Collisions Of Rydberg Atoms Research Articles

Dipole-allowed transitions in collisions of charged particles and Rydberg atoms: method of virtual quanta in low velocity collisions

Experiments involving collisions between charged particles and Na Rydberg atoms have been undertaken to study the projectile-velocity dependence of collisionally induced dipole-allowed transitions. Measurements being carried out in this laboratory, involving H +, H 2 +, He +, Ne +, and e − projectiles incident on Na(30s) and Na(29d), cover the reduced projectile velocity range from 1.7 to 200 times the mean orbital velocity of the excited electron. Absolute cross sections for the dipole-allowed transitions, 30s to 30p and 29d to 30p, have been measured and exhibit pronounced and unique projectile-velocity dependent thresholds. The Weizsäcker-Williams method of virtual quanta has been extended to include low projectile velocities and is found to be in good agreement with the observed projectile-velocity dependent dipole-allowed thresholds.

Resonant collisions of Na Rydberg atoms in electric and magnetic fields.

We have observed 10-MHz-wide energy-transfer resonances between colliding Na Rydberg atoms. The narrow linewidth allows us to apply a modest 5--40-G static magnetic field in addition to a static tuning electric field and resolve new resonances arising from magnetic-field-induced splitting of the Stark energy levels. When B is parallel to E, the nine resolved resonances observed with only an E field are further split into 144 possible resonances. However, only the 21 resonances corresponding to collisions in which the spins are unchanged are observable. When B is perpendicular to E, the spectrum of collisional resonances is similar to that of the vertical field case, except that the final spin states cannot be determined. With both parallel and perpendicular fields present, linear and quadratic terms of the B field interaction create more complicated spectra than with either case alone. The experimental spectra agree quantitatively with the spectra calculated using a simple Zeeman model of the energy levels.

Frequency evolution of radiatively assisted collisions of K Rydberg atoms.

Ramsey interference fringes are observed in the line shape of resonant Rydberg-atom--Rydberg-atom collisions occurring in the presence of a radiation field of frequency comparable to the inverse of the collision time. At low frequencies the fringes are phase dependent, but at high frequencies they are not. We present a theoretical interpretation based on a quasistatic field description which connects the low- and high-frequency regimes. In the high-frequency regime, this model gives results equivalent to those obtained using a ``dressed-atom'' approach.

Classical mechanics and the Born theory of the collisions of electrons and Rydberg atoms

A purely classical method is presented for the calculation of the cross sections for transitions between the Rydberg atomic states n1, L1 to n2, L2 by electron impact. The state of the electron in the atom is described by the microcanonical distribution function. The cross sections are obtained in a quite appealing form after a very simple calculation. The accuracy of the approximation appears to be reasonable.

Angular-momentum distributions following impact of slow ions on Na(nd) Rydberg atoms.

The degree of dipole-forbidden l change in Na(26d) and Na(28d) Rydberg atoms caused by slow ${\mathrm{Na}}^{+}$ ion impact at reduced velocity v\ifmmode \tilde{}\else \~{}\fi{}=0.1--0.9 has been measured. ${\mathrm{Na}}^{+}$ beam energies 7.3--684 eV were used. Previous measurements were greatly improved by reducing stray electric fields in the Rydberg collision region to about 10 mV/cm. The distributions of final-angular-momentum states were analyzed using directly measured diabatic selective-field-ionization profiles for l=3, 4, and 5. l-change results in Na(26d) and Na(28d) targets are very similar and clearly show an increasing amount of dipole-forbidden behavior for decreasing reduced velocities v\ifmmode \tilde{}\else \~{}\fi{} and partial agreement with the coupled-channel calculation of Beigman and Syrkin (Zh. Eksp. Teor. Fiz. 89, 400 (1985) [Sov. Phys. JETP 62, 226 (1985)]). The dipole-forbidden process is the main channel for angular-momentum change in ion--Rydberg-atom collisions at v\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\le}0.7. At very low energy (v\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\le}0.3) the measurements demonstrate that the final-state distributions are dominated by very high angular-momentum states.

Collisional ionization and destruction cross sections for hydrogen and helium Rydberg atoms on Ar, Xe, and N2: Failure of the independent-particle model.

Absolute cross sections for the collisional ionization and the total destruction of hydrogen and helium atoms in low-lying Rydberg states colliding with Ar, Xe, and ${\mathrm{N}}_{2}$ have been measured over the energy range from 1.0 to 30 keV/amu. The independent-particle model fails to explain the behavior of the Rydberg atom's outer electron in ionization collisions below 3 keV/amu for He (12\ensuremath{\le}n\ensuremath{\le}15) on Xe and ${\mathrm{N}}_{2}$, but not on Ar. This represents an observation of the model's failure in collisions of Rydberg atoms with neutral perturbers in this energy range and shows that at low energies, where a molecular picture of the core-target scattering is valid, the Rydberg electron is sensitive to details of the molecular interactions between the target and the Rydberg atom's core.

Rydberg-atom collisions with SF6 and CCl4 at very high n.

Rate constants for Rydberg-atom destruction through electron transfer in collisions between K(np) Rydberg atoms and ${\mathrm{SF}}_{6}$ and ${\mathrm{CCl}}_{4}$ have been measured for values of principal quantum number n up to n\ensuremath{\sim}400 using an apparatus specifically developed for very-high-n collision studies. Analysis of the data using the free-electron model provides cross sections for electron attachment to ${\mathrm{SF}}_{6}$ and ${\mathrm{CCl}}_{4}$ at electron energies down to \ensuremath{\sim}10 \ensuremath{\mu}eV, which are far below those accessible using any alternate technique. The measured cross sections are inversely proportional to electron velocity and are consistent with the Wigner threshold law for s-wave attachment.

Semiclassical study of the collision of a highly excited Rydberg atom with the molecules HF and HCl.

The semiclassical impact-parameter method is applied to the processes of state changing and energy transfer in the collision of a highly excited Rydberg atom (n\ensuremath{\ge}20) with the polar molecules HF and HCl. The relative motion of the molecule and atomic nucleus is taken to be rectilinear; the electron-molecule and ion core-molecule interactions are represented by cutoff dipole forms. Cross sections for transitions involving quantum numbers n and l of the atom and rotational quantum number j of the molecule are obtained for a range of collision energies and initial atomic and molecular states. Comparisons are made with the results of earlier classical studies and with the quantum-mechanical impulse approximation. Collision rates are calculated and compared with experimental values for l mixing and n and j changing. The agreement between experiment and theory is shown to be satisfactory, within the uncertainties of both the measurements and the theory. Cases of agreement and disagreement between various theories are examined. One finding of the present work is that the quantum-mechanical impulse approximation appears to significantly overestimate the values of various state-changing cross sections when the internal energy defect is small. The validity of the impulse approximation for collisions of Rydberg atoms with polar molecules is discussed.

Intracollisional interference in resonant collisions of Na Rydberg atoms.

The theory of resonant dipole-dipole energy transfer in an electric field E indicates substantial differences between the collision velocities v\ensuremath{\parallel}E and v\ensuremath{\perp}E. For v\ensuremath{\parallel}E the cross section vanishes at exact resonance due to a destructive interference in the transition amplitude, producing a double-peaked line shape. The presence of the permanent dipole moments in the field produces slight shifts and asymmetries of the collisional resonance line shapes. A high-resolution experimental study of the cases v\ensuremath{\parallel}E and v\ensuremath{\perp}E indicates that the theoretical predictions are correct and further allows the resolution of the spin-orbit structure in the resonant collisions.

Radiative Rydberg-atom-Rydberg-atom collisions in the strong-field regime.

We describe the experimental observation of the resonant collisional energy transfer Na(ns)+Na(ns)+qh\ensuremath{\nu}\ensuremath{\rightarrow}Na(np)+Na[(n-1)p]+(q○)h\ensuremath{\nu} which occurs when the Na atoms in a strong microwave field are tuned in energy with a static field so that the energy resonance ${E}_{\mathrm{ns}}$+${E}_{\mathrm{ns}}$=${E}_{\mathrm{np}}$+${E}_{n\mathrm{\ensuremath{-}}1}$,p\ifmmode\pm\else\textpm\fi{}mh\ensuremath{\nu} occurs. We describe our experimental observations and provide a detailed description of a nonperturbative theory which we have developed to explain our observations. The theory is essentially developed in terms of the resonant dipole-dipole collisions between ``microwave dressed'' atoms. Using an extension of the approach we used previously to describe resonant dipole-dipole collisions in the absence of microwaves, we are able to take into account all the coherences in the problem in a completely straightforward way.

Study of low-energy electron attachment to 1,1,1-C2Cl3F3 using K(n d) Rydberg atoms

Studies of K(nd) Rydberg atom collisions (10≤n≤100) are used to investigate electron attachment to trichlorotrifluoroethane 1,1,1-C2Cl3F3 (TCTFE) at subthermal energies. The Rydberg atom data indicate that electron attachment to TCTFE occurs predominantly through dissociative capture and that the cross section for this process is large. Differences between the present results and those obtained using the threshold photoelectron spectroscopy (TPSA) technique are discussed.

Fast Rydberg hydrogen atom collisions with neutral atoms and molecules

Absolute cross sections for collisional depopulation of 2.5-40 keV u-1 Rydberg hydrogen atoms have been measured in Ar, N2, CO2 and SF6. The data establish unambiguously that both the Rydberg electron and the proton core scatter quasi-freely. This result confirms Matsuzawa's (1980) theoretical description of fast collisions of Rydberg atoms with neutrals. Consequently, under certain conditions the dominant Rydberg atom depopulation mechanism is that of core collisions and the total scattering cross section of a fast Rydberg atom reaches a natural limit, independent of the principal quantum number n, given by the independent-particle model.

Ionization in Rydberg-atom-SF6 collisions at high n.

Absolute rate constants for ionization of Rb(ns,nd) atoms in collisions with SF/sub 6/ have been measured for values of n in the range 38--106. In addition, the kinematics of such reactions have been investigated using Xe(nf) atoms. The data confirm that collisions of Rydberg atoms with targets that attach free low-energy electrons are well described by the ''essentially-free''-electron model. Analysis of the data shows that the cross section sigma(v) for electron attachment to SF/sub 6/ at very low electron energies (< or approx. =10 meV) is inversely proportional to the electron velocity v and, if v is expressed in cm sec/sup -1/, is given by sigma(v) = (4.2 +- 1.0) x 10/sup -7//v cm/sup 2/.

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.

Analytical expressions for cross sections of Rydberg-neutral inelastic collisions

The author obtains simple analytical expressions for the cross sections of the state-changing and ionisation collisions of Rydberg atoms with neutral atomic particles due to the Rydberg-electron-perturber interaction. The theoretical investigation is based on the impulse approximation and binary encounter approach for the form factor. The earlier results of Alekseev and Sobel'man (1965) and Omont (1977) turn out to be the limiting cases of the author's expressions. The calculated quenching cross sections for the Rydberg states agree well with experimental results and numerical calculations.

Collisions of Xe(n f) Rydberg atoms with electron attaching targets

This paper reports a study of Xe (nf) destruction in collisions with 1,1,1-C2Cl3F3 and C5F8. The primary Rydberg atom destruction process is collisional ionization that results in the formation of a positive and a negative ion. Rate constants for this reaction have been measured for Xe (nf) over the range 26 ≤n≤43, and the major negative ion products identified. Cross sections for Rydberg electron attachment derived from these and earlier measurements are in good agreement with recent free electron attachment cross sections obtained using swarm techniques. This agreement further confirms that the study of Rydberg atom collisions can provide information on very-low-energy electron interactions.

Collisions of Rydberg atoms in an electric field: Calculations using hydrogenic wave functions in parabolic coordinates

The theory of collisions of Rydberg atoms with rare-gas atoms is generalized to include the effects of an applied static electric field. Two effects occur. First, the field causes the states to split, and the increased energy separations lead to a decrease in the inelastic cross sections. Second, there is a change in the form of the wave functions, because the various Stark states in the field have electron charge distributions that may be localized in very different regions of space. The present work focuses on this second effect. With the description of the initial and final Stark states of the Rydberg atom by the proper electronic wave functions obtained in parabolic coordinates, cross sections are obtained for a variety of specific transitions of Rydberg atoms ($n=10, 15, \mathrm{and} 20$) caused by collisions. The calculations are strictly valid for hydrogen, but exhibit general features that are expected to apply to other Rydberg atoms as well. The results show a dramatic effect on the cross section for particular transitions depending on the degree of overlap of the initial and final charge distributions. In addition, it is observed that transitions are favored that involve small changes in the azimuthal quantum number $m$. The origin of this tendency is discussed.

Application of the Faddeev-Watson expansion to thermal collisions of Rydberg atoms with neutral particles

Application of the Faddeev-Watson expansion to thermal collisions of Rydberg atoms with neutral particles

Collisions of Rydberg Atoms with Molecules

Collisions of Rydberg Atoms with Molecules

Comments on 'Thermal collisions of Rydberg atoms with neutrals'

Various comments by Matsuzawa (1981) on validity criteria for application of the impulse approximation to A-B(n, l) collisions are examined and are shown to be without basis. The weak-binding and quasi-classical conditions are discussed and a connection between classical momentum and angular momentum changes is developed.