Physical Review Letters

Studies of Electron-Molecule Scattering at Microelectronvolt Energies Using Very-High- n Rydberg Atoms

M. T. Frey, S. B. Hill, K. A. Smith, F. B. Dunning, I. I. Fabrikant

Published Jul 31, 1995• 39 citations

Abstract

Atoms in very high Rydberg states, $100\ensuremath{\lesssim}n\ensuremath{\lesssim}1100$, are used to investigate electron-molecule interactions at electron energies extending down to a few microelectronvolts. At such energies the cross section for electron capture by CC${\mathrm{l}}_{4}$ is observed to vary inversely with electron velocity, indicative of an $s$-wave process. Studies with the polar target C${\mathrm{H}}_{3}$Cl suggest that dipole-supported states may be important in inelastic electron-polar molecule scattering at very low electron energies.

DOI: 10.1103/physrevlett.75.810

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Studies of threshold behavior in electron-molecule collisions using ultra-high-n Rydberg atoms

M. T. Frey

Jan 1, 19950 citations

Studies of electron-molecule scattering at microelectronvolt energies using very-high-n Rydberg atoms

M. T. Frey, S. B. Hill, K. A. Smith +2 more

Recent advances in experimental techniques now allow studies with atoms in very high‐lying Rydberg states, 100≲n≲1100. Such atoms provide a unique tool with which to investigate electron‐molecule scattering at ultra‐low electron energies and the new capabilities this affords are illustrated by considering collisions with molecules that attach free low‐energy electrons, and with molecules that possess large electric dipole moments.

AIP conference proceedingsJan 1, 19963 citations

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Ionization of low-Rydberg-state He atoms by polar molecules. I. Velocity dependence of the cross sections

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Physical Review ANov 1, 19888 citations

Ionizing collisions of He( n1 P) atoms in a low-Rydberg state with SF6 and NO2 electron-attaching molecules

A. Pesnelle, C. Ronge, M. Perdrix +1 more

Physical Review AJul 1, 199015 citations

Physical Review Letters

Studies of Electron-Molecule Scattering at Microelectronvolt Energies Using Very-High- n Rydberg Atoms

M. T. Frey, S. B. Hill, K. A. Smith, F. B. Dunning, I. I. Fabrikant

Published Jul 31, 1995• 39 citations

Publisher

Abstract

Atoms in very high Rydberg states,

100\ensuremath{\lesssim}n\ensuremath{\lesssim}1100

, are used to investigate electron-molecule interactions at electron energies extending down to a few microelectronvolts. At such energies the cross section for electron capture by CC

{\mathrm{l}}_{4}

is observed to vary inversely with electron velocity, indicative of an

s

-wave process. Studies with the polar target C

{\mathrm{H}}_{3}

Cl suggest that dipole-supported states may be important in inelastic electron-polar molecule scattering at very low electron energies.

DOI: 10.1103/physrevlett.75.810

Related papers

Studies of threshold behavior in electron-molecule collisions using ultra-high-n Rydberg atoms

M. T. Frey

Potassium atoms in selected high-lying Rydberg states (n $<$ 1200) are used as a tool to probe threshold behavior in electron-molecule collisions. Collisions with non-polar electron attaching molecules such as CCl $\\sb4$ are dominated by electron capture in a binary interaction between the Rydberg electron and target molecule allowing the study of electron attachment at electron energies of only a few $\\mu$ eV. Analysis of the data shows the cross section for electron capture is consistent with the Wigner threshold law for an inelastic s-wave process. Collisions with polar molecules can lead to Rydberg atom destruction through transfer of molecular rotational energy to the Rydberg electron. Rydberg atom data obtained with polar targets are not consistent with scattering from a static dipole. However, the data are consistent with a threshold law that takes into account a rotationally-averaged induced dipole interaction that can possibly support bound or virtual states.Potassium atoms in selected high-lying Rydberg states (n $<$ 1200) are used as a tool to probe threshold behavior in electron-molecule collisions. Collisions with non-polar electron attaching molecules such as CCl$\\sb4$ are dominated by electron capture in a binary interaction between the Rydberg electron and target molecule allowing the study of electron attachment at electron energies of only a few $\\mu$eV. Analysis of the data shows the cross section for electron capture is consistent with the Wigner threshold law for an inelastic s-wave process. Collisions with polar molecules can lead to Rydberg atom destruction through transfer of molecular rotational energy to the Rydberg electron. Rydberg atom data obtained with polar targets are not consistent with scattering from a static dipole. However, the data are consistent with a threshold law that takes into account a rotationally-averaged induced dipole interaction that can possibly support bound or virtual states.

Jan 1, 19950 citations

Studies of electron-molecule scattering at microelectronvolt energies using very-high-n Rydberg atoms

M. T. Frey, S. B. Hill, K. A. Smith +2 more

AIP conference proceedingsJan 1, 19963 citations

Studies of ultra-low energy electron-molecule collisions using very-high-n Rydberg atoms

M. T. Frey, Xiaohui Ling, K. A. Smith +1 more

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.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.

Bulletin of the American Physical SocietyMay 1, 19930 citations

A study of ultra-low-energy electron-molecule collisions using very-high-n Rydberg atoms

Xiaohui Ling

In the present work very-high-n Rydberg-atoms $(n \\sim 100 - n \\sim 400)$ are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low $\\rm({\\sim}85\\mu eV - {\\sim}1.4meV),$ analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\\sim $1$ \\sp\\circ$K, which are far below those accessible using any alternate approach.\nRate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions $\\rm K({\\it np\\/}) + XY \\to K\\sp+ + (XY)\\sp{-*}\\ or\\ \\to K\\sp+ + X\\sp- + Y\\eqno(1)$ and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization $\\rm K({\\it np\\/}) + XY({\\it J\\/}) \\to K\\sp+ + {\\it e}\\sp- + XY({\\it J\\/}-1)\\eqno(2)$ \nThe n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process.\nThe essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\\eqalignno{&e\\sp- + \\rm XY \\to (XY)\\sp{-*}\\ or\\ \\to X\\sp- + Y&(1\\sp\\prime)\\cr &e\\sp- + \\rm XY({\\it J\\/}) \\to {\\it e}\\sp- + XY({\\it J\\/}-1)&(2\\sp\\prime)\\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.In the present work very-high-n Rydberg-atoms $(n \\sim 100 - n \\sim 400)$ are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low $\\rm({\\sim}85\\mu eV - {\\sim}1.4meV),$ analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\\sim$1$\\sp\\circ$K, which are far below those accessible using any alternate approach.\nRate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions $$\\rm K({\\it np\\/}) + XY \\to K\\sp+ + (XY)\\sp{-*}\\ or\\ \\to K\\sp+ + X\\sp- + Y\\eqno(1)$$and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization $$\\rm K({\\it np\\/}) + XY({\\it J\\/}) \\to K\\sp+ + {\\it e}\\sp- + XY({\\it J\\/}-1)\\eqno(2)$$\nThe n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process.\nThe essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\\eqalignno{&e\\sp- + \\rm XY \\to (XY)\\sp{-*}\\ or\\ \\to X\\sp- + Y&(1\\sp\\prime)\\cr &e\\sp- + \\rm XY({\\it J\\/}) \\to {\\it e}\\sp- + XY({\\it J\\/}-1)&(2\\sp\\prime)\\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.

Jan 1, 19930 citations

Ionization of low-Rydberg-state He atoms by polar molecules. I. Velocity dependence of the cross sections

C. Ronge, A. Pesnelle, M. Perdrix +1 more

The ionization of low-Rydberg-state He atoms in the n $^{1}P$ state (n=14) in collisions with the polar molecules ${\mathrm{NH}}_{3}$ , ${\mathrm{SO}}_{2}$ , and acetone ( ${\mathrm{C}}_{3}$ ${\mathrm{H}}_{6}$ O) is carried out in a crossed-beam experiment at thermal kinetic energies. The dependence of the experimental ionization cross sections ${\ensuremath{\sigma}}_{i}^{\mathrm{expt}}$ on the relative velocity of the reactants is obtained by a time-of-flight method. We observe that it varies mainly as 1/ ${v}_{r}^{2}$ in a large part of the studied velocity range, while a 1/ ${v}_{r}$ velocity dependence is predicted by the free-electron model for the higher Rydberg levels.The ionization of low-Rydberg-state He atoms in the n $^{1}P$ state (n=14) in collisions with the polar molecules ${\mathrm{NH}}_{3}$, ${\mathrm{SO}}_{2}$, and acetone (${\mathrm{C}}_{3}$${\mathrm{H}}_{6}$O) is carried out in a crossed-beam experiment at thermal kinetic energies. The dependence of the experimental ionization cross sections ${\ensuremath{\sigma}}_{i}^{\mathrm{expt}}$ on the relative velocity of the reactants is obtained by a time-of-flight method. We observe that it varies mainly as 1/${v}_{r}^{2}$ in a large part of the studied velocity range, while a 1/${v}_{r}$ velocity dependence is predicted by the free-electron model for the higher Rydberg levels.

Physical Review ANov 1, 19888 citations

Ionizing collisions of He( n1 P) atoms in a low-Rydberg state with SF6 and NO2 electron-attaching molecules

A. Pesnelle, C. Ronge, M. Perdrix +1 more

The ionization of He atoms, prepared in a well-defined (n=14) $^{1}$ P Rydberg state, in collisions with electron-attaching molecules like ${\mathrm{SF}}_{6}$ and ${\mathrm{NO}}_{2}$ , is studied in a crossed-beam experiment at thermal energies. The ionization cross section is observed to be large for ${\mathrm{SF}}_{6}$ , reaching 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{2}$ , and to be smaller for ${\mathrm{NO}}_{2}$ , close to ${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$ . The dependence of these two cross sections on the relative velocity of the reactants exhibits a characteristic behavior with a saturation at low velocities, a dependence that is completely different from that obtained with polar molecules. The observation of such a dependence for ${\mathrm{NO}}_{2}$ indicates that, for this molecule, the reaction product is an ion pair, as for ${\mathrm{SF}}_{6}$ , and that ionization is not due to rotational energy transfer even though ${\mathrm{NO}}_{2}$ has a dipole moment. This dependence cannot be explained by the ``free-electron model'' usually used in Rydberg-atom--molecule collisions; it clearly shows that the interaction of the molecule with the ion core cannot be neglected.The ionization of He atoms, prepared in a well-defined (n=14) $^{1}$P Rydberg state, in collisions with electron-attaching molecules like ${\mathrm{SF}}_{6}$ and ${\mathrm{NO}}_{2}$, is studied in a crossed-beam experiment at thermal energies. The ionization cross section is observed to be large for ${\mathrm{SF}}_{6}$, reaching 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{2}$, and to be smaller for ${\mathrm{NO}}_{2}$, close to ${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$. The dependence of these two cross sections on the relative velocity of the reactants exhibits a characteristic behavior with a saturation at low velocities, a dependence that is completely different from that obtained with polar molecules. The observation of such a dependence for ${\mathrm{NO}}_{2}$ indicates that, for this molecule, the reaction product is an ion pair, as for ${\mathrm{SF}}_{6}$, and that ionization is not due to rotational energy transfer even though ${\mathrm{NO}}_{2}$ has a dipole moment. This dependence cannot be explained by the ``free-electron model'' usually used in Rydberg-atom--molecule collisions; it clearly shows that the interaction of the molecule with the ion core cannot be neglected.

Physical Review AJul 1, 199015 citations

Atoms in very high Rydberg states,

100\ensuremath{\lesssim}n\ensuremath{\lesssim}1100

, are used to investigate electron-molecule interactions at electron energies extending down to a few microelectronvolts. At such energies the cross section for electron capture by CC

{\mathrm{l}}_{4}

is observed to vary inversely with electron velocity, indicative of an

s

-wave process. Studies with the polar target C

{\mathrm{H}}_{3}

Cl suggest that dipole-supported states may be important in inelastic electron-polar molecule scattering at very low electron energies.

Potassium atoms in selected high-lying Rydberg states (n

<

1200) are used as a tool to probe threshold behavior in electron-molecule collisions. Collisions with non-polar electron attaching molecules such as CCl

\\sb4

are dominated by electron capture in a binary interaction between the Rydberg electron and target molecule allowing the study of electron attachment at electron energies of only a few

\\mu

eV. Analysis of the data shows the cross section for electron capture is consistent with the Wigner threshold law for an inelastic s-wave process. Collisions with polar molecules can lead to Rydberg atom destruction through transfer of molecular rotational energy to the Rydberg electron. Rydberg atom data obtained with polar targets are not consistent with scattering from a static dipole. However, the data are consistent with a threshold law that takes into account a rotationally-averaged induced dipole interaction that can possibly support bound or virtual states.

In the present work very-high-n Rydberg-atoms

(n \\sim 100 - n \\sim 400)

are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low

\\rm({\\sim}85\\mu eV - {\\sim}1.4meV),

analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\\sim

1

\\sp\\circ$K, which are far below those accessible using any alternate approach.\nRate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions

\\rm K({\\it np\\/}) + XY \\to K\\sp+ + (XY)\\sp{-*}\\ or\\ \\to K\\sp+ + X\\sp- + Y\\eqno(1)

and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization

\\rm K({\\it np\\/}) + XY({\\it J\\/}) \\to K\\sp+ + {\\it e}\\sp- + XY({\\it J\\/}-1)\\eqno(2)

\nThe n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process.\nThe essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\\eqalignno{&e\\sp- + \\rm XY \\to (XY)\\sp{-*}\\ or\\ \\to X\\sp- + Y&(1\\sp\\prime)\\cr &e\\sp- + \\rm XY({\\it J\\/}) \\to {\\it e}\\sp- + XY({\\it J\\/}-1)&(2\\sp\\prime)\\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.

The ionization of low-Rydberg-state He atoms in the n

^{1}P

state (n=14) in collisions with the polar molecules

{\mathrm{NH}}_{3}

{\mathrm{SO}}_{2}

, and acetone (

{\mathrm{C}}_{3}

{\mathrm{H}}_{6}

O) is carried out in a crossed-beam experiment at thermal kinetic energies. The dependence of the experimental ionization cross sections ${\ensuremath{\sigma}}_{i}^{\mathrm{expt}}$ on the relative velocity of the reactants is obtained by a time-of-flight method. We observe that it varies mainly as 1/

{v}_{r}^{2}

in a large part of the studied velocity range, while a 1/

{v}_{r}

velocity dependence is predicted by the free-electron model for the higher Rydberg levels.

The ionization of He atoms, prepared in a well-defined (n=14)

^{1}

P Rydberg state, in collisions with electron-attaching molecules like

{\mathrm{SF}}_{6}

and

{\mathrm{NO}}_{2}

, is studied in a crossed-beam experiment at thermal energies. The ionization cross section is observed to be large for

{\mathrm{SF}}_{6}

, reaching 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$

{\mathrm{cm}}^{2}

, and to be smaller for

{\mathrm{NO}}_{2}

, close to ${10}^{\mathrm{\ensuremath{-}}15}$

{\mathrm{cm}}^{2}

. The dependence of these two cross sections on the relative velocity of the reactants exhibits a characteristic behavior with a saturation at low velocities, a dependence that is completely different from that obtained with polar molecules. The observation of such a dependence for

{\mathrm{NO}}_{2}

indicates that, for this molecule, the reaction product is an ion pair, as for

{\mathrm{SF}}_{6}

, and that ionization is not due to rotational energy transfer even though

{\mathrm{NO}}_{2}

has a dipole moment. This dependence cannot be explained by the ``free-electron model'' usually used in Rydberg-atom--molecule collisions; it clearly shows that the interaction of the molecule with the ion core cannot be neglected.