Saturation of three-photon ionization of atomic hydrogen and deuterium at 243 nm

Two-photon-resonant, three-photon ionization of atomic hydrogen and deuterium is performed at 243 nm, and the ions are detected by time-of-flight mass spectrometry. The power dependence of the 1S--2S two-photon excitation and one-photon ionization of 2S hydrogen and deuterium are determined. The ionization step saturates at 4 x 106 W/cm2. The two-photon step-power dependence varies with laser linewidth, with saturation occurring between 1 x 107 and 3 x 107 W/cm2. The extraction electric field in the mass spectrometer does not affect the 2S excitation. Strong signals are recorded at atomic concentrations of 105 atoms/cm3. Nonresonant ionization of background gases producing H+ and D+ is minimized