A joint experimental and theoretical study of near-threshold electron-impact excitation of the 33S and 31S states in helium is reported. A high-resolution electron spectrometer is used to study integral cross sections in the energy region of the n = 3 − 5 negative-ion resonances. Photons are detected from the decay of the two states and the observed intensities are normalized to theoretical predictions using a new B-spline R-matrix (close-coupling) method that allows for non-orthogonal orbitals to improve the target description. Remarkable agreement between experiment and theory is demonstrated in both the overall energy dependence of the cross section and the fine details of a wealth of resonance structures. A detailed list of the resonances in the individual partial waves, along with their widths, is presented.

A low-energy crossed-beam electron spectrometer is used to investigate excitation of ammonia. Spectra obtained at low residual energies and large scattering angles in both constant residual energy mode and energy loss mode show excitation of two triplet states of ammonia, labelled a and b. A triplet b state is observed for the first time in the present work, while for the a state several lower, previously unobserved vibrational levels are detected. The threshold electron spectrum obtained by detecting zero residual energy electrons reveals a strong influence of threshold resonance in the excitation of he lowest Rydberg a state. Also a very large threshold peak in the excitation of the v=1 level of the symmetric stretch mode (v1) of the ground electronic state suggests a resonant process at very low energies.

We investigated effects of spatially separated laser fields on amplitudes and widths of electromagnetically induced transparency in M-type atomic system. We have shown narrowing of the electromagnetically induced transmission resonance and therefore enhancement of medium non-linearities that can be explained by diffusion-induced Ramsey narrowing. Depending on the polarization of two laser beams, narrow probe Hanle transmission resonance changes its sign in respect of the profile obtained when the pump laser beam is off.

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