It is shown that laser generation is possible in principle upon coherent excitation according to the V-scheme of the (1s2p)3 P j levels with j = 1, 2 of a helium atom from the (1s2s)3 S 1 level of the same atom when the latter level is, in turn, populated by electron or proton impact.
We have calculated the excitation spectra of a single electron in the excited state of a four-level atom interacting with three laser fields at low intensities, where two of them are on two-photon resonance with the three levels of the atom, while the third laser is a weak one operating between the fourth and excited level of the atom. It is shown that light is switched off at certain frequency due to the coherent population trapping while light is turned on at another frequency, where stimulated emission occurs leading to light amplification without population inversion.
Spectroscopy of helium is discussed in the context of methods to determine the fine structure constant α. Resonances corresponding to the fine structure separation are observed in the coherent excitation of a 4 He metastable beam by two laser fields. A quantitative analysis of the lineshapes is performed for the three level model ("V" scheme). A comparison with similar Doppler free line-center determinations from saturation spectroscopy is discussed. This work provides a modern demonstration in the field of coherent phenomena pioneered by George Series.
Stimulated radiative recombination has been demonstrated experimentally in and collisions using a merged electron - ion beams apparatus with field ionization detection of the excited neutral products. Enhancement of the recombination over spontaneous recombination to form the n=11, 12 and 13 levels of atomic hydrogen and the n = 11 and 12 levels of atomic helium by factors of between 1000 and 3000 have been found using a laser power of 8 W. Evidence for the resolution of fine-structure levels has been seen for the case of helium.
The collision of a proton with a hydrogen atom in the presence of electromagnetic radiation which is nearly resonant with the atomic levels is investigated theoretically. The non-perturbative quasi energy method is used to describe the laser-atom interaction and the proton-dressed atom collision is solved using a diagonalization technique. We have calculated the transition probabilities and the total cross-section for various transitions. We discuss the influence of various laser and collision parameters on the excitation process.