Polarization transfer in then 2 P-excitation of alkali atoms by longitudinally polarized electrons

In (ns — np)-excitation of alkali atoms by polarized electrons some of the spin orientation of the primary electron beam is transfered to the excited atoms because of exchange collisions. In the present work the polarization transfer to Na, K, Rb, and Cs respectively is studied by looking for the circular light polarization of (np — ns)-decay radiation emitted in forward direction following impact excitation by a beam of longitudinally polarized electrons. Maximum polarization transfers are observed at collision energies with values about 1.5 times that ofnp-thresholds. The transfer increases with increasing atomic number. In case of (6s — 6p)-excitation of cesium 45% of primary electron spin polarization is transfered to the atoms at maximum. The agreement with available close coupling data is satisfactory.     

Light absorption during alkali atom-noble gas atom interactions at thermal energies: a quantum dynamics treatment

The absorption of light during atomic collisions is treated by coupling electronic excitations, treated quantum mechanically, to the motion of the nuclei described within a short de Broglie wavelength approximation, using a density matrix approach. The time-dependent electric dipole of the system provides the intensity of light absorption in a treatment valid for transient phenomena, and the Fourier transform of time-dependent intensities gives absorption spectra that are very sensitive to details of the interaction potentials of excited diatomic states. We consider several sets of atomic expansion functions and atomic pseudopotentials, and introduce new parametrizations to provide light absorption spectra in good agreement with experimentally measured and ab initio calculated spectra. To this end, we describe the electronic excitation of the valence electron of excited alkali atoms in collisions with noble gas atoms with a procedure that combines l-dependent atomic pseudopotentials, including two- and three-body polarization terms, and a treatment of the dynamics based on the eikonal approximation of atomic motions and time-dependent molecular orbitals. We present results for the collision induced absorption spectra in the Li-He system at 720 K, which display both atomic and molecular transition intensities.     

Effects of angular momentum recoupling on depolarization spectra in alkali-rare gas optical half collisions

The depolarization mechanisms of excited alkali atoms interacting with ground state rare gas atoms are investigated using the method of far wing spectroscopy of collision pairs under single collision conditions. From a semiclassical theory explicite expressions for the spectra of alkali multipole components σ _k ^(?) (ω _L ) are derived assuming quasistatic excitation at a localized internuclear separationR _L(ω _L ) related to the laserfrequency ω _L as well as realistic models for the half collision following excitation. The collision models are characterized by different Hund's coupling regions, where excitation takes place and which are traversed on the collision path. Due to selection rules for excitation of populations and coherences and for support of multipoles the σ _k ^(?) (ω _L ) are shown to depend strongly on the collision model. From the spectra thus a labelling of the initial molecular states and mapping of the change in coupling case is possible. Estimations of the contributions of the various angular momentum recoupling effects are given.     

Spectroscopy on Rydberg states of sodium atoms on the surface of helium nanodroplets

One- and two-photon excitation spectra of sodium atoms on the surface of helium droplets are reported. The spectra are recorded by monitoring the photoionization yield of desorbed atoms as function of excitation frequency. The excitation spectra involving states with principal quantum number up to n = 6 can be reproduced by a pseudodiatomic model where the helium droplet is treated as a single atom. For the lowest excited states of sodium, the effective interaction potentials for this system can be approximated by the sum of NaHe pair potentials. For the higher excited states, the interaction of the sodium valence electron with the helium induces significant configuration mixing, leading to a failure of this approach. For these states, effective interaction potentials based on a perturbative treatment of the interactions between the valence electron, the alkali positive core, and the helium, as described in detail in the accompanying publication, yield excellent agreement with experiment.     

Spectroscopy of alkali atoms (Li,Na, K) attached to large helium clusters

Large liquid helium clusters (He_n, n ≈ 10⁴) produced in a supersonic jet are doped with alkali atoms (Li, Na, K) and characterized by means of laser induced fluorescence. Each cluster contains, on average, less than one dopant atom. Both excitation and emission spectra have been recorded. The observed excitation spectra are analyzed, calculating the transitions within an approach based on the hypothesis that the chromophores are trapped in a dimple on the cluster’s surface as predicted by the theoretical calculations of Ancilotto et al. [9]. The results of the model calculations are in good qualitative agreement with the experimental findings. In spite of the very weak binding energy (a few cm^?1), some of the excited atoms remain bound to the surface, provided the excitation occurs at frequencies not too far from the alkali’s gas phase absorptions. These bound-bound excitations produce very broad, red shifted emission spectra. At other, blue shifted frequencies, the excited atoms desorb from the cluster’s surface, giving rise to unshifted, free atom, emission spectra. The heavier alkali metals (Na, K) show, compared to the calculations, an additional broadening which is attributed to surface excitations on the helium droplet.     

Polarization and velocity dependence of associative ionization in Na(3p)+Na(3p) collisions

We have excited Na atoms of two counterrunning thermal beams by means of linearly polarized laser light and have investigated associative ionization processes. To this end we measured the total ionization signal as a function of the angle ? between light polarization and the relative collision velocity. Velocities of the excited atoms were selected by exploiting the Doppler effect. We found an increasing polarization dependence at increasing collision velocities. At all velocities the preparation of the collision partners in the |M _j |=1/2 sublevel of the Na² P _3/2 state is most efficient in producing ionization.     

Collisional transfer of population and orientation in NaK

Collisional satellite lines with |ΔJ| ≤ 58 have been identified in recent polarization spectroscopy V-type optical-optical double resonance (OODR) excitation spectra of the Rb(2) molecule [H. Salami et al., Phys. Rev. A 80, 022515 (2009)]. Observation of these satellite lines clearly requires a transfer of population from the rotational level directly excited by the pump laser to a neighboring level in a collision of the molecule with an atomic perturber. However to be observed in polarization spectroscopy, the collision must also partially preserve the angular momentum orientation, which is at least somewhat surprising given the extremely large values of ΔJ that were observed. In the present work, we used the two-step OODR fluorescence and polarization spectroscopy techniques to obtain quantitative information on the transfer of population and orientation in rotationally inelastic collisions of the NaK molecules prepared in the 2(A)(1)Σ(+)(v' = 16, J' = 30) rovibrational level with argon and potassium perturbers. A rate equation model was used to study the intensities of these satellite lines as a function of argon pressure and heat pipe oven temperature, in order to separate the collisional effects of argon and potassium atoms. Using a fit of this rate equation model to the data, we found that collisions of NaK molecules with potassium atoms are more likely to transfer population and destroy orientation than collisions with argon atoms. Collisions with argon atoms show a strong propensity for population transfer with ΔJ = even. Conversely, collisions with potassium atoms do not show this ΔJ = even propensity, but do show a propensity for ΔJ = positive compared to ΔJ = negative, for this particular initial state. The density matrix equations of motion have also been solved numerically in order to test the approximations used in the rate equation model and to calculate fluorescence and polarization spectroscopy line shapes. In addition, we have measured rate coefficients for broadening of NaK 3(1)Π ← 2(A)(1)Σ(+)spectral lines due to collisions with argon and potassium atoms. Additional broadening, due to velocity changes occurring in rotationally inelastic collisions, has also been observed.     

Classical path study of excitation of a collision system by ultrashort laser pulses

The interaction of an ultrashort laser pulse with a two-level collision system is investigated. An increased overall photon efficiency of ultrashort pulses is confirmed in many cases by the calculation. The dependence of the excitation process on duration, intensity and shape of the laser pulse is studied on the basis of calculations with the classical-path method. Realistic potentials modeling theXΣ → AΠ transition in Na-Ar are employed. Numerical trajectories were generated on these potentials that either switch at the Condon points from the ground state to the excited state or are propagated on the average potential. The role of the nonresonant excitation of free atoms is discussed in detail and found to be an important factor accompanying collision pair excitation. However a drastic reduction of this effect occurs when considering the propagation of the pulse through the medium before collision. Parallel computation of related collisions leads to a calculational procedure by which the necessary average over the pulse onset time is performed efficiently.     

The role of intermediate state polarization in determination of vector properties of the ground state using multiphoton excitation

We present a general theory for calculating the vector and geometrical properties of the multiphoton excitation of an arbitrary atomic or molecular system. The results are applied to study the influence of the polarization of the two-photon excited state, which is usually neglected, on the intensity of (2 + 1) resonant multiphoton ionization in atoms. Two examples of specific atomic systems of practical importance are presented: oxygen and chlorine. For some cases, the effect of the polarization of the pre-ionized state can be significant and must be properly treated.     

Inelastic electronic excitation and electron transfer processes in collisions between Mg(31S0) atoms and K+(1S0) ions studied by crossed beams in the 0.10-3.80-keV energy range

Inelastic and charge-transfer excitation processes in collisions between ground-state neutral Mg atoms and K+ ions have been studied by means of a crossed molecular-beam technique. Decay fluorescent emissions from Mg(3 1P1),Mg(4 3S1), and Mg(3s(1)3d(1), 3(3)D3,2,1) as well as the phosphorescent emission due to Mg(3 3P1) have been observed from excited Mg atoms and the charge-transfer emission decays from K(4 2P 3/2,1/2), K(5 2P 3/2, 1/2), K(6 2S 1/2), and K(4 2D 5/2, 3/2) for excited K atoms. The corresponding absolute cross-sections values versus collision energy functions were determined in the 0.10-3.80 keV laboratory energy range. In order to interpret the experimental results, accurate ab initio full configuration-interaction calculations using pseudopotentials have been performed for the (Mg-K)+ system, giving a manifold of adiabatic singlet and triplet potential-energy curves correlating with the different collision channels, which allow a qualitative interpretation of the emission excitation functions measured for the different processes studied. A comparative study with other Mg-alkali ion systems previously studied is also included.     

Impact excitation of 42 P-potassium by polarized electrons

The present work investigates the 4² P impact excitation of potassium by longitudinally polarized electrons. Exchange scattering causes a polarization transfer from the primary electron beam to the ensemble of excited atoms which results in a non zero circular polarization of the fluorescence light emitted in foreward direction. The experimental values of light polarization agree well with values calculated using close coupling data of Moores.     

A molecular-beam study of the collision dynamics of methane and ethane upon a graphitic monolayer on Pt(111)

Utilizing a supersonic molecular-beam scattering technique, the angular intensity distributions of alkane molecules (CH4 and C2H6) have been measured, which are scattered from a chemically inert and highly oriented monolayer graphite (MG) on Pt(111). A MG which covers the Pt(111) surface with a full monolayer is found to induce a large energy loss of alkanes during collision with the surface by phonon creation due to the large mass ratio of an alkane molecule with respect to MG. Based on the classical cube model, only applicable to the molecules without internal mode excitation, the effective masses of MG of 76 (six atoms of carbon) and Pt(111) of 585 (three atoms of platinum) are determined from rare-gas atom scattering data. Despite the difference in the degree of freedom between CH4 and rare-gas atoms, CH4 scattering is found to be well described by the simple hard-cube model as a result of the high symmetry of the CH4 structure. With the recently developed ellipsoid-washboard model, an extension of the hard-cube model to include some internal mode excitation of impinging molecules in addition to the surface corrugation, it is found that unlike CH4 the cartwheel rotation mode of C2H6 is significantly excited during collision, while the helicopter mode excitation is negligible on a flat MG surface.     

Spectroscopy of laser-excited indium,gallium and indium–gallium systems

The spectroscopy of dense Ga, In and In–Ga vapours is studied via resonant pulsed laser excitation at 403.4 nm (Ga) and 410.3 nm (In). Besides some known atomic Rydberg levels emission, satellite wings on the blue side of the fundamental transitions show up both in the homonuclear and heteronuclear vapours. They are due to the presence of excited homonuclear dimers that form in the collision between excited and ground-state atoms. The formation of heteronuclear dimers is inferred by the time-resolved analysis of some atomic fluorescences.     

Laser control of product electronic state: desorption from alkali halides

We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin-orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the yield of excited state chorine atoms, Cl(2P(1/2)), can be made to vary from near 0 to 80% for KCl and from near 0 to 50% for NaCl relative to the total yield of Cl atoms. We describe the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models.     

(2+1) laser-induced fluorescence of spin-polarized hydrogen atoms

We report the measurement of the spin polarization of hydrogen (SPH) atoms by (2+1) laser-induced fluorescence, produced via the photodissociation of thermal HBr molecules with circularly polarized 193 nm light. This scheme, which involves two-photon laser excitation at 205 nm and fluorescence at 656 nm, offers an experimentally simpler polarization-detection method than the previously reported vacuum ultraviolet detection scheme, allowing the detection of SPH atoms to be performed more straightforwardly, from the photodissociation of a wide range of molecules and from a variety of collision experiments.     

Perturbation method to calculate the interaction potentials and electronic excitation spectra of atoms in He nanodroplets

A method is proposed for the calculation of potential energy curves and related electronic excitation spectra of dopant atoms captured in/on He nanodroplets and is applied to alkali metal atoms. The method requires knowledge of the droplet density distribution at equilibrium (here calculated within a bosonic-He density functional approach) and of a set of valence electron orbitals of the bare dopant atom (here calculated by numeric solution of the Schr?dinger equation in a suitably parametrized model potential). The electron-helium interaction is added as a perturbation, and potential energy curves are obtained by numeric diagonalization of the resulting Hamiltonian as a function of an effective coordinate z(A) (here the distance between the dopant atom and center of mass of the droplet, resulting in a pseudodiatomic potential). Excitation spectra are calculated for Na in the companion paper as the Franck-Condon factors between the v = 0 vibrational state in the ground electronic state and excited states of the pseudodiatomic molecule. They agree well with available experimental data, even for highly excited states where a more traditional approach fails.     

Penning and associative ionizations in collisions of laser-excited Na atoms with K atoms

Collisional ionization has been observed in the collision of electronically excited sodium atoms with potassium atoms. By means of the crossed-beam and stepwise laser excitation technique and time-of-flight measurements, the folowing reactions have been investigated: Penning ionization, Na^* + K → Na + K⁺ + e^?, and associative ionization, Na^* + K → NaK⁺ + e^?.     

Doubly-excited states for the Na2 molecule: application to the dynamics of the associative ionization reaction

In a recent paper, we have performed accurate model potential calculations for the adiabatic excited states of the Na₂ molecule. In view of dynamical calculations we propose a method to extract doubly excited potential curves, singly excited Rydberg series, and electronic couplings. In contrast to the previous work, the diabatic curves are correctly matching the adiabatic curves at large internuclear distances, and the electronic interactions do vanish in the asymptotic region. The adequacy of scaling laws to extract reduced parameters relevant to MQDT treatment of molecular autoionization is discussed. These parameters are used to calculate cross-sections for the associative ionization reaction at thermal collision energies when fine structure splitting can be neglected. In the framework of a simple model for the treatment of the long range dynamical couplings, we show the strong influence of the initial preparation of the atoms, and of long range population transfers. In contrast to the usual interpretation of experimental data, we predict that at least four doubly excited states may contribute efficiently to the process. Nevertheless, we may reproduce the marked variation of the observed ion signal as a function of the polarization of the exciting light. This work is a step towards a full treatment of the associative ionization between two cold atoms.     

Significant nonlinear‐optical switching capacity in atomic clusters built from silicon and lithium: A combined ab initio and density functional study

Starting from a hypothetical but fundamental charge/discharge sequence, the topic of nonlinear optical switching in atomic clusters built from silicon and alkali metals is opened up. The outcomes presented in this work, obtained with ab initio methods of exceptional predictive capabilities, offer strong evidences that sizable hyperpolarizability contrasts between neutral and charged alkali metal doped cluster forms might be simultaneously accomplished. The observed switching procedure involves redox polyatomic clusters formed by Si atoms. These centers function as electron acceptors at the ground state and as electron donors at the excited states facilitating low energy charge transfer transitions upon electronic excitation. © 2014 Wiley Periodicals, Inc.