7 - Pulsed field ionization

Summary

Because it can be efficient and selective, field ionization of Rydberg atoms has become a widely used tool. Often the field is applied as a pulse, with rise times of nanoseconds to microseconds, and to realize the potential of field ionization we need to understand what happens to the atoms as the pulsed field rises from zero to the ionizing field. In the previous chapter we discussed the ionization rates of Stark states in static fields. In this chapter we consider how atoms evolve from zero field states to the high field Stark states during the pulse. Since the evolution depends on the risetime of the pulse, it is impossible to describe all possible outcomes. Instead, we describe a few practically important limiting cases.

Although we are not concerned here with the details of how to produce the pulses, it is worth noting that several different types of pulse, having the time dependences shown in Fig. 7.1, have been used. Fig. 7.1(a) depicts a pulse which rises rapidly to a plateau. Atoms in a fast beam experience this sort of pulse when passing into a region of high homogenous field. Fig. 7.1(b) shows a rapidly rising pulse which decays rapidly after reaching its peak. While not elegant, such pulses are easily produced. For pulse shapes such as those of Figs. 7.1 (a) and (b) the ability to discriminate between different states comes mostly from adjustment of the amplitude of the pulse.