Velocity-gauge theory for the treatment of strong-field photodetachment

A recent experimental study of the photodetachment of negative fluorine ions by a strong, circularly polarized laser field has been used as a test case for comparison of different gauges in a strong-field theory. Earlier studies concluded that the length gauge was the more suitable theory, but it was necessary to view the experimental peak intensity as a fitting parameter, and this had to be altered to 145% of the measured peak intensity. That amount of variation is not possible. There were two independent experimental measurements of the peak intensity: one direct and the other through a measurement of the momentum distribution of the photodetached electron. These two measurements give the same result for the peak intensity, and so the 45% increase required to have a fit to the length gauge is not an acceptable theoretical explanation for the experimental results. In contrast, the velocity gauge calculation provides a good match to the spectrum peak with no alteration necessary for the measured peak intensity. The theoretical spectrum is somewhat narrower than the measured spectrum, but that is consistent with the fact that the intensity parameters are not quite large enough to ensure detailed accuracy of the strong-field approximation.