Light-induced ejection of alkali atoms in polysiloxane coated cells

Summary An intense laser-induced fluorescence of sodium and of other alkali atoms (K, Rb) at room and lower temperatures has been observed in polysiloxanecoated cells, producing a vapor density of the metal which is much higher than that corresponding to the thermal equilibrium between the vapor and the condensed phase. This enhancement is attributed to the light-induced ejection of atoms absorbed by the polysiloxane coating. The atomic density of the vapor can be manipulated by changing either the laser power or frequency. The dependence of the atomic density on these and other parameters as well as its time-dependent behavior are studied experimentally. A tentative interpretation of the phenomenon is discussed in terms of the properties of solutions of the alkali metals in several solvents.     

Magnetic resonance frequency shifts upon spin-exchange collisions of alkali atoms

Magnetic resonance frequency shifts caused by spin-exchange collisions between ⁸⁷Rb and ¹³³Cs atoms are calculated, and temperature dependences of magnetic resonance frequency shifts are drawn for two hyperfine states (F = 1, 2) of ⁸⁷Rb atoms. The obtained dependences are compared with experimental data.     

The effect of spin-orbit coupling upon elastic collisions between alkali atoms

Within the framework of helicity formalism we discuss the scattering between alkali atoms at thermal energies, taking account of the spin orbit interaction. Polarised cross sections show some small spin effect, unpolarised cross sections remain almost unchanged.     

n-changing collisions of Rydberg atoms with ground-state atoms

The inelastic transitions between hydrogenic energy levels of a Rydberg atom induced in collisions with ground-state atoms are considered. The corresponding transition probability and cross section are calculated using the Fermi pseudopotential to describe the atom-Rydberg-atom interaction and a quasiclassical expression for the density distribution of the Rydberg electron.     

Fragmentation of fast positive and negative C ions in collisions with rare gas atoms

The fragmentation of anions and cations resulting from 50 keV collisions with rare gas targets is studied. Positive ion fragment patterns are recorded, and dramatic changes in these patterns are observed as a function of target atom number. The fragment pattern dependence on the target atom size is investigated within a simple model, normally used for stopping power calculations. Fair agreement is obtained between calculated and experimental spectra. From these comparisons we conclude that the range of the screened atomic potentials, as e.g., the Thomas-Fermi potential, is an essential parameter in the collisional induced fragmentation process. Received: 13 February 1998 / Revised: 27 October 1998 / Accepted: 29 October 1998     

On the interaction of excited alkali atoms with rare gas targets in scattering processes

A model potential calculation has been applied to evaluate scattering experiments for Na and K in the groundstate and the resonance state interacting with Ar. The model potential has only two free parameters which are determined by a best fit of the interatomic potentials to experimental results. Satisfactory agreement between calculated and experimental results is found for the differential cross sections in the groundstate and the excited state, for satellites in theK(4P),K(5P) and Na(3P) line profile, for the van der Waals constantsC ⁽⁶⁾ andC ⁽⁸⁾, the alkali ion-rare gas interaction and the vibrational energy levels of the Na-Ar molecule. As maior advantages we point out that for a given pair of atoms all these calculated data are given with one single pair of values for the free parameters and that with this set also the interatomic potentials for the higher alkali states (specifically up to 5f for Na and K) are obtained.     

Experimental studies of two-photon absorption of sodium perturbed by collisions with noble gas atoms

This study was performed with sodium vapor at about 300°C perturbed by noble gases at densities between 0.5 x 10¹⁹ and 6 x 10¹⁹ atoms/cm³. We have measured the rate of two-photon absorption for step-wise processes, which occur if the exciting frequency is resonant with one of the transition frequencies ω_ri or ω_fr with i, r, and f referring, respectively, to the initial, the relay and the final state. By varying the exciting frequency, we obtain collision profiles which may be difficult to obtain by other methods. The shifts and widths for the transitions 3S-3P_{$\text{3}\text{2}$}, 3S-3P_{$\text{1}\text{2}$}, and 3P_{$\text{3}\text{2}$}-5S have been measured for several noble gases. The observed variation of the profiles with perturber gas pressure suggests possible quenching of the relay state by noble gas molecules present in the mixture.     

Inelastic electron collisions with Rydberg atoms

The standard classical method of computer simulation is used for evaluation of the inelastic cross section in electron collisions with a highly excited (Rydberg) atom. In the course of collision, the incident and bound electrons move along classical trajectories in the Coulomb field of the nucleus, and the scattering parameters are averaged over many initial conditions. The reduced ionization cross section of a Rydberg atom by electron impact approximately corresponds to that of atoms in the ground states with valence s-electrons and coincides with the results of the previous Monte Carlo calculations. The cross section of an atom transition between Rydberg atom states as a result of electron impact is used for finding the stepwise ionization rate constant of atoms in collisions with electrons or the rate constant of three-body electron-ion recombination in a dense ionized gas because these processes are determined by kinetics of highly excited atom states. Surprisingly, the low-temperature limit of electron temperatures is realized when the electron thermal energy is lower than the atom ionization potential by about three orders of magnitude, as follows from the kinetics of excited atom states. The article is published in the original.     

Resonant control of elastic collisions in an optically trapped fermi gas of atoms

We have loaded an ultracold gas of fermionic atoms into a far-off resonance optical dipole trap and precisely controlled the spin composition of the trapped gas. We have measured a magnetic-field Feshbach resonance between atoms in the two lowest energy spin states, /9/2,-9/2> and /9/2,-7/2>. The resonance peaks at a magnetic field of 201.5+/-1.4 G and has a width of 8.0+/-1.1 G. Using this resonance, we have changed the elastic collision cross section in the gas by nearly 3 orders of magnitude.     

Total cross section prediction of the collisions of positrons and electrons with alkali atoms using Gradient Tree Boosting

A relatively new computational technique, namely gradient tree boosting (GTB), is presented for modeling the total cross sections of the scattering of positrons and electrons by alkali atoms in the low and intermediate energy regions. The calculations have been performed in the framework of gradient tree boosting (GTB). The GTB has been running based on the experimental data of the total collisional cross sections to produce the total cross sections for each alkali atom as a function of the incident energy of the projectile as well as the atomic number and the static dipole polarizability of the atom. Moreover our GTB model is used to predict the experimental data for total collisional cross sections that are not used in the training session. The calculated and predicted total collisional cross sections are compared with the experimental data. We find that the GTB technique shows a good match to the experimental data. To our knowledge, this is the first application of the GTB technique to the data of positron and electron collisions with alkali atoms at low and intermediate energies.     

Polarizability of alkali atoms

The scalar and tensor components of static polarizability of alkali atoms Na, K, and Rb were calculated. The results were compared with available theoretical and experimental data.     

Frequency of collisions between electrons and gas and vapor atoms and molecules

A resonant method based on the phenomenon of electron cyclotron resonance is used to determine the frequency of collisions between electrons and He, Ar atoms and H₂S, SO₂, H₂O, and CsCl molecules. Electron distribution over energy is measured under experimental conditions and the mean electron energy determined therefrom. Existing techniques of collision frequency measurement and calculation are discussed. Experimental dependences of reduced collision frequency on mean electron energy are obtained and the parameter E/N is compared to values presented in the literature.     

Transfer and ionization in collisions of positrons with atoms

Kinematic singularities in certain few-body reactions may provide special features that are both conceptually simple and observable. Particle transfer is such a reaction. At high velocities, it is characterized by singularities that are manifested by peaks and ridges in differential cross sections. For electrons captured by protons, some of these singular features have been observed. For capture by positrons, i.e. positronium formation, new features are predicted.     

Protonium formation in collisions of antiprotons with hydrogen atoms

The formation of an antiprotonic hydrogen atom $\{ p\bar p\} $ (protonium) in the ground state and in excited states (nlm) in collisions of 1-to 250-keV antiprotons with hydrogen atoms is considered theoretically. Partial cross sections (both differential and integrated ones) for this reaction are calculated for various values of n, l, and m up to n～100; cross sections summed over l and m are also obtained. Statistical (polarization) tensors of the orbital angular momentum of protonium atoms are evaluated. Corrections due to strong proton-antiproton interaction in the protonium ground (1s) state are analyzed.     

One-electron capture in collisions of fast ions with atoms

The properties of one-electron charge transfer between ions and atomsB ^Z++A →B ^(Z?1)+ A ⁺ are studied at relative velocities of the colliding particles higher than target electron velocities. Calculations of partial and total cross sections in collisions of protons and multiply-charged ions with neutral atoms are performed and compared with experimental data. The universal curve for the capture of the targetK- andL-electrons is given. In all cases at sufficiently high collision energies the electron capture from outer shells decreases and the capture of electrons from inner shells of the target atom becomes predominant.     

42P12↔42P32 mixing in potassium induced in collisions with noble gas atoms

Cross sections for$\text{4}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{-4}{}^2\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ mixing in potassium, induced by collisions with noble gas atoms, were determined using the methods of fluorescence spectroscopy. The experiments were carried out at an optical depth of ≤3×10^-2 in the absence of radiation trapping and at noble gas pressures in the mtorr region and yielded the cross sections $\text{Q}{}_1\text{(4}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{→4}{}^2\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ and $\text{Q}{}_2\text{(4}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{« 4}{}^2\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ (in units of 10^-16 cm²) respectively, as follows: for He, 26.3 and 17.8; for Ne, 6.4 and 4.4; for Ar, 16.0 and 11.2; for Kr, 24.9 and 17.7; for Xe, 44.9 and 31.7. These values supersede those reported previously by Chapman and Krause (1966).