Stark-PNC Experiments Cesium and Thallium

As mentioned already, experiments using highly forbidden Ml transitions in Cs and T1 were first suggested by Bouchiat and Bouchiat, and PNC measurements have been underway since then with Cs at Paris, and with T1 at Berkeley. Observations of PNC have been reported in both experiments. Other experiments with Cs began more recently at Zurich and at Michigan, but they have not yet reported results accurate enough to observe PNC. (See note added at end of this article.) 

In order to increase the transition rate for these forbidden transitions, all groups use an electric field E. In Section 3.1, we pointed out that the combination (a, x ) is the observable pseudoscalar, which is odd under parity. Here T/ = 17k [see Eq. (21)] is the circular polarization of the pump laser that excites the atom using the forbidden transitions enhanced by Stark mixing due to E. J, is the angular momentum polarization of the 

An additional distinction between ApNc and may be made by reversing the direction k of the pump beam. A is odd and ApNc is even under k reversal. All the distinctions can be understood by noting that A measures the scalar (fe x JB) j/. while ApNc measures the pseudoscalar 

As an example of these principles, the group in Paris working on cesium boosts the pump rate while simultaneously decreasing the size of A by using a multipass system for their pump beam. They put the cesium cell inside a cavity in which the pump beam makes about 50-70 double passes. The net direction of the pump approaches zero, reducing A by about a factor of 180 with no reduction in ApNc- This system also increases the net power in the pump beam therefore increasing the signal by that same factor. 

A second class of systematic effects can come from imperfect electric field reversal. For example, if there is some stray (nonreversing) electric field acting on the atoms, with a component parallel to the pump laser beam, then a portion of the detected polarization A can reverse with o, and E just like the PNC portion. Fortunately, this effect can be measured independently, as can other spurious electric field effects.  

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