Selective field ionization

Selective field ionization (SFI) of Rydberg atoms has been used to observe radiofrequency transitions of both and The selectivity 

At n = 20, EjE = 1%, a usable difference. Even greater sensitivity exists 

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Because it can be efficient and selective, field ionization of Rydberg atoms has become a widely used tool.1 Often the field is applied as a pulse, with rise times of nanoseconds to microseconds,2"4 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. 

The second approach is to use thermal beams of alkali atoms as shown in Fig. 10.2.4 A beam of alkali atoms passes into a microwave cavity where the atoms are excited by pulsed dye lasers to a Rydberg state. A1 /zs pulse of microwave power is then injected into the cavity. After the microwave pulse a high voltage pulse is applied to the septum, or plate, inside the cavity to analyze the final states after interaction with the microwaves. By adjusting the voltage pulse it is possible to detect separately atoms which have and have not been ionized or to analyze by selective field ionization the final states of atoms which have made transitions to other bound states. 

The other commonly used technique is selective field ionization. The atoms can be in a beam or in a cell of the type shown in Fig. 11.4, in which the detector is in a separately pumped region connected to the interaction region, which may have a high pressure, 10 3torr, of added gas.27 The basic principle is to use the known... 

Fig. 13.5 Adiabatic and diabatic selective field ionization (SFI) for -changed ensembles produced from Na 39p, 40s, 39d, and 40p states by 43 eV Na+ impact. The adiabatic peaks occur at 170-180 V/cm, and the diabatic features occur above 250 V/cm (note change of vertical scale). The diabatic SFI from -changed 50s targets most closely resembles that from 39d. In contrast, 40p and 39p targets yields SFI that indicates a different distribution of Stark sublevels lying high in the n = 39 and 38 manifolds, respectively (from ref. 10).

Fig. 13.13 Typical selective field ionization data for laser excited Na 50d atoms (a) data with electron beam gated off ( ), data following collisions with 25 eV electrons (+) corrected for electron-induced background signals (b) net signal due to electron impact. The horizontal bars indicate the range of field strengths over which n = 50 atoms are expected to ionize adiabatically and diabatically (from ref. 36).

A way of doing microwave spectroscopy peculiar to the study of Rydberg atoms is to use selective field ionization to discriminate between the initial and final states of the microwave transition. An example of the application of this technique is the measurement of millimeter wave intervals between Na Rydberg states by Fabre et a/.13 using the arrangement shown in Fig. 16.5. 

A good example of the use of the electric resonance technique is the measurement of the Na nd fine structure intervals and tensor polarizabilities.38 These transitions were observed using selective field ionization, although they appear to be unlikely prospects for field ionization detection because of the small separations of the levels, 20 MHz. The nd3/2 states were selectively excited from the 3p1/2 state in a small static electric field and the = 0 transitions to the nd5/2 states induced by a... 

Fig. 16.10 Quantum beat signals of high lying 2D states of Na obtained by time resolved selective field ionization. The variation of the beat frequency with principal quantum number is shown. Several quantum beat frequencies appear due to a Zeeman splitting of the fine structure levels in the earth s magnetic field (from ref. 43).

The second technique, selective field ionization, was used by Gentile et al.H to measure many intervals between low states of Ca. In their experiments the laser excitation, microwave transition, and detection were separated spatially as well as temporally, an approach similar to the one of Goy et al.9 described in Chapter 16. 

The experiments are done by exciting Na atoms in an atomic beam to the 17s state, by the route 3s —> Sp —> 17s, using two 5 ns dye laser pulses. The atoms are allowed to collide for 3 ps, and then the final states are analyzed by selective field ionization. In particular, atoms in the 17p state are detected as the tuning electric field is slowly swept over many shots of the laser. In Fig. 2 we show the resonances observed in this way. There are four resonances corresponding to the two possible rni values of the up states, mi = 0 and 1. It is convenient to label the four resonances by the m values of the upper and lower p states, e.g., (0,0) has m = 0 in both. 

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