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distribution of atomic velocities in the beam causes a spread (of ≈ 2 ms) in the time of flight between the in- teraction region and the probe region. This, along with the finite bandwidth of the PD, leads to a reduction of the signal-modulation contrast (see below). The choice of the modulation frequency of 76.2 Hz is a tradeoff be- tween this contrast degradation and the frequent E-field reversal. 

In Fig. 7 a profile of the B-field-split 408-nm spectral line of the $^{174}$Yb is shown. The 649-nm-light-induced flu- orescence was recorded during a single profile scan. Sta- tistical error bars determined directly from the spread of data are smaller than the points in the figure. The pecu- liar asymmetric line shape of the Zeeman components is a result of the dynamic Stark effect [7]. During a typical 

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experimental run 100 profiles are recorded for each com- bination of the magnetic field and the polarization angle (400 profile scans in total). In order to compute the nor- malized amplitude, r$_{q}$ , of a selected Zeeman component, the actual first-harmonic signal near the Zeeman peak is divided by the respective second-harmonic signal and then averaged over a number of the data points $^{1}$. Then, the combination K of Eq. (14) is computed for each pro- file scan followed by averaging the result over all the scans at a given B- θ configuration. This procedure is repeated for all four reversals, and all B- θ symmetrical contribu- tions, K 1 − $_{4}$, are determined. In the present experiment, the values of K 2 , 3 , $_{4}$-terms are found to be consistent with 

## VI. RESULTS AND ANALYSIS 

zero within the statistical uncertainty, which is the same as that of the PV-asymmetry (see below). 

As can be seen from Table II, terms in K 1 associated with the fields imperfection are of crucial importance: 

In order to measure the contribution of these terms, arti- ficially exaggerated E-field imperfections both static and oscillating, e ex z , e ex y , ˜ e ex y and ˜ e ex z , are imposed by use of the “correction electrodes” (see Fig. 6), and two sets of the experiments were performed. In the first one, a DC- voltage was applied to the correction electrodes, and the measurements were done reversing e ex y and e ex z . These experiments yield values of ˜ e$_{y}$ and ˜ e$_{z}$ + 2 ˜ E$_{0}$ ˜ b$_{x}$/B . In the second set, an AC-voltage modulated synchronously with the main E-field is applied to the correction electrodes. In order to reverse the sign of the parasitic terms a π - phase-shift of ˜ e ex y and ˜ e ex z with respect to the modulation signal is employed by switching the wiring of the correc- tion electrodes. Thus, values of the DC-imperfections, e$_{y}$ and e$_{z}$ , are determined. The magnitudes of the applied electric fields and their distributions are calculated using a 3D-numerical-model of the interaction region. The re- sults of the experiments are presented in Table III. The 

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net contribution of these imperfections to K 1 in the ab- sence of the exaggerated fields is found to be $^{2}$: 

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