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Collisions of laser excited xenon Rydberg atoms with ammonia

F. G. Kellert

Published Jan 1, 1980• 0 citations
OpenAlex

Abstract

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.

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•Jan 1, 1986•0 citations
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.
Thermal collisions between laser excited Xe(nf) Rydberg atoms and a variety of neutral atomic and molecular targets have been investigated. In Xe(nf)-HF collisions, n-changing, 4-changing and collisional ionization were observed. For n = 26 and 27, absolute collisional cross sections for these processes have been determined using time-resolved selective field ionization. In addition, the first evidence of molecular rotational excitation through the transfer of energy of electronic excitation from a Rydberg atom is presented. In Xe(nf) - Xe, Kr, Ar, CO2, CO and N2 collisions the dominant mechanism was observed to be 4-changing: no evidence of n-changing or collisional ionization was apparent. Absolute collisional cross sections have been determined for a range of n between 24 and 37. Comparison of these cross sections with those obtained using a theoretical model in which effects due to the Rydberg core are ignored appears to lead to satisfactory results only for Xe, Kr and CO2, whereas when core effects are included satisfactory agreement between experiment and theory is found in all cases.
Thermal collisions between Xe(nf) Rydberg atoms and HF molecules have been investigated. The highly excited collision products were detected using selective field ionisation (SFI). Both n-changing and l-changing collisions were identified and the absolute rate constants for these processes, and for collisional ionisation, have been measured for n=26, 27. In addition, the first evidence of molecular rotational excitation through the transfer of energy of electronic excitation from a Rydberg atom is presented.
In thermal-energy collisions between sodium atoms in ns\mathrm{ns}ns or np\mathrm{np}np states (32< n< 5232<~n<~5232< n< 52) and xenon atoms, depopulation of the Rydberg states is observed to occur predominantly through state-changing reactions of the type $\mathrm{Na}(ns or np)+\mathrm{Xe}\ensuremath{\rightarrow}\mathrm{Na}(n\ensuremath{-}1, l>~2)+\mathrm{Xe}$. For the present range of nnn, the measured cross sections increase with nnn and lie between 1500 and 2600 A˚2{\mathrm{\AA{}}}^{2}A˚2 for ns\mathrm{ns}ns states and between 2000 and 4200 A˚2{\mathrm{\AA{}}}^{2}A˚2 for np\mathrm{np}np states. Implications of the data are discussed.
The excited products of collisions between xenon atoms and Na(nd)\mathrm{Na}(\mathrm{nd})Na(nd) atoms with n=37n=37n=37 and n=42n=42n=42 have been investigated by use of selective field ionization (SFI). It is observed that, at an ionizing-field slew rate of \ensuremath{\sim} 109{10}^{9}109 V/cm sec, most of the collision products follow predominantly diabatic paths to ionization which occurs at field strengths consistent with those predicted by quantum mechanical tunneling calculations. Detailed analysis of the SFI data indicates that the collision products have a wide (possibly complete) range of values of lll and mj{m}_{j}mj​, although it is not clear whether the observed distribution results from single or multiple collisions. No evidence of nnn-changing collisions was seen. Similar data for collisions involving Xe(nf)\mathrm{Xe}(\mathrm{nf})Xe(nf) atoms are briefly discussed.
Collisional and thermal ionization of sodium n S and n D Rydberg atoms with n = 8–20 has been studied. The experiments were performed using a two-step pulsed laser excitation in an effusive atomic beam at atom density of about 2 × 10 10 cm −3 . Molecular and atomic ions from associative, Penning and thermal ionization processes were detected. It was found that the atomic ions were created mainly due to photoionization of Rydberg atoms by photons of blackbody radiation at the ambient temperature of 300 K. Blackbody ionization rates and effective lifetimes of Rydberg states of interest were determined. The molecular ions were found to be from associative ionization in Na( nL ) + Na(3S) collisions. Rate constants of associative ionization have been measured using an original method based on relative measurements of Na + 2 and Na + ion signals.
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.