Photodetachment Dynamics and Dissociative Photodetachment

RLM 7.104
Joint CQS/CNLD Seminar: Robert Continetti
Department of Chemistry and Biochemistry, University of California, San Diego
Photodetachment Dynamics and Dissociative Photodetachment

Molecular anions are exotic species that exhibit a wide range of chemical and photophysical phenomena. In this talk experimental studies of two molecular anions will be discussed: HOCO’? and NO2′?. The first half of the talk will focus on using the photodetachment of the hydroxycarbonyl anion, HOCO’?, to prepare and study the HOCO radical, an important species in combustion. The hydroxycarbonyl anion HOCO’? is stable relative to HOCO by ~1.5 eV. High–‘?resolution near–‘?threshold photoelectron spectra in concert with high–‘? level theory in the Stanton group at UT Austin have been used to accurately determine the energetics and a number of vibrational frequencies for the HOCO radical. At higher photon energies (3.2 eV) the HOCO radical is produced with sufficient excitation that it dissociates to H + CO2 and OH + CO products, and studies of this dissociative photodetachment process has provided an experimental measure of the tunneling barrier for decomposition of HOCO to H + CO2.

In the second part of the talk, the focus will be on the photodetachment dynamics of molecular anions. The large dipole moment of the cis–‘? and trans–‘? isomers of HOCO leads to a dense manifold of short–‘?lived dipole–‘?bound HOCO’? states embedded in the photodetachment continuum. These states readily serve as the intermediate state in two–‘? photon photodetachment, and wavelength–‘?dependent studies of the photodetachment process reveal a shift in the apparent electron affinity that must be a result of rotational effects on the photodetachment cross–‘?section. A weak non–‘? resonant two–‘?photon photodetachment process of NO2’? is also observed with the first photon 0.6 eV below the photodetachment threshold and a modest photon flux of ~2×1012 W/cm2. The angular distributions in this two–‘?photon photodetachment can be understood in terms of the energy dependence of a three–‘?center s–‘? and d–‘? wave interference model for the photodetached electron. Finally, non–‘?resonant two–‘? photon photodetachment of NO2’? with elliptically polarized light reveals photoelectron angular distributions in the polarization plane that display a striking streak effect in the direction of polarization rotation. As ellipticity increases, these perpendicular features streak towards the direction of rotation,  until the angular distribution reaches circular symmetry for circular polarization. It is speculated that these effects arise from the short lifetime of the non–‘? resonant virtual state accessed by absorption of a first photon, on the order of a quarter optical cycle of 100s of attoseconds.