A joint experimental and theoretical study of low-energy electron collisions with methyl iodide molecules with emphasis on dissociative attachment (DA) over the range 0-170 meV has been carried out. Below the onset for the symmetric C-I stretch (ν 3 = 1) vibrational excitation, a prominent narrow peak is observed followed by a cusp at threshold. On approach to the ν 3 = 2 onset, the DA cross section shows a rapid drop terminated by a cusp at threshold. The experimental findings are interpreted by an R-matrix calculation which includes non-local effects and involves realistic model potentials for the neutral molecule and negative ion. Mediated by the combined effects of dipolar and polarization attraction in the electron CH 3 I interaction, the peak below the ν 3 = 1 onset is identified as a vibrational ('nuclear-excited') Feshbach resonance which so far may represent the clearest example of its kind. The higher vibrational onsets are free of such resonances. The effects of initial vibrational excitation are discussed and found to be significant. Predictions are made for elastic (σ -wave) and vibrationally inelastic cross sections and structure at the ν 3 = 1 onset is discussed. Absolute DA cross sections and DA rate coefficients for Maxwellian electron gases with variable electron temperature (for the fixed gas temperature T G = 300 K) are presented.

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The effect of the polarization of the atomic core by the outer electron on near threshold photoionization of excited alkali atoms Ak(np) (Ak = Na-Cs; n = 3−6) is investigated. Partial and total cross-sections for photo-ionization of the np-electron were computed utilizing the configuration interaction technique with Pauli-Fock atomic orbitals (CIPF) and including the long range core polarization potential (CP). To calculate the core polarization potential the variational principle is applied. Comparison with previous theoretical results and with available experimental data is made for the total cross-section σ, for the electron angular distribution parameter β, for the ratio ν = |D d /Ds| of the reduced electric dipole matrix elements and for the phase shift difference ∆ = δ d − δs, associated with the d-wave and s-wave continua, respectively. In the comparison, new experimental results for σ, ν, and ∆, measured for laser-excited, polarized 39 K(4p 3/2 ) atoms, have been included.PACS. 32.80.Fb Photoionization of atoms and ions -33.60.-q Photoelectron spectra -31.50.+w Excited states

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Using transverse resonant two-photon, single-mode laser excitation of metastable 40 Ar(4s 3 P 2 ) atoms in a collimated beam in combination with a calibrated travelling Michelson interferometer involving digital fringe interpolation, we have measured the energies for the Ar(4p, J = 3 → nd, J = 4) transitions with principal quantum numbers n = 12-100 with a relative uncertainty of 8 × 10 −8 . The Rydberg atoms are formed in a region of low electric and magnetic fields and detected by electron transfer to SF 6 molecules in a skimmed supersonic beam. A single-channel quantum defect analysis of the experimental data is performed, revealing weak perturbations of the n = 12 and 20 levels by interaction with the Ar( 2 P 1/2 7g , J = 4) and Ar( 2 P 1/2 8g , J = 4) levels, respectively. The value for the ionization energy of the 40 Ar + ( 2 P 3/2 ) limit has been determined as 21 647.076(2) cm −1 relative to the 40 Ar(4p, J = 3) level and as 127 109.836(3)(±0.05) cm −1 relative to the ground state of 40 Ar (see also note added in proof). Low-lying autoionizing ng , J = 4 resonances (n = 9-13) have been investigated and their quantum defects and reduced width determined for the first time. A multichannel quantum defect theory analysis yields an estimate (0.76 cm −1 ) for the energy separation between the perturbed 20d level and the perturbing 8g level, predicts the transition intensity to the 8g level to be much weaker than that to the 20d level and shows that the g -g coupling in the autoionization region is much stronger than the g -d coupling determined from the 8g -induced perturbation in the discrete nd, J = 4 spectrum.