Recoil frame photoemission in multiphoton ionization of small polyatomic molecules: photodynamics of NO 2 probed by 400 nm fs pulses

We report a general method for the complete analysis of the recoil frame photoelectron angular distribution (RFPAD) in n-photon dissociative ionization of small polyatomic molecules, resulting from (n − 1) bound-to-bound transitions plus one-photon ionization of a neutral excited state of the target. This method relies on the decomposition of the RFPAD in terms of the R K (χ , θe) recoil frame azimuthal harmonics (RFAHs) which are the components of its Fourier expansion in φe, where χ and θe are the polar angles referring to the polarization axis P and the photoelectron momentum k relative to the ion fragment recoil direction, respectively, and φe is the azimuth of k relative to P. The RFAH expansion method is illustrated by a detailed experimental and theoretical study of one-colour multiphoton dissociative and non-dissociative ionization of the NO2 molecule of C2v symmetry induced by 400 nm fs laser pulses, which involve electronic and nuclear dynamics within the pulse duration of the order of 70 fs. The reaction mechanism proposed to account for five-photon dissociative ionization of NO2 involves the role of [R ∗ (6a1) −1 ] Rydberg states populated by three-photon absorption, subsequently ionized by a fourth photon into the NO2 + (X 1 � g + , v1,v2,v3) manifold involving autoionization of [R ∗ (4b2) −1 ] Rydberg states, and linear versus bent geometry selective dissociation of NO2 + (X 1 � g + , v1,v2,v3) by a fifth photon. The reported calculations provide a coherent picture of the experimental findings although all features are not yet well reproduced.