Multiphoton ionization spectroscopy: A theoretical analysis of the NO spectrum

Abstract

The multiphoton ionization spectrum of the NO molecule is described theoretically by calculations of absorption and ionization cross-sections. Both perturbation theoretic (short-time) and rate equation (long-time) descriptions of the stepwise excitation and ionization process are considered. Under the present experimental conditions the two approaches are shown to give qualitatively similar results. Two cases of four-photon absorption are treated: in the first, called a 2+2 process, resonance occurs on absorption of the second photon; and in the second, 3+1 process, resonance occurs at the third photon level. For the wavelength range considered, there is one state, the A 2Σ+, in the former category and six possible choices for the latter. The 2+2 process is shown to have the higher intensity. For most of the resonant states, relative calculated ionization cross sections are in accord with experiment.