s-wave threshold in electron attachment: Observations and cross sections inCCl4andSF6

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

“…Using energy-variable photoelectrons from VUV photoionization of rare gas atoms (energy range 0-160 meV, energy width 6-8 meV), Chutjian and Alajajian (1985) obtained clear evidence for s-wave behavior of the attachment cross-section for SF 6 and CCl 4 at low energies. They used the following analytical form for the attachment cross-section to describe the TPSA data over the range from 0 to 140 meV (see also Chutjian, 1992;Chutjian et al, 1996): It contains three fit parameters a, l, and and a normalization constant N TPSA ; the latter is determined by use of Eq.…”

“…31 we present a comparison of the LPA cross-section (normalized in absolute size to the thermal rate coefficient due to Petrovic´and Crompton, 1985) with the VW result for s-wave attachment (p-wave capture would reach its peak at an electron energy of about 330 meV and is neglected), with the fit (39) to the TPSA data (Chutjian and Alajajian, 1985) and with the empirical fit (40) of Klar et al (1992b). The fit (40) gives a good description of the LPA data up to the threshold for 1 ¼ 1 vibrational excitation where the cross-section exhibits a sharp downward cusp, predicted theoretically by Gauyacq and Herzenberg (1984) and confirmed by Klar et al (1992a, b).…”

“…The yield is proportional to the absolute cross-section e (E) ¼ NY e (E) for anion formation due to free electron attachment. The normalization constant N is conveniently determined with reference to known thermal energy attachment rate coefficients k e (T) (Smith and Sˇpanel, 1994;Christophorou, 1996) using the expression (Chutjian and Alajajian, 1985;Klar et al, 1992bKlar et al, , 2001aChutjian et al, 1996;Schramm et al, 1999Schramm et al, , 2002 …”

Attachment of Electrons to Molecules at meV Resolution

“…So far, the most advanced experiments on threshold attachment of free electrons have been carried out by Chutjian and colleagues (Chutjian et al 1984(Chutjian et al , 1985(Chutjian et al , 1987Chutjian 1992), who used VUV photoionisation of Kr atoms above the 2P1/2 threshold to generate photoelectrons of defined and variable energy. With an effective electron energy width of 4-8 meV (FWHM) , they studied relative attachment cross sections for a variety of molecules in the range 0-150 me V and produced absolute cross sections by normalisation to rate constants obtained in swarm experiments (Chutjian and Alajajian 1985).…”

“…With an effective electron energy width of 4-8 meV (FWHM) , they studied relative attachment cross sections for a variety of molecules in the range 0-150 me V and produced absolute cross sections by normalisation to rate constants obtained in swarm experiments (Chutjian and Alajajian 1985). For several molecules Chutjian et al (1984Chutjian et al ( , 1985Chutjian et al ( , 1987 interpreted a strong peak in their measured negative ion yields close to zero energy as evidence for, and to be compatible with, the E-1 / 2 rise expected for s-wave attachment at energies ;:; 10 meV.…”