Microwave ionization circularly polarized microwaves

While ionization by linearly polarized fields has been well studied, there is only one report of ionization by a circularly polarized field, the ionization of Na by an 8.5 GHz field.36 In the experiment Na atoms in an atomic beam pass through a Fabry-Perot microwave cavity, where they are excited to a Rydberg state using two pulsed tunable dye lasers tuned to the 3s — 3p and 3p —> Rydberg transitions at 5890 A and —4140 A respectively. The atoms are excited to the Rydberg states in the presence of the circularly polarized microwave field which is turned off 1 fis after the laser pulses. Immediately afterwards a pulsed field is applied to the atoms to drive any ions produced by microwave ionization to a microchannel plate detector. To measure the ionization threshold field the ion current is measured as the microwave power is varied. 

A much higher circularly polarized field is required to ionize the atoms than a linearly polarized field, as shown by Fig. 10.21, a plot of the threshold fields, where 50% ionization occurs, for linearly and circularly polarized 8.5 GHz fields. As shown by Fig. 10.21, the circularly polarized microwave ionization threshold field is very nearly E = l/16n4, the same as the the static field required to ionize a Rydberg Na atom and much higher than the field required for ionization by... 

Fig. 10.21 Ionization threshold fields for linear (+) and circular ( ) polarization as a function of n when Na atoms are excited in the 8.5 GHz microwave field (from ref. 36).

The second kind of cavity which has been used is Fabry-Perot cavity [10]. These cavities are completely tunable and have the advantage that they are open, allowing far better access to the atoms than the closed cavity shown in Fig. 1. The improved access was critical for measurements of angular distributions of the electrons ejected in microwave ionization [11]. Their cylindrical symmetry is useful for measurements involving circularly polarized microwaves, but a closed cylindrically symmetric cavity is equally good [12]. 

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