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Journal of Physics B Atomic and Molecular Physics

A study of n changing, l changing and ionisation in collisions of Xe(nf) Rydberg atoms with HF

C Higgs, K. A. Smith, G. B. McMillian, F. B. Dunning, R. F. Stebbings

Published Apr 28, 1981• 22 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/14/8/004

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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.

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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.

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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.

•Jan 1, 1986•0 citations
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.
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.
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.
In collisions with NH3{\mathrm{H}}_{3}H3​ molecules it is observed that Xe(nf)\mathrm{Xe}(\mathrm{nf})Xe(nf) Rydberg atoms are selectively excited to discrete, more highly excited states. These states are identified by the technique of field ionization and are found to be displaced in energy from the initial Rydberg states by amounts equal to the rotational spacings of the NH3{\mathrm{H}}_{3}H3​ molecule. These measurements demonstrate, for the first time, the conversion of rotational energy to electronic energy in collisions.
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.
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.
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.