Microwave excitation and ionization

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. 

As we shall see, microwave ionization can be thought of as a multiphoton absorption or as a process driven by a time varying field. We first discuss microwave ionization of alkali atoms, which can be described by the notions used to describe pulsed field ionization. To show the connection between the time varying field point of view and the photon absorption point of view we then discuss 

The experiment is done using the apparatus shown in Fig. 10.2. Two tunable dye lasers are used to excite K atoms in a beam to the (n + 2)s state. A static field can be applied to vary the separation between the 18s and (16,k) states. The atoms are exposed to a microwave pulse, and subsequently to a field ionization pulse which ionizes atoms in the (n,k) Stark state, but not those in the (n + 2)s state. The (n,k) field ionization signal is then monitored as either the microwave field amplitude or the static field is swept over many shots of the laser. To the extent that both Stark shifts are linear, the static field alters the energy separation between the two states, but not their wavefunctions. 

The similarity of the K (n + 2)s — (n,k) transitions to the n— n + 1 transitions of microwave ionization is verified by measuring the number of atoms making the former transition as a function of the microwave field amplitude.10 As the 

Sweeping the static field, which alters the energy spacing between the levels, while keeping the microwave field fixed leads to the observation of resonant multiphoton transitions. An example, shown in Fig. 10.9, is the set of the 18s— (16,k) transitions observed by sweeping the static field with different strengths of the 10.35 GHz microwave field.8 The sequence of 18s— (16,3) transitions 25 V/cm apart is quite evident. At the top of Fig. 9 there is a scale in terms of the number of 10.35 GHz photons required to drive the 18s— (16,3) transition. 

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