Coherent Oscillatory Femtosecond Dynamics in Multichannel Photodynamics of NO2 Studied by Spatially Masked Electron Imaging

The femtosecond multiphoton photoiomization and dissociation dynamics of NO2 have been studies in a two-color pump-probe experimetn at 400 and 266 nm using velocity map ton imaging in conjunction with photoelectron imaging. We report here a series of experiments focusing on the oscillatory patterns in pump-probe transients of the photoelectron signal. By using the technique of spatially masked imaging detection, we can select different photoelectron channels enabling the rapid measurement of energy selected transisents with good signal-to-noise ratio. At short delay times (300 fs) the dommant process is dissociative multiphoton ronization by 3 x 400 nm + 1 x 266 nm excitation to a repulsive potential energy surface of the NO2+ cation correlating to NO+((1)Sigma(+)) + O(P-3) and the ejection of a 0.37 eV electron. At longer delay times (>400 fs), the release of high-energy electron (0.88 eV) is observed attributed to a three-photon absorption at 400 nm to Rydberg and valence type excited states of netural NO2 leading to predissociation and the production of NO+ + O(P-3) from a one-photon ionization at 266 nm. At longer delay times (>400 fs) a second slow (near 0 eV) photoelectron channel is observed that is associated with one photon excitation at 400 nm to the first excited (A) over bar B-2(2) state NO2 followed by two photon excitation at 266 nm leading to near threshould tomization and dissociation to NO- + O(P-3). Distinctive oscillatory patterns were found in the pump-probe transients of the photoelectron yield for both the slow and the fast photoelectron channels but with different periods of about 750 fs (slow) ro 590 fs (fast) Extensive polarization experiments are reported for both linerar and circular polarized pump and probe laser geometries. We discuss the oscillatory mechanism in relation to ab initio calculations of relevant Rydberg and valence type excited states of NO2 near 9.3 eV. We propose that an oscillating wavepacked of mixed Rydberg and valence character that predissociates is reponsible for the observed osicillations in the transients of the fast (0.88 eV) photoelectron channel.