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*(ms³ P ₂,³ P ₀) (Rg=Ne, Ar, Kr, Xe) and ground state sodium atoms Na(3s ² S _1/2), we have measured the energy spectra of electrons released in the respective Penning ionization processes at thermal collision energies. For Rg*(³ P ₂)+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*(³ P ₀)+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 ratiosq ₂/q ₀ of the ionization cross sections for Rg*(³ P ₂)+Na and Rg*(³ P ₀)+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).     

The interaction of metastable helium atoms with alkali atoms

Using crossed beams of ground state alkali atoms A (A = Li, Na, K, Rb, Cs) and metastable He(2³ S), He(2¹ S) atoms, we have measured the energy spectra of electrons resulting in the respective Penning ionization processes at: thermal collision energies. The data are interpreted to yield the well depthD _e ^* of the²Σ interaction potentials as follows: He(2³ S)+A:D _e ^* (A=Li)=868(20) meV;D _e ^* (Na)=740(25) meV;D _e ^* (K)=591(24) meV;D _e ^* (Rb)=546(18) meV;D _e ^* (Cs)=533(18) meV. He(2¹ S)+A:D _e ^* (Li)=330(17) meV;D _e ^* (Na)=277(24) meV;D _e ^* (K)=202(23) meV;D _e ^* (Rb)=219(18) meV;D _e ^* (Cs)=277(18) meV. The well depth for He(2³ S)+A(²Σ) is always close to 80% of the well depth for Li(2s)+A(X ¹Σ). The ionization cross sections for He(2¹ S)+A are about 3 to 4 times larger than those for He(2³ S)+A.     

Accurate comparison of Hel,NeI photoionization and He(23, 1 S), Ne(3s 3 P s,3 P 0) penning ionization of argon atoms and dimers

Using crossed beams and mass spectrometric ion detection, we have investigated the ionization of argon atoms and dimers in a skimmed supersonic beam by HeI (58.4 nm) and NeI (73.6, 74.4 nm) photons and by He(2^3,1 S) and state selected Ne(3s ³ P ₂,³ P ₀) metastable atoms. The cross section ratioq ₂₂/q ₁ (i.e. the cross sectionq ₂₂ for formation of Ar ₂ ⁺ ions from Ar₂ divided by the total ionization cross sectionq ₁ for Ar atoms), arbitrarily normalized to 1 for HeI impact, is found to vary weakly as follows: HeI/NeI/He(2^{3, 1} S)/Ne(³ P ₀)Ne(³ P ₂)=1/1.136(9)/0.893(4)/1.034(12)/0.985(9). The results are qualitatively interpreted using available information on the intermolecular potentials and the two different ionization processes. The observation thatq ₂₂/q ₁ is 5% larger for Ne(³ P ₀) than for Ne(³ P ₂) is attributed to anomalies in the respective branching ratios for formation of the Ar+(² P _3/2)/Ar⁺(² P _1/2) ion states in conjunction with differences in the stability of the formed Ar-Ar⁺(² P _3/2) and Ar-Ar⁺(² P _1/2) molecular ions.     

Collisions of metastable Ne*, He* atoms with ground-state He,Ne atoms studied by atomic beam and laser techniques

A crossed nozzle-beam experiment is used to investigate thermal energy collisions: Ne*(2p ⁵3s,³ P _{0, 2})+He(1s ²,¹ S ₀), almost purely elastic, and He*(1s2s,^{1, 3} S)+Ne(2p ⁶,¹ S ₀), in which inelastic excitation transfers occur. State and velocity selection of the scattered Ne* atoms is performed using a tunablecw dye laser frequency locked on a definite Zeeman component of the transition 1s ₅→2p ₆ (λ=614.3 nm) of²⁰Ne or²²Ne. In the purely elastic case, this technique allows the selection of one of the two final velocities, and then an unambiguous LAB-CM transformation. The differential cross section at 62 meV tallies on accords with a calculation using a single effective potential. In He* on Ne collisions, the main inelastic processes are endothermic excitation transfers from He*(2¹ S). Experimental results obtained at different energies (62, 95, 109, 124 meV) show that the transfers essentially result in levels 3s and 4d of Ne.     

Absorption and photoluminescence spectra of Tm3+-doped fluorophosphate glasses

We have carried out absorption and photoluminescence spectra of six newly prepared TmF₃ (2 M%) doped fluorophosphate glasses of composition: (NaPO₃)6BaF₂ZnF₂RF where R = Li, Na, K, (LiNa), (NaK) and (KLi). By evaluating the Judd—Ofelt intensity parameters for the measured absorption levels, the recorded photoluminescence spectra have been analysed to estimate the fluorescent radiative properties (A, A_T, β_R) and the stimulated emission cross-sections (σ^E_P) of the following luminescence transitions:¹G₄→³H₆ (λ=451 nm)¹D₂→³H₄ (λ=468 nm)³P₀→³F₂ (λ=482 nm)¹D₂→³H₅ (λ=492 nm)These have been determined in order to understand the alkali effects on the Tm³⁺-optical glasses studied.     

The interaction of metastable helium atoms with alkaline earth atoms: He*(23 S,21 S)+Mg,Ca, Sr and Ba

We have carried out experimental and theoretical studies of Penning ionization processes occurring in thermal energy collisions of state-selected metastable He*(2³ S) and He*(2¹ S) atoms with ground state alkaline earth atoms X(X=Mg, Ca, Sr, Ba). Penning ionization electron energy spectra for these eight systems, measured with a crossed-beam set-up perpendicular to the collision velocity at energy resolutions 40–70 meV, are reported; relative populations of the different ionic X ⁺ (ml) states are presented and well depths D*_e for the He*+X entrance channel potentials with uncertainties around 25 meV are derived from the electron spectra as follows: He*(2³ S)+Mg/Ca/Sr/Ba: 130/250/240/260 meV; He*(2¹ S) +Mg/Ca/Sr/Ba: 300/570/550/670 meV. The spectra show substantial differences for the three ionic states X ⁺(² S), X ⁺(² P) and X ⁺(² D) and reveal that transitions to a repulsive potential — attributed to He+X ⁺(² P)² Σ formation — are mainly involved for the X ⁺(² P) channel. Ab initio calculations of potential curves, autoionization widths, electron energy spectra and ionization cross sections are reported for the systems He*(2³ S)+Ca and He*(2¹ S)+Ca. The respective well depths D _e ^* are calculated to be 243(15) meV and 544(15) meV; the ionization cross sections at the experimental mean energy of 72 meV amount to 101 Ų and 201 Ų, respectively. Very good overall agreement with the experimental electron spectra is observed.     

The collision cross sections for the fine-structure mixing of caesium 6P levels induced by collisions with potassium atoms

The cross section for the fine-structure excitation transfer Cs(6P _1/2) → Cs(6P _3/2), induced by collisions with the ground state potassium atoms, has been measured by resonant Doppler-free two-photon spectroscopy. The population densities of caesium 6P _J (J=1/2, 3/2) levels were probed by thermionic detection of the collisionally ionized caesium atoms from the Cs(6P _J ) → Cs(10S _1/2) excitation channel. The cross section for the transfer process at the temperatureT=503 K has been found to be σ(1/2 → 3/2)=45 Ų ± 20%. The result is compared with previously published experimental cross sections for fine-structure transfer in resonance states of other alkali elements perturbed by potassium and a thoeretical value of the Li(2P _J )-K system calculated in a simple approach.     

Thermal energy collisions between metastable Ne⋆(2p 53s,3 P 0, 2) and H2(X,1Σ g + )

A crossed beam experiment is used to investigate the Ne*(2p ⁵ 3s,³ P _{0, 2}) ? H₂(¹Σ _g ⁺ ) collision at thermal energy (67 meV). The H₂ beam is supersonic, the Ne* beam is thermal. Different collision processes have been analyzed separately by means of a double chopping technique combined with a time of flight measurement. Ions produced by Penning effect and chemi-ionization have been separated from scattered metastable atoms by an accelerating electric field small enough to preserve a reasonable angular resolution: δ?(ions)=±5.5°, δ?(Ne*)=±1°, which allows a determination of differential cross sections. The attenuation method, combined with an absolute measurement of the total H₂ flux, has been used to measure the total cross section: σ _t =940±220a ₀ ² . Differential cross sections have been obtained, in arbitrary but unique unit, for the following processes: (1) elastic collisions, for a mixture (1:3) of para- and ortho-hydrogen; (2) rotationally inelastic collisions:J=0→2; (3) Penning ionization resulting into H ₂ ⁺ ions; (4) chemiionization yielding NeH⁺ ions.     

Laser photoionization of metastable Xe* (J=0) atoms in the range 462–430 nm

Absolute cross sections for photoionization of metastable Xe* (5p ⁵ 6s′³ P ₀) atoms are presented over the wavelength range 462–430 nm (photoelectron energy ε=0–200 meV), including the 7p′[3/2]₁ and 7p′[1/2]₁ autoionization resonances. In addition the angular distribution of the photoelectrons is reported across the 7p′ resonances (ε=90–180 meV). The experimental data are compared with Hartree-Fock calculations of the photoionization process.     

Absolute detection of metastable rare gas atoms by a cw laser photoionization method

A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient γ for impact of the respective metastable atom on a conducting surface. γ is reliably obtained by a cw two-photon ionization — depletion technique: the reduction ΔI _S in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current ΔI _P (γ = ΔI _S/ΔI _P). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s ³ P ₂), Ar* (4s ³ P ₂), and Kr* (5s ³ P ₂) atoms, and corresponding results for γ, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of γ (≈0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (2³ S, 2¹ S) beam), strong variations in γ, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.     

Symmetry constraints in energy transfer between state-selected Ar*(3P2, 3P0) metastable atoms and ground state H atoms

The excitation-transfer reaction in thermal energy collisions of state-selected metastable Ar*(³P₂) and Ar*(³P₀) atoms with ground state H atoms, giving excited H*(n = 2) atoms, has been studied with the stationary afterglow technique. The rate constant for the excitation of H atoms by Ar*(³P₂) has been found to be more than one order of magnitude larger than in excitation by Ar*(³P₀). This difference in the reactivity of two metastable species is explained to be a consequence of the attractive nature of the D(²II) and E(²Σ⁺) potentials that develop from the Ar*(³P₂)+H entrance channel and which give curve crossing with the B(²II) and C(²Σ⁺ potentials, respectively, leading to the Ar+H*(n=2) exit channel, whereas only a repulsive ⁴II (Ω=$\text{1}\text{2}$) potential develops from the Ar*(³P₀+H entrance channel.     

Collision cross sections for excitation energy transfer in Na* (3P 1/2)+K(4S 1/2)⇔Na*(3P 3/2)+K(4S 1/2) processes

The cross sections for the excitation energy transfer between the 3² P _J states of sodium atoms by collisions with ground-state potassium atoms have been measured by resonant Doppler-free two-photon spectroscopy, where the population densities of directly pumped and collisionally excited Na(3P _J )(J=1/2, 3/2) levels were probed by counter-propagating Na(3P _J ) → Na(4D _{3/2, 5/2}) excitation and detected with the thermionic diode. Cross sections of σ(3P _1/2 → 3P _3/2)=190 Ų±20% and σ(3P _3/2 → 3P _1/2)=100 Ų±20% were found. The theoretical calculations taking into account the long-range interaction termsR ^?6,R ^?8 andR ^?10 yield a value σ(3P _1/2 → 3P _3/2)=165 Ų. On the basis of these long-range interaction potentials the differential cross section has been calculated and compared with recently published experimental data. Very good agreement between the theoretical and experimental data was found.     

Angle-dependent electron energy spectra resulting from autoionizing Ne⋆ (3s 3 P 2) +H(12 S) collisions

Experimental angle-dependent electron energy spectra for the autoionization complex Ne*(3s ³ P ₂)+H(1² S), leading to Penning and associative ionization, are reported. The data, measured at thermal collision energies (ē _rel～51 meV), clearly show an angular variation of the spectral shape, indicating that electrons with angular momentuml>0 participate in the autoionization process. The corresponding non-isotropic electron emission leads to a correlation between the impact parameter-dependent heavy-particle dynamics and the observed electron energy spectrum at a certain detection angle. The experimental results are qualitatively discussed in connection with previous work on the system He*(2³ S)+H(1² S). Furthermore, we present quantum mechanical model-calculations for the electron energy spectrum on the basis of available potential data.     

Electron transfer from laser excited rydberg atoms to molecules. Absolute rate constants at low and intermediate principal quantum numbers

Using mass spectrometric detection of positive and negative ions, we have investigated ionizing reactions of Ne(ns,nd) Rydberg atoms, efficiently excited by resonant two-photon excitation of metastable Ne(3s ³ P ₂) atoms, with electron attaching moleculesBC (BC=SF₆, CCl₄, CS₂, O₂) at thermal collision energies. Absolute rate constants have been determined in the range of low and intermediate principal quantum numbersn(5≦n?30) by utilizing the photoionization signal caused by room temperature black-body radiation and the loss of Ne(3s ³ P ₂) atoms, associated with the laser excitation. Substantially differentn-dependences of the electron transfer cross section have been found for the larger molecules (BC = SF₆, CCl₄) and the smaller molecules (BC = CS₂, O₂). Simple model calculations have been performed to gain new insight into the dynamics of the electron transfer process; forBC = SF₆, our results at lown(5 ≦n ≦ 10) suggest that internal energy conversion in the Coulombic complex Ne⁺ — SF ₆ ^? is important for the formation of the detected ions.     

Kinetics of photo-induced dissociation of Na clusters deposited on Mica

Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energyE _kin of the photodesorbed atoms at the surface temperatureT _S =300 K was found to beE _kin=18±5 meV, independent of laser irradiance (3 µJ/cm²...20 mJ/cm²) and wavelength (450 nm, 505 nm, 658 nm). With increasing orientation angle between detection axis and surface normal (0°≦Θ≦90°)E _kin was observed to decrease slightly, while it was nearly independent of surface temperature betweenT _S =30 K andT _S =300 K. Also, with increasing radius of the Na clusters the desorbing Na atoms slowed down. The angular distribution of the Na atoms was of cos²-type with respect to the surface normal. These observations suggest that laser-induced desorption of Na from Na clusters bound to mica surfaces involves an initial rate-limiting step of direct surface plasmon excitation followed by a final step of delayed thermal desorption.     

Fine structure excitation transfer between the potassium 42P states induced by collisions with caesium atoms

Applying diode-laser resonant fluorescence method, the cross sections for the excitation energy transfer of the collisional process K*(4² P _1/2)+Cs(6² S _1/2)?K*(4² P _3/2)+Cs(6² S _1/2) have been measured. The values we have obtained are σ(1/2→3/2)=77 Ų and σ(3/2→1/2)=48 Ų. These results complete the sequence of data for the fine-structure mixing of the first-resonance states of alkali atoms colliding with the ground-state caesium atoms.     

The low energy collision between metastable Ne*(3 P 0,2) and He(1 S 0): ab initio potentials,differential cross sections and polarization effects

Potential energies for molecular states dissociating into Ne*(¹ P ₁,³ P _0,1,2) + He(¹ S ₀) have been calculated ab initio within the distance range 4–100a ₀. The SCF energy (without spin-orbit interaction) is optimized on the lowest³Σ state. After CI, the four Λ-states (^1,3Σ,^1,3Π)are obtained. They dissociate into Ne*(^1,3 P) + He(¹ S). All of them are repulsive atR ? 8a ₀, they exhibit shallow wells around 12a ₀ and have a correct asymptotic behaviour (～ -R ^?6). The spin-orbit interaction is introduced, using the Cohen-Schneider scheme, and adiabatic Ω-potentials are derived. The collision at low energy (E ≦ 124 meV) is described in the frame of a fragment-state basis. By means of a deflation procedure, it is shown that states dissociating into Ne*(¹ P ₁) + He can be eliminated, which lead to a 9 × 9 interaction matrix dynamically equivalent to the original 12 × 12 matrix, in the subspace of interest. Collision channels are defined by angular momenta,J (total),j (of Ne*) andl (of the relative motion). Scattering radial equations are solved by the algorithm of Gordon and theS matrix is derived. Two sets of physically meaningful scattering amplitudes (and differential cross sections) are constructed, referred to the incident axis or to the initial and final directions of the internuclear axis. Polarization effects are discussed. The case of a quantization axis perpendicular to the collision plane is also mentioned.     

Formation of negatively-charged cluster ions in thermal energy collisions with state-selected rydberg atoms

Using crossed atomic, molecular cluster, and cw laser beams in conjunction with mass spectrometric ion detection, we have obtained for the first time results for electron transfer fromstate-selected Rydberg atoms to molecular clusters. We report negative ion mass spectra for (CO₂) _k ^? (4≦k≦25) and (O₂) _k ^? (1≦k≦13) cluster ions, resulting from collisions of Ar** (nd) Rydberg atoms (12≦n≦40) with (CO₂) _m and (O₂) _m clusters at relative velocities around 830 m/s, and, for comparison, positive ion mass spectra due to Ne(3s ³ P _{2, 0}) Penning ionization. For both CO ₂ ^? and O₂-clusters, the negative and the positive ion mass spectra are very different. For (CO₂) _k ^Emphasis>/? cluster ions, the mass spectra show distinct variations with principal quantum number of the Rydberg atom, corresponding to differentn-dependences of the effective rate constant for selected cluster ions, as measured relative to the knownn-dependence for SF ₆ ^? formation in collisions with SF₆. For (O₂) _k ^? cluster ions, on the other hand, the mass spectra are almost independent ofn with ion intensities, which clearly reflect their thermochemical stabilities (O ₄ ^? as dominant species).     

Experimental and theoretical cross sections for photoionization of metastable Xe*. (6s 3 P 2, 3 P 0) atoms near threshold

Using high resolution laser photoelectron spectrometry we have determined absolute cross sections σJ ₀ J ₁ and the electron angular distribution parameter for one photon ionization of metastable Xe^*(6s ³ P _J0, J ₀ = 2, 0) atoms to the resolved Xe₊ (² P _J1, J ₁ = 3/2, 1/2) ion states at several wavelengths near threshold. For comparison with the present and future experimental data we have calculated partial cross sections and ß-parameters for photoionization of Xe^*(6s ³ P _J0, J ₀ = 2, 0) and of the analogous alkali atom Cs(6s) over the photoelectron energy range (0–5) eV. We have used both a term-dependent Pauli-Fock (PF) approach and a configuration interaction method involving Pauli-Fock atomic orbitals (CIPF). Through the PF method we include relativistic effects on the atomic orbitals; the CIPF method was designed to take into account the important electron correlation effects which are found to be essential for obtaining good agreement between the theoretical and experimental results.     

Kinetic study of electronically excited silicon atoms,Si(3p2(1S0)), by time-resolved attenuation of atomic resonance radiation

The collisional behaviour of electronically excited silicon atoms in the 3p²(¹S₀) state, 1.909 eV above the 3p²(³P₀) ground state, is investigated by time-resolved attenuation of atomic resonance radiation at λ = 390.53 nm (4s(¹P^o₁)←3p² (¹S₀)). The optically metastable Si(3¹S₀) atoms were generated by the repetitive pulsed irradiation of SiCl₄ and their decay monitored in the presence of added gases. Absolute quenching rate constants (k_Q, cm³ molecule^?1 s^?1, 300 K) are reported for the following collision partners: He (?1.3 × 10^?15), SiCl₄ ((9.1 ± 1.4) × 10^?11), O₂ ((1.5 ± 0.2) × 10^?11) and N₂O ((4.3 ± 0.4) × 10^?11). The results for O₂ and N₂O are compared with analogous data reported hitherto for Si(3p²(³P_J)) and with those for the other np²(¹S₀) states of the group IV atoms C, Ge, Sn and Pb. The rate data for the silicon atoms are considered in terms of the nature of the potential surfaces arising from symmetry arguments based on the weak spin orbit coupling approximation.