Physical Review A

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

C. Ronge, A. Pesnelle, M. Perdrix, G. Watel

Published Nov 1, 1988• 8 citations

Abstract

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.

DOI: 10.1103/physreva.38.4552

Related papers

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

Ionization of low Rydberg He atoms by rotational deexcitation of N H3

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

Physical Review ADec 1, 19869 citations

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

Xiaohui Ling

Jan 1, 19930 citations

Ionization of Rydberg atoms in thermal collisions with polar molecules

Toshizo Shirai, Hiroki Nakamura

Physical Review ANov 1, 19875 citations

Ionization of low-Rydberg-state He atoms by dipole-forbidden rotational deexcitation of NH3

Toshizo Shirai, Hiroki Nakamura

Physical Review ADec 1, 19890 citations

Electron Impact Ionization of Sodium Rydberg Atoms below 2 eV

Kannadaguli Nagesha, Keith B. MacAdam

Physical Review LettersSep 12, 200318 citations

Physical Review A

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

C. Ronge, A. Pesnelle, M. Perdrix, G. Watel

Published Nov 1, 1988• 8 citations

Publisher

Abstract

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.

DOI: 10.1103/physreva.38.4552

Related papers

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

Ionization of low Rydberg He atoms by rotational deexcitation of N H3

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

Ionization of excited helium atoms in $n^{1}P(14\ensuremath{\le}n\ensuremath{\le}16)$ states due to collisions with the polar molecules N ${\mathrm{H}}_{3}$ has been studied in a thermal energy crossed-beam experiment. A large ionization signal from these low Rydberg states is observed even though there is insufficient energy available from rotational $\ensuremath{\Delta}J=\ensuremath{-}1$, $\ensuremath{\Delta}K=0$ relaxation transitions of the molecule. In order to explain our observations, large $|\ensuremath{\Delta}J|$ transitions must be postulated.Ionization of excited helium atoms in $n^{1}P(14\ensuremath{\le}n\ensuremath{\le}16)$ states due to collisions with the polar molecules N${\mathrm{H}}_{3}$ has been studied in a thermal energy crossed-beam experiment. A large ionization signal from these low Rydberg states is observed even though there is insufficient energy available from rotational $\ensuremath{\Delta}J=\ensuremath{-}1$, $\ensuremath{\Delta}K=0$ relaxation transitions of the molecule. In order to explain our observations, large $|\ensuremath{\Delta}J|$ transitions must be postulated.

Physical Review ADec 1, 19869 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 Rydberg atoms in thermal collisions with polar molecules

Toshizo Shirai, Hiroki Nakamura

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.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.

Physical Review ANov 1, 19875 citations

Ionization of low-Rydberg-state He atoms by dipole-forbidden rotational deexcitation of NH3

Toshizo Shirai, Hiroki Nakamura

A theoretical study of the ionization of He(n=14 $^{1}P$ ) atoms in thermal collisions with ${\mathrm{NH}}_{3}$ was carried out within the semiquantal approximation. The charge-dipole Glauber differential cross section was employed for the electron-molecule rotational deexcitation process. Calculated thermally averaged cross sections show quite a strong n dependence, especially for n\ensuremath{\le}17. A comparison is made between the present results and recent experiments.A theoretical study of the ionization of He(n=14 $^{1}P$) atoms in thermal collisions with ${\mathrm{NH}}_{3}$ was carried out within the semiquantal approximation. The charge-dipole Glauber differential cross section was employed for the electron-molecule rotational deexcitation process. Calculated thermally averaged cross sections show quite a strong n dependence, especially for n\ensuremath{\le}17. A comparison is made between the present results and recent experiments.

Physical Review ADec 1, 19890 citations

Electron Impact Ionization of Sodium Rydberg Atoms below 2 eV

Kannadaguli Nagesha, Keith B. MacAdam

We have observed electron impact ionization of highly excited sodium Rydberg atoms in $ns$ and $nd$ states, $n=35--51$ , below $E=2\text{ }\text{ }\mathrm{e}\mathrm{V}$ electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from ${\ensuremath{\sigma}}_{35d}\ensuremath{\approx}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ to ${\ensuremath{\sigma}}_{50d}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{2}$. The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and ${\ensuremath{\sigma}}_{n}$ follows a power law in $n$ . The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.We have observed electron impact ionization of highly excited sodium Rydberg atoms in $ns$ and $nd$ states, $n=35--51$, below $E=2\text{ }\text{ }\mathrm{e}\mathrm{V}$ electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from ${\ensuremath{\sigma}}_{35d}\ensuremath{\approx}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ to ${\ensuremath{\sigma}}_{50d}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{2}$. The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and ${\ensuremath{\sigma}}_{n}$ follows a power law in $n$. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.

Physical Review LettersSep 12, 200318 citations

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.

Ionization of excited helium atoms in $n^{1}P(14\ensuremath{\le}n\ensuremath{\le}16)$ states due to collisions with the polar molecules N

{\mathrm{H}}_{3}

has been studied in a thermal energy crossed-beam experiment. A large ionization signal from these low Rydberg states is observed even though there is insufficient energy available from rotational $\ensuremath{\Delta}J=\ensuremath{-}1$, $\ensuremath{\Delta}K=0$ relaxation transitions of the molecule. In order to explain our observations, large $|\ensuremath{\Delta}J|$ transitions must be postulated.

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.

A theoretical study of the ionization of He(n=14

^{1}P

) atoms in thermal collisions with

{\mathrm{NH}}_{3}

was carried out within the semiquantal approximation. The charge-dipole Glauber differential cross section was employed for the electron-molecule rotational deexcitation process. Calculated thermally averaged cross sections show quite a strong n dependence, especially for n\ensuremath{\le}17. A comparison is made between the present results and recent experiments.

We have observed electron impact ionization of highly excited sodium Rydberg atoms in

ns

and

nd

states,

n=35--51

, below

E=2\text{ }\text{ }\mathrm{e}\mathrm{V}

electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from ${\ensuremath{\sigma}}_{35d}\ensuremath{\approx}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ to ${\ensuremath{\sigma}}_{50d}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{2}$. The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and ${\ensuremath{\sigma}}_{n}$ follows a power law in

n

. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.