Berlin 2001 – scientific programme
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AMPD: EPS AMPD
AMPD 5: Sitzung 5
AMPD 5.2: Talk
Wednesday, April 4, 2001, 09:35–10:10, H105
VIBRATIONAL FESHBACH RESONANCES IN COLLISIONS OF ELECTRONS WITH MOLECULES AND CLUSTERS — •H. Hotop1, E. Leber1, J. M. Weber1, A. Schramm1, S. Barsotti1, M.-W. Ruf1, and I.I. Fabrikant2 — 1Fachbereich Physik, Universität Kaiserslautern, D-67663 Kaiserslautern, Germany — 2Dept of Physics and Astronomy, Univ. Nebraska, Lincoln, NE 68588-0111, USA
Temporary negative ion states XY−, formed in low energy electron collisions with molecules XY, are crucial for vibrational excitation as well as for formation of anions X− through dissociative attachment (DA). Both shape resonances (electrons trapped within a centrifugal barrier) and Feshbach resonances (electrons attached to electronically excited states XY* of the neutral molecule) are representatives of such temporary negative ion states, typically located at incident electron energies of 1 to 10 eV. It has recently become evident that at very low energies (in the range of vibrational onsets) another type of resonance can occur which is mediated by attractive long-range interactions in the electron-target system and may exist without the supporting influence of a centrifugal barrier, i.e. for s–wave electrons. These resonances are found just below vibrational onsets of the molecule in its electronic ground state and are therefore addressed as vibrational Feshbach resonances (VFR [1], replacing the term nuclear–excited FR [2,3]). In cross sections for elastic and vibrationally inelastic scattering VFRs show up as narrow features (typically dips), as experimentally observed, e.g., for XY = HF [3,4]. Using laser photoelectron attachment (resolution 1 meV, energy range 0-200 meV), a prominent VFR was observed in DA to methyl iodide molecules just below the onset for excitation of one quantum of the C–I stretch vibration (ν3 = 1) and interpreted through R–matrix calculations [1]. Quite unexpectedly, very narrow VFRs were recently found in electron attachment to molecular clusters of N2O [5] and CO2 [6]. In the yield for N2O cluster anions VFRs as narrow as 2.3 meV were detected with locations slightly red–shifted from vibrational excitation energies. In contrast the widths of the VFRs, observed in the yield for CO2 cluster anions, are distinctly broader and the redshifts per added molecule more than 10 times larger than for N2O clusters. These findings are compatible with a simple model which describes the VFRs as bound states in an isotropic electron–cluster potential, using the known long–range polarization attraction outside the cluster and an average short range interaction within the cluster which is around -0.5 eV (i.e. attractive) for CO2 clusters and about +0.2 eV for N2O clusters, in qualitative agreement with what is known on the short range interaction of electrons with the two individual molecules [6]. In addition, findings for other molecules and molecular clusters (including oxygen [7]) will be briefly addressed.
This work has been supported by the Deutsche Forschungsgemeinschaft and by the Laserzentrum at Universität Kaiserslautern.
[1] A. Schramm et al., J. Phys. B32, 2153 (1999).
[2] J. Gauyacq, A. Herzenberg, Phys. Rev. A 25, 2959 (1982).
[3] G. Knoth et al., Phys. Rev. Lett. 62, 173 (1989).
[4] A.-C. Sergenton et al., Phys. Rev. A 61, 062702 (2000).
[5] J. M. Weber et al., Phys. Rev. Lett. 82, 516 (1999).
[6] E. Leber et al., Eur. Phys. J. D 12, 125 (2000).
[7] S. Matejcik et al., J. Chem. Phys. 111, 3548 (1999).