Photoionization spectrum of H in strong dc electric fields

We studied the $m_l$=0 photoionization channel of H(n=2, $m_l$=0) red and blue states in a dc electric field F theoretically and experimentally in the energy region between the two ionization thresholds: the Stark-induced classical threshold at E=-2 √F and the threshold of the field-free atom at E=0. We find that the ionization yield via the ‘‘rapidly’’ ionizing $m_l$=0 channels concentrates at the two thresholds. Such concentration has not been observed in similar experiments on complex atoms due to a large degree of electron scattering from the ion core that produces significant amounts of mixing among the excitation amplitudes and among the ionization channels. Our results show that the hydrogenic Stark levels of fixed principle quantum number n become sharper as their parabolic quantum number $n_1$ increases. On the other hand, the levels of fixed $n_1$ get wider as their quantum number n increases. Moreover, our results show a remarkable property: The ratio of the cross sections from the red and from the blue $m_l$=0 states of n=2 fall on a universal curve when plotted as a function of $Z_1$, the fraction of the nuclear charge that drives the bound motion. Also, the ratio is found to be larger than 1 in two regions where $Z_1$=0–(1/4) and (1/2)–(3/4), less than 1 in the rest of the $Z_1$ regions, $Z_1$=(1/4)–(1/2) and (3/4)–1, and is equal to unity at the common boundaries at $Z_1$=(1/4), (1/2), and (3/4), thus indicating special properties of these fractional charges.