Abstract

The Coulomb field of an electron passing an atom generates a transverse electric field corresponding to a half cycle of electromagnetic radiation and a longitudinal field corresponding to a single cycle pulse. These fields induce transitions within the atom. The power spectrum of the radiation of a single-cycle pulse is very broad. The field of the passing electron can induce transitions in the atom to all states for which the energy difference to the initial state, ΔE, is within the generated power spectrum. As a result, collisional excitation lacks selectivity in the population of the excited states. We investigate how, in such a collisional process, the transfer to an excited state can be enhanced. In our search for a scheme to control the excited state population we are inspired by the success of controlled photoexcitation using narrow-band lasers, having many oscillations of the e.m. radiation field. We investigated [1] if enhanced excitation of a selected state is also possible for electron collisions and thus turn the (inefficient) into controlled excitation to a selected final state. To this end we consider the impact of a train of electron pulses with a repetition frequency v. As in the optical case, this will induce a series of N broadband bursts of radiation spectrum. As can be seen in the figure the analogy with photons holds and N electron bursts spaced with time τ_t will also favour transitions with frequency v=1/τt.

© 2000 IEEE