Abnormal behavior in the three-photon resonance enhanced multiphoton ionization spectroscopy of CO E (1)Pi <- X (1)Sigma(+)(0,0)

The abnormal behavior of CO E (1)Pi (v=0)<--X (1)Sigma (+)(v=0) three-photo n resonance enhanced multiphoton ionization (REMPI) spectroscopy is studied , including the anomalous band structures and adverse pressure effects on t he spectral intensity. One- and two-color experiments with laser beams in c o- and counterpropagations have been performed to elucidate the mechanisms. It is demonstrated that the abnormalities originate from two mechanisms, i .e., the reabsorption of the resultant third harmonic generation (THG) in t he one-color case or four wave mixing sum frequency generation (FWSFG) in t he two-color case, and the interference cancellation between the one-photon excitation channel induced by THG/FWSFG reabsorption and the three-photon direct excitation channel. The former mechanism dominates in the wing regio n of a resonant transition profile, while the latter is restricted within t he kernel portion of the transition. (C) 2001 American Institute of Physics .