Resonant infrared two-photon ionization of D(n=8) atoms.

Abstract

Two-photon ionization of hydrogenic atoms with principal quantum number n=8 has been studied experimentally over the wavelength range 10.45--10.59 \ensuremath{\mu}m. The ionization was resonantly enhanced by the n=12 intermediate state, with a large n=8 to n=12 coupling responsible for a fractional resonance width of about 0.1%. On resonance, ionization rates of order 10 MHz were achieved with nominal 2-mJ, 40-nsec laser pulses, with 40 mJ being required at wavelengths 1% away from resonance. The data are interpreted in terms of the extended two-level model for multiphoton ionization. The peak laser electric field strength reached values where the top of the instantaneous potential energy barrier was near the intermediate-state energy value.