State-changing in collisions of Rb(nS) - HF are studied using a crossed atomic beam/modulated CW laser technique and selective field ionization for values of principal quantum number n where 40 (LESSTHEQ) n (LESSTHEQ) 49. Collisions between a rotating polar molecule and a Rydberg atom often result in a near-resonant interchange of the rotational energy of the molecule, and a corresponding change in the electronic energy of the atom. A critical parameter is the energy imbalance of the reaction, defined as the difference between the atom's initial and final electronic energy and the molecule's initial and final rotational energy. The molecule used in this experiment, hydrogen fluoride, has a large rotational constant which permits study of n-changing rate constants for transitions from an initially populated s-state to a single n-manifold of high-L states. Because only one final manifold is populated, the reaction involves a single energy imbalance, and thus proves a useful test of theoretical models. n-Changing rate constants for collisions with energy imbalances ranging from 0.057 cm('-1) to 3.447 cm('-1), and ionization and L-mixing rate constants are compared to those predicted by an impulse approximation model and a semiclassical approach.
State-changing in collisions of Rb(nS) - HF are studied using a crossed atomic beam/modulated CW laser technique and selective field ionization for values of principal quantum number n where 40 (LESSTHEQ) n (LESSTHEQ) 49. Collisions between a rotating polar molecule and a Rydberg atom often result in a near-resonant interchange of the rotational energy of the molecule, and a corresponding change in the electronic energy of the atom. A critical parameter is the energy imbalance of the reaction, defined as the difference between the atom's initial and final electronic energy and the molecule's initial and final rotational energy. The molecule used in this experiment, hydrogen fluoride, has a large rotational constant which permits study of n-changing rate constants for transitions from an initially populated s-state to a single n-manifold of high-L states. Because only one final manifold is populated, the reaction involves a single energy imbalance, and thus proves a useful test of theoretical models. n-Changing rate constants for collisions with energy imbalances ranging from 0.057 cm('-1) to 3.447 cm('-1), and ionization and L-mixing rate constants are compared to those predicted by an impulse approximation model and a semiclassical approach.
Rate constants for state changing in collisions of Rb(ns) Rydberg atoms (40\ensuremath{\le}n\ensuremath{\le}48) with HF molecules have been measured and have been calculated using a semiclassical approach. Agreement between the total rate constants measured experimentally and derived theoretically is very good. The results point to the near-resonant nature of n-changing collisions, but indicate that the rate constants for such reactions are less dependent on energy imbalance than suggested by calculations based on the impulse approximation.
Potassium atoms in selected high-lying np Rydberg states (80 {<=} n {<=} 400) are being used as a tool to examine electron-polar molecule collisions at ultra-low energies ({approximately}80 {mu}eV - 1 meV). This energy regime has not been investigated previously and is of interest because the corresponding electron de Broglie wavelength is large, {approximately}1300 - 330 {Angstrom}, and because the collision time is comparable to the period of molecular rotation. A variety of target molecules has been studied, including simple rotors such as HF, and more complex species such as NH{sub 3}, CH{sub 3}I, CH{sub 2}Br{sub 2} and C{sub 6}H{sub 5}NO{sub 2}. These targets encompass a range of dipole moments and rotational behavior, but analysis of the data shows that in each case the cross section {sigma}({epsilon}) for rotationally-inelastic electron-polar molecule scattering varies approximately as 1/{epsilon} where {epsilon} is the electron energy. Such threshold behavior is not predicted by the Born approximation and possible reasons for this discrepancy will be discussed. In the case of NH{sub 3}, the data show that near-resonant energy transfer associated with transitions between different inversion levels is an important process leading to collisional state changing.
In thermal collisions between Xe(nf) Rydberg atoms and NH(,3) collisional depopulation of the nf state occurs through at least three distinct mechanisms: n-changing collisions, l-changing collisions, and ionizing collisions. Using selective field ionization, the total collisional depopulation rate constants k(,d) are measured for laser-excited Xe(nf) atoms in the n range of 22 to 39. The magnitudes of k(,d) are large, corresponding to reaction cross sections comparable to the geometric size of the Xe(nf) atom at n = 22. In the n range of 25 to 40, Xe('+) ion production provides an absolute measure of the rate constants for collisional ionization. Approximate rate constants for n-and l-changing collisions are also presented for the 31f state. In these experiments, one finds that the state-changing collision rate constants are fairly independent of n and are determined largely by the l-changing collisions. The collisional ionization rate constants measured are smaller than the state-changing rate constants and increase with increasing n, ranging between 0.2 and 4.6 x 10('-7) cm('3)/sec for n between 25 and 40. The present results are compared to those of recent theoretical calculations.
The title process with simultaneous rotational deexcitation of molecules was investigated theoretically by using the semiquantal approximation. The dipole Born and dipole Glauber differential cross sections were employed for the electron-molecule rotational deexcitation process. Numerical applications were made to the following systems: Xe+CO, HCl, HF, and LiF. A comparative study of the two approximations was carried out concerning the dependences of the cross sections on the following various quantities: n (principal quantum number), J (initial rotational quantum number), \ensuremath{\Delta}J (rotational quantum number change), D (dipole moment), V (relative collision velocity). As the molecular dipole moment becomes larger, there appears a prominent difference in the n dependence of the cross section. The Glauber approximation brings about a decrease in the cross section and yields a sawlike n dependence in contrast to a steplike n dependence in the case of the Born approximation. In the collisions involving lowly excited Rydberg atoms, ionization with simultaneous dipole-forbidden transition of a molecule becomes dominant compared to the ionization with simultaneous dipole-allowed transition. The cross section was found to be inversely proportional to the relative collision velocity; thus the ionization rate becomes independent of temperature. A comparison was also made between the experiment for the ionization rate at 300 K for Xe(27f)+HF and Xe(31f)+HCl.
Using state-selective field ionization, the state distributions of Rydberg atoms in cold Rydberg gases are measured for various initially excited Rydberg levels, populations, and evolution times. We provide direct experimental evidence for l-changing collisions that we previously observed indirectly [S. K. Dutta, D. Feldbaum, A. Walz-Flannigan, J. R. Guest, and G. Raithel, Phys. Rev. Lett. 86, 3993 (2001)]. We also observe n-mixing and find that its effects are largely in agreement with recent theoretical work on n-changing collisions between electrons and Rydberg atoms, thus enabling an estimation of the electron temperature. Unexpectedly large populations of atoms are found in states with principal quantum numbers much lower than that of the initially excited atoms. We explain this observation by collisions between high-l Rydberg atoms, which are highly polar and can collide due to static electric-dipole forces between them.
A classical model for l mixing of Rydberg states in atomic collisions is described. The primary assumption of the model is that l changing occurs in a single hard-sphere collision between the Rydberg electron and the perturbing atom which is represented by the zero-energy scattering length L. Predictions of the model, which are in the form of cross sections as functions of principal quantum number n, are compared with data for l mixing of Na(nd) and Rb(nf) by rare-gas atoms. Agreement is, in most cases, quite good, comparable with that of other, more sophisticated, models. It is shown that, because the calculation must be carried out in the reference frame of the ion core, the energy deficit between low angular momentum states and the nearest hydrogenic manifold can be made up in a single collision between the Rydberg electron and the perturbing atom, an effect that is analogous to the gravitational slingshot effect used to alter the orbits of a spacecraft under the influence of the Sun's gravitational field. Comparison of data for l mixing by ground-state Rb atoms with model calculations in which L is treated as a free parameter leads to an estimate of 10 a.u. for the zero-energy scattering length of Rb.