Journal of Physical Chemistry, Vol.99, No.6, 1666-1670, 1995
Interference Effects in the (2+1) Photon Ionization Spectrum of the H and H’ Rydberg States of No
The (2 + 1) photon ionization spectrum of NO via the H-2 Sigma(+), 3d sigma and H’(II)-I-2, 3d pi (v = 0) Rydberg states has been recorded in the UV range between 317.7 and 320 nm. The rotational analysis and the Line intensity calculation have been performed by using a propensity rule approach including the dominant contribution from the intermediate quasi-resonant (CII)-I-2 and D-2 Sigma(+) states (v = 0) to the two-photon transition moment. The line positions have been taken from the upper stares and ground state rotational term values extracted from earlier absorption data. The calculations include l-mixing between the do and s sigma Rydberg states as well as l-uncoupling between the close lying d sigma and d pi components. In addition to the interference effects due to the mixing of the upper levels, a new type of interference occurs in the two-photon transition amplitude through the two different pathways via the C and D states. The mixing coefficients for the upper levels and the oscillator strengths for the C-X, D-X, H,H’-C, and H,H’-D transitions have been taken from the literature. Therefore, our calculation has been performed without any fitting of the molecular parameters. The resulting simulated two-photon spectrum agrees reasonably well with the observed one. This approach has been applied to reinvestigate a recently published analysis of the same system involving v = 1 in the upper states. We propose a completely revised analysis of this (1,0) two-photon band, showing a very good agreement between observed and calculated rotational profiles. This work demonstrates the ability of the propensity rule model for predictions of upper state rotational and parity relative populations in multiphoton excitation experiments. These predictions may be essential when these upper levels are used as intermediate levels for two-color experiments toward highly excited states or the ionization continuum.