Laser Photoelectron Attachment to Molecules in a Skimmed Supersonic Beam: Diagnostics of Weak Electric Fields and Attachment Cross Sections Down to20μeV

Abstract

Combining a tunable single mode laser -atomic beam photoelectron source with a skimmed supersonic beam target we have carried out ultrahigh resolution studies of threshold electron attachment to SF 6 molecules. Monitoring SF 6 2 formation around the onset for field ionization of high n Rydberg atoms, residual electric fields are diagnosed and reduced to levels around 0.01 V͞m, allowing the first study of free electron attachment cross sections s͑E͒ for energies down to about 20 meV at energy widths as low as 20 meV. The present results for SF 6 2 production conclusively demonstrate the convergence towards the limiting s-wave attachment behavior s~E 21͞2 at energies below 1 meV.[S0031-9007(98)06711-8] PACS numbers: 34.80.Lx, 34.60. + z, 82.30.Fi The attachment of slow electrons to molecules(1) with formation of long-lived negative ions XY 2 or dissociated products X 2 1 Y (dissociative attachment) is an important process in gaseous dielectrics [1,2]. Many halogen-containing molecules such as SF 6 and CCl 4 exhibit very large electron attachment cross sections at low energies, and it is of both fundamental and applied interest to study the threshold behavior ͑E ! 0͒ with high resolution. For sufficiently short-ranged interaction potentials ͓V ͑r͒~r 2m , m . 2͔, capture of particles with orbital angular momentum L is theoretically predicted to exhibit a threshold energy dependence s͑E͒~E L21͞2 [3,4]. For s-wave attachment ͑L 0͒ one therefore expects a diverging cross section s͑E; L 0͒~E 21͞2ỹ 21 , as also observed for neutron capture by nuclei at low velocities y [3]. Using a vacuum ultraviolet photoelectron attachment technique with an energy resolution around DE ഠ 6 meV (FWHM) Chutjian and coworkers provided evidence for s-wave attachment to occur in several systems including SF 6 and CCl 4 [2,5]. Electron transfer studies with laser excited, state selected Rydberg atoms A ‫ءء‬ ͑nl͒, most notably those by Dunning and colleagues [6,7], demonstrated that the rate coefficient for Rydberg electron attachment ͓ f nl ͑y͒ velocity distribution of nl Rydberg electron͔is essentially constant at high principal quantum numbers ͑n . 40͒ for several molecules including SF 6 and CCl 4 as expected for s-wave attachment. Using a novel laser photoelectron attachment (LPA) method, Klar et al. investigated free electron attachment processes with sub-meV resolution [8][9][10][11]. Their results clearly indicated that the limiting E 21͞2 dependence is reached only at very low energies ͑#1 meV͒; moreover, they detected cusp structures at onsets for vibrational excitation [8,9].At these low energies the major problem is associated with the influence of residual electric fields in the reaction volume. Klar et al. [9,10]-using time-delayed ion extraction following pulsed electron attachment at high repetition rate-carried out model calculations for the effective energy dependence of negative ion formation, taking into account the effects of a homogeneous residual field (of fitted strength) on the electron trajectories. In this way they...

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

“…(40) (Klar et al, 1992b) and ¼ 0.59(6) for CCl 4 (Hotop et al, 1995;Klar et al, 2001a), in both cases distinctly larger than the prediction obtained from the Klots formula (21), namely K ¼ 0.228 for SF 6 and K ¼ 0.299 for CCl 4 . For SF 6 , Schramm et al (1998) measured the attachment yield at residual electric fields as low as 0.01 V/m and negligible laser bandwidth for electron energies from 10 meV down to 20 meV; they confirmed the results of Klar et al (1992b) for the parameter . The limiting LPA rate coefficient k e (E !…”

“…Subsequently, Klar et al (1992aKlar et al ( , b, 2001a, Hotop et al (1995) and Schramm et al (1998) used the laser photoelectron attachment (LPA) method to investigate anion formation from these two molecules over a similar energy range, but with substantially reduced energy width (below 1 meV). As an important ingredient and improvement over previous work, they analyzed the effects of residual electric fields (reduced to values below 1 V/m) on the near-threshold attachment yield through model calculations of the attachment yield (Klar et al, 1992b;1994a, b;2001a, b;Schramm et al, 1998).…”

“…As an important ingredient and improvement over previous work, they analyzed the effects of residual electric fields (reduced to values below 1 V/m) on the near-threshold attachment yield through model calculations of the attachment yield (Klar et al, 1992b;1994a, b;2001a, b;Schramm et al, 1998). In the spirit of the s-wave capture formula (21) due to Klots they used the analytical cross-section…”

Dissociative electron attachment to C2F5 radicals

“…With an electron beam resolution of ∼20 μeV it has been experimentally demonstrated for SF 6 that from <10 μeV to ∼10 meV the s-wave electron attachment cross section, σ , follows the theoretically predicted σ ∝ E −1/2 form. 41 Thus, swave attachment cross sections rise as the energy decreases to theoretically infinite values at zero energy. By contrast, it has also been observed experimentally, in dissociative electron attachment to Cl 2 , that from ∼1 to 50 meV the p-wave attachment cross section follows the theoretically predicted σ ∝ E 1/2 form.…”

On the accuracy of thermionic electron emission models. I. Electron detachment from SF6−

“…This leads to the reduced energy scale i = 1.80͓͑E − E 0i ͒ / eV͔ 1/2 . Suggesting 15 that both the experimental nondissociative attachment cross sections [24][25][26] and the thermal detachment rate constants 15,27,28 for SF 6 − are not only determined by pure electron capture but also include contributions from IVR, we then modify P͑ i ͒ by an additional, empirical, factor P IVR ͑ i ͒. In Ref.…”