Sequential ionization in ^{3}He with a 1.5-ps 1-mm laser pulse

Helium and other atoms can, in principle, become doubly ionized when exposed to an intense laser pulse through two different channels: a direct process whereby both electrons are ejected simultaneously from the atom and a sequential process whereby the second charge state is created only after the first charge state. Crance and Aymar [1] calculated for He that if the number of photons required for direct ionization is less than the number of photons required for sequential ionization, then, depending on the duration and intensity of the applied field pulse, one or the other ionization channel may be favored. Their analysis used high-energy photons such that in the sequential ionization channel the removal of the first electron required one photon and the removal of the second required two. However, they suggested that the result should qualitatively hold for lower-energy photons where a large number are required for ionization. They predicted that the direct-ionization channel should become apparent in He at 1.06 pm for sufficiently short pulse durations. A rough estimate that they give [Ref. [1], Eq. (52)], when applied to typical ionization-field intensities for He, suggests that the pulse duration must be about 10 fs or shorter before direct ionization can occur significantly. Lambropoulos [2] made a strong case that any neutral atom exposed to intensities much above 10' W/cm at near-infrared or higher frequencies cannot survive for a longer duration than about 100 fs. Helium has the highest ionization potential of any atom, but it also has a large difference in ionization potentials between the first and second charge states. Therefore the neutral atom would h'ave to survive well past its first ionization threshold to allow for a simultaneous ejection of both electrons from the atom. If a laser pulse is too long, then by the time the field reaches sufficient intensity to produce doubly ionized helium, the neutral atoms are depleted, restricting ionization to the sequential channel. L'Huillier et al. [3—5] performed a pioneering series of experiments studying direct versus sequential ionization of noble gases. They observed evidence for direct ionization in xenon at wavelengths of both 1.06 and 0.53 pm and a pulse duration of 50 ps. They also reported evidence in support of direct ionization in other noble gases, including helium, under these conditions. They extended [5] the work at 0.53 pm in xenon to include pulse dura106-. . . I I II II I I I I I II II I I I I I II ll I