Multiphoton ionization photoelectron spectroscopic study on NO: Autoionization pathway through dissociative superexcited valence states

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Abstract
In the present work we have carried out measurements of total ion–current and photoelectrons to study autoionization of NO molecule through the two‐photon resonant, valence‐excited B 2Π state at the v’=9 level (designated as B‐9). This state was selected so as to remove a possibility of direct ionization of producing the ground‐state NO+ ion. It has been found that the ion–current spectrum of the B‐9 state shows several anomalous‐intensity rotational lines as well as normal Q‐branch rotational lines. Each of the photoelectron spectra obtained at the normal rotational lines exhibits three energetically accessible vibrational bands with branching ratios of 0.65 (v+=2), 0.30 (v+=1), and 0.05 (v+=0). On the other hand, each of the photoelectron spectra obtained at the anomalous‐intensity rotational lines suggests a relatively high yield of the v+=0 ion, and there seems to be no photoelectron angular dependence for the v+=0 band. From these experimental results, we have deduced the following conclusions: (1) The overall process of producing the normal rotational lines is represented by X2hν→B‐9 hν→I*→NO+, where I* means the superexcited valence I 2Σ+ state and → indicates electronic autoionization forming the ground electronic state of ions. (2) The ionization scheme of producing the anomalous‐intensity rotational lines is expressed by X2hν→B‐9hν→N‐6↔B’*→NO+, involving an accidental double resonance, where N is the Rydberg N(4dδ) 2Δ state, B’* is the superexcited valence B’ 2Δ state, and ↔ means an electronic coupling between the Rydberg and the valence states. The N‐6 state has further been supported by experimental fact that the v+=6 photoelectron band appears at a higher laser power by four‐photon direct ionization. In the above two schemes, the electronic coupling between the dissociative valence and the ionization continuum state is considered to play an important role in the present autoionization phenomena.