Influence of the laser character on the resonance multiphoton ionization efficiency of NO molecule

Abstract

The analytic formula of the ionization efficiency in the process of resonance enhanced multiphoton ionization is derived from the population rate equation. Based on this formula, the ionization efficiency of NO molecule, which is ionized via A²Σ, E²Σ intermediate resonant states and by (2+2) or (3+1) multiphoton process, versus laser intensity and pulse duration is analyzed by computer simulation. It is shown that the ionization efficiency of NO molecule increases with the laser intensity and pulse duration. When the photon flux is 2×10²⁹photon•cm^-2•s^-1, all of the two steps in both processes are not get saturation as the pulse duration of the laser is 35ps. While the second excitation step is already saturated when the pulse duration is 6ns. And both of the two steps get saturation when the pulse duration is 50ns.Owing to the higher absorption transition cross section in the (2+2) process, the ionization efficiency via A²Σresonant state is with a much larger value than that of via E²Σstate. The ionization efficiency of NO molecule reaches saturation under lower laser intensity when it is ionized via A²Σresonant state. The optimum ionization pathway is decided when one detect NO by the technique of REMPI and with visible light as excitation source. It is the (2+2) multiphoton ionization process and via A²Σ intermediate resonant state. We wish the results can provide useful information for the detection of NO molecule.