[CT1.001] Electron-molecular ion interaction studied in storage rings
Mats Larsson (Department of Physics,
Stockholm University, Box 6730, S-113 85 Stockholm, Sweden)
Ion storage rings have emerged as an excellent tool for the study of
electron-molecular ion interaction. Molecular ions are stored at Mev
energies in the ring for tens of seconds, which effectively removes
vibrational excitations. They are allowed to interact with a cold electron
beam in one section of the ring. Neutral or charged particles arising from
the interaction of the stored ions with the electrons are detected. The
collision energy is controlled by the cathode voltage of the electron gun.
In Stockholm, the heavy-ion storage ring CRYRING is used to study
molecular ions. Dissociative electron-molecular ion recombination will be
described, and studies of molecular oxygen and water ions will used as
examples.
[CT1.002] Preliminary observations of
the dissociative recombination reaction of He^+_2 from the v=4 vibrational
level.
Kenneth Hardy, I. Boniche, L. Coman, M.
Faxas, L. Simons (Physics Department, Florida International University,
Miami, Fl 33199)
Measurements of the velocities of the final
product state atoms from the dissociative recombination (DR) reaction of
the He^+_2 molecular ion in the v=4 vibrational state have been made. The
reaction populated the 1s3s and 1s3p atomic levels at 184864.9 cm^-1 and
185564.6 cm^-1. We observed DR of He_2^+ from different rotational states
of the v=4 vibrational level. Using TOF techniques we determined the
velocities of the final product atoms and from this directly measured the
kinetic energy released in the DR reaction from which the rotational
energies can be determined. Using the energies of the rotational states
F_v(J) and the expression, F_v(J)=B_vJ(J+1)+D_vJ^2(J+1)^2, the rotational
constants B_v and D_v of the v=4 level of the molecular ion were
determined. Here, new experimental results from studies of the DR reaction
in helium are presented and the preliminary results for the rotational
constants are B_v = 6.11\pm0.2 cm^-1 and D_v = 9.5 x 10^-5\pm0.0004 cm^-1
for the v=4 vibrational level.
[CT1.003] Electron Scattering Processes from Plasma Processing Gases:
CF3I, C3F8 and C4F8
Mineo Kimura (Graduate School of Science
and Engineering, Yamaguchi University)
Brief review of the current level of
understanding of electron scattering processes from plasma processing
gases such as CF3I, C3F8 and C4F8 will be presented. Experimental as well
as theoretical studies for these gases have been carried out last a few
years by several groups, and although the amount of cross-section data is
still far from sufficient, we are able to establish the data set for some
processes in these gases. I will briefly discuss possible next-generation
processing gases.
[CT1.004] Dissociative Electron Attachment in Chlorofluoromethanes
Paul Burrow, K. Aflatooni (University of
Nebraska-Lincoln)
The total dissociative electron attachment
(DEA) cross sections of the chlorofluoromethanes are determined in an
electron beam experiment and correlated with the vertical attachment
energies (VAE) for formation of the lowest temporary negative ion states
of these compounds. The latter are determined independently by electron
transmission spectroscopy (ETS) and correspond to the energies of the
anions at the equilibrium geometries of the neutral molecules. As we
observed previously in the chloroalkanes (K. Aflatooni and P.D. Burrow, J.
Chem. Phys. in press, July 2000.), the peak DEA cross sections are well
correlated with VAE. For values of VAE between 0.9 to 3.0 eV, the peak DEA
cross sections vary by more than four orders of magnitude. The correlation
is attributable to the remarkably monotonic variation of the temporary
anion widths, arising from their finite lifetimes, over this range of VAE.
[CT1.005] Dissociative electron attachment process to CF_3 radicals
D. Hayashi, G.M.W. Kroesen (Eindhoven
University of Technology)
To elucidate the electron attachment
process to CF_3 radicals, electronic and geometrical structures of CF_3^-
and its parent CF_3 radicals were calculated by GAUSSIAN 98 package. All
computations were carried out by Hartree-Fock method with the standard
6-311++G^** basis. The geometrical structures were optimised by the
second-order Moller-Presset perturbation theory. When an low-energy
electron (<5 eV) is captured by the ground-state CF_3(C_3v), it is
plausible that they first form a negative ion state (denoted as CF_3^-*)
with the same geometry (bond length of C-F (r_CF) and angle FCF and
symmetry as those of CF_3, and then CF_3^-* changes its geometry. The
potential energy of CF_3, CF_3^-* and the ground-state negative ion
CF_3(C_3v) were examined by scanning r_CF. The potential energy of CF_3^-*
at the equilibrium r_CF of CF_3 is the same as that of CF_3, while that of
CF_3^- is 0.5 eV lower than that of CF_3. The potential energy surface of
dissociation channel CF_3^-\toF^-(^1S)+CF_2(^1A_1) was calculated as a
function of C-F^- (r_CF-). The dissociation state F^-+CF_2 locates a few
tens meV higher to the equilibrium state of CF_3. These results imply that
the attachment of low-energy electrons (\sim1 eV) results in the formation
of CF_3^-*. Since the total energy of the CF_3^-* is slightly exothermic
against the dissociation, it automatically dissociates into F^-+CF_2.
[CT1.006] Electron Interactions with CF_3I
Loucas Christophorou, James Olthoff (NIST)
CF_3I is a potential plasma processing gas that has a short environmental
lifetime and that produces copious quantities of CF_3^+ and CF_3 in low
temperature plasmas. Low-energy electron collision data for CF_3I are
sparse. Limited cross section data are available only for total and
differential elastic electron scattering, electron-impact ionization, and
electron attachment. These data have been synthesized and assessed and
will be presented and discussed. There is a need for further scrutiny of
these data and for measurements of the cross sections of all other main
electron-collision processes. There is a need also for measurements of the
electron transport, ionization, and attachment coefficients of this
molecule. |