour mixture

Applying the Boltzmann equation to a He-Cd mixture discharge the electron energy distribution functions, kinetic coefficients and collision frequencies are numerically calculated. Calculations are made for a homogeneous and stationary discharge plasma subjected to an externally applied electric field. The collision processes which have been taken into account are elastic and inelastic collisions of electrons with He and Cd atoms as well as mutual encounters of electrons. In this case the electron energy distribution and all the quantities calculated from it are dependent on the reduced electric field, the ionization degree and the relative cadmium concentration.     

Nonadiabatic effects in the lattice dynamics of compressed rare-gas crystals

Electron-ion contributions to the energy of rare-gas crystals are discussed from first principles in the framework of the Tolpygo model and its variants. The frequencies of phonons in a neon crystal at pressures p ≠ 0 are calculated in terms of models that go beyond the scope of the adiabatic approximation. Analysis of the contributions from different interactions to the lattice dynamics of the crystals demonstrates that the phonon frequencies calculated in the framework of the simplest model (allowing only for the nearest neighbors) and the most complex model (with the inclusion of the nearest neighbors, next-nearest neighbors, nonadiabatic effects, etc.) for small wave vectors are close to each other. The difference between the phonon frequencies calculated within the above models is most pronounced at the Brillouin zone boundary. Under strong compression, the phonon spectrum along the Δ direction is distorted and the longitudinal mode is softened as a result of the electron-phonon interaction. The contribution from terms of higher orders in the overlap integral S at p ≠ 0 to the phonon frequencies is more significant than that obtained in the band-structure calculations of the neon crystal.