Rare Gas Atoms Rg Research Articles

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 ArN, KrN, NeN and HeN could be studied in great detail. In the interior of KrN and ArN the Xe atoms are located in substitutional sites with 12 nearest neighbours and internuclear distances comparable to that of the host matrix. In NeN and HeN 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 Rgm-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 Rgm clusters. This effect is sensitive to electron affinities of the guest and the host cluster.

Experimental and theoretical investigation of even mp51/2 np′ autoionizing resonances of rare gas atoms

Using experimental set-ups involving single photon excitation of metastable rare gas atoms Rg(mp5(m + 1)s, J = 2, 0) by a pulsed frequency-doubled dye laser in conjunction with time-of-flight ion detection, we have investigated several even Rg(mp51/2 np′) autoionizing resonances of the heavier rare gas atoms Rg = Ne − Xe (m = 2–5) at experimental bandwidths of 0.06 cm−1 (Ne) and about 0.25 cm−1 (Ar, Kr, Xe). New level energies (quantum defects) and widths for these resonances have been derived by a Fano-type lineshape analysis, thus yielding reduced resonance widths. The experimental spectra and the resonance parameters are compared with theoretical calculations which are based on the configuration interaction Pauli–Fock approach, including core polarization. For Rg = Ne, good agreement between the measured and the calculated results is observed. For the heavier rare gases, significant deviations between some of the existing experimental data and the theoretical results are found, indicating the need for further studies.

Ab initio potentials for the S(3Pj)–rare gas dimers: Implementation for elastic and inelastic collisions and comparison with scattering potentials

The interaction potentials between the ground state S(3P) atom and rare gas atoms Rg (He, Ne, Ar, Kr, and Xe) in Π3 and Σ−3 states are calculated ab initio using an unrestricted CCSD(T) level of theory and extended correlation consistent basis sets augmented by bond functions. For NeS, the effects of extending the basis set, of a more accurate treatment of triple excitations within the coupled cluster method, and of the frozen core approximation are analyzed. The spin–orbit interaction is taken into account by the commonly used atomic model, whose validity is verified by the direct ab initio calculations of spin–orbit coupling matrix elements. The ab initio potentials are tested in the calculations of the absolute total scattering cross sections measured in molecular beams and compared with the potentials derived from the same data. This comparison, along with an analysis in terms of correlation formulas, proves the high accuracy of ab initio potentials and characterizes the sensitivity of scattering cross sections to the properties of interaction potentials. Both ab initio and scattering derived potentials are implemented in the study of inelastic fine structure transitions in S+Rg collisions. The relaxation rate constants are calculated and compared with those for O+Rg collisions.

Diatomic potential well depths from analyses of high-resolution electron energy spectra for autoionizing collision complexes

High-resolution energy spectra of electrons released in Penning ionization collisions of metastable rare gas atoms Rg*(ns) (Rg = He, Ne, Ar, Kr, Xe) with several open-shell and closed-shell atoms are analyzed to determine the well depth of the potential energy curve which describes the respective autoionizing collision complex. We thereby elucidate trends in the chemical interaction of Rg* with closed-shell target atoms A(ns2) and establish a basis for detailed comparison with the respective interactions involving open-shell, ground state alkali atoms A(ns). From electron energy spectra due to␣associative ionization (RgH+ formation) in Rg* + H(1s) collisions, we determine binding energies for the RgH+(1Σ) ground state potential (Rg = Ne, Ar, Kr, Xe) with uncertainties around 0.03 eV.

On theJ-dependence of the odd (ns?,J=0,1) autoionization resonances of the rare gases Ne, Ar, Kr, and Xe

Using resonant two-photon ionization of metastable rare gas atoms Rg(mp5(m + 1)s3P2,0) in a collimated beam by two transverse cw dye lasers, we have performed a comprehensive high resolution study of the odd Rg(ns′,J=0,1) autoionization resonances of all the heavier rare gases Rg=Ne, Ar, Kr and Xe. In this way, we have determined the variation of the nearlyn-independent reduced resonance widths Γr=n*3 · Γn (n*=effective quantum number) with atomic size and — for the first time — with the angular momentumJ for all Rg. The reduced widths for the Rg(ns′,J=1) resonances vary by about a factor of three, being lowest for Ne(371 cm−1) and largest for Kr(1185 cm−1). Compared toJ=1, the Rg(ns′,J=0) resonances are three times narrower for Ne, but up to two times broader for Ar, Kr, and Xe. The results of ab initio calculations for the widths are reported, in which the relevant transition matrix elements are carefully evaluated. Good overall agreement of the calculated widths with the experimental data is achieved. For Rg(ns′,J=0), the first orders′−d coupling amplitude provides a satisfactory description only for Rg=Ne, whereas for Rg=Ar−Xe, correlation effects are important. For Rg(ns′,J=1), the additional amplitudes due tos′−d exchange coupling and due tos′−s decay are found to be of varying importance with atomic number. Destructive interference between the different amplitudes and the influence of Rg(ns′,J=1)−Rg(nd′,J=1) coupling are responsible for the fact that the (ns′,J=1) resonances are narrower than the (ns′,J=0) resonances for Ar, Kr, and Xe.

Measurement of yields for electron emission from surfaces upon impact of laser-excited Ar*(4p) and Kr*(5p) atoms

A laser method for the determination of the electron emission coefficient gamma for the impact of slow laser-excited rare gas atoms Rg*(mp5(m+1)p 3D3) on surfaces is described. It is based on controlled partial depletion of metastable Rg*(mp5(m+1)s 3P2) atoms in a collimated beam by two-step laser photoionization in combination with saturated laser excitation of the metastable (3P2) atoms to the (3D3) level at the surface. The method simultaneously yields the coefficients gamma for the metastable Rg*(3P2) and the laser-excited Rg*(3D3) atoms. As an application, the authors report gamma values corresponding to the impact of Ar*(3P2, 3D3) and Kr*(3P2, 3D3) atoms on several gas-covered metal surfaces. The data show that the electron emission process involves Auger de-excitation and can be interpreted as Penning ionization of adsorbed molecules. Comparison of the gamma values for the four excited rare gas species shows a complex excitation energy dependence.

The state dependence of the interaction of metastable rare gas atoms Rg*(ms3 P 2,3 P 0) (Rg=Ne, Ar, Kr, Xe) with ground state sodium atoms

Using crossed beams of metastable rare gas atoms Rg*(ms3P2,3P0) (Rg=Ne, Ar, Kr, Xe) and ground state sodium atoms Na(3s2S1/2), we have measured the energy spectra of electrons released in the respective Penning ionization processes at thermal collision energies. For Rg*(3P2)+Na(3s), the spectra are quite similar for the different rare gases, both in width and shape; they reflect attractive interactions in the entrance channel with well depthsD* e [meV] decreasing slowly from Rg=Ne to Xe as follows: 676(18); 602(23); 565(26); 555(30). For Rg*(3P0)+Na(3s), the spectra vary strongly with the rare gas, indicating a change in the character of the interaction from van der Waals type attraction (Ne) to chemical binding for Kr and Xe with well depthsD* e [meV] of: 51(19); 107(25); 432(30); 530(50). These findings are explained through model calculations of the respective potential curves, in which the exchange and the spin orbit interaction in the excited rare gas and the molecular interaction between the two valences-electrons in terms of suitably chosen singlet and triplet potentials are taken into account. These calculations also explain qualitatively the experimental finding that the ratiosq2/q0 of the ionization cross sections for Rg*(3P2)+Na and Rg*(3P0)+Na vary strongly with the rare gas from Ne to Xe as follows: 15.8(3.2); 2.6(4); 1.4(2); 1.6(4).