Three-photon resonance ionization of H2

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Abstract
Calculations are performed for the ionization of para‐H2 by absorption of three 6.6 eV photons. Two photons are absorbed to excite H2 to the E, F 1Σ+g state, and a third photon is absorbed to ionize this state. The two‐photon Rabi rate and one‐photon ionization rate are linear in the laser intensity (I) and are thus competitive. This competition produces a strong dependence of the three‐photon ionization rate on the vibrational‐rotational levels of the bound–bound electronic transition and on the electron ejection angle. For example, for I = 1 GW cm−2 the strong (maximum ionization rate 6×108 s−1/molecule) vJ = 00→v′J′ = 30 transition is 70% saturated at 0° and 100% saturated at 90° ejection angle relative to the linear polarization direction of the photon. The weak (maximum ionization rate 2×1 07 s−1/molecule) vJ = 00→v′J′ = 32 transition is 0.3% saturated at 0° and 80% saturated at 90° ejection angle. These calculations suggest use of multiphoton ionization data to study one‐photon excited‐state angular distributions. The latter occur for an excited state which has a nonstatistical distribution of rotational M′ sublevels belonging to the J′ level of the excited state (in contrast to the photoionization of a ground state having a statistical distribution of M sublevels belonging to J).