Multiphoton ionization spectroscopy: A theoretical analysis of the NO spectrum

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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.