Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2

Abstract

The multiphoton multichannel photodynamics of NO2 has been studied using femtosecond time-resolved coincidence imaging. A novel photoelectron-photoion coincidence imaging machine was developed at the laboratory in Amsterdam employing velocity map imaging and “slow” charged particle extraction using additional electron and ion optics. The NO2 photodynamics was studied using a two color pump-probe scheme with femtosecond pulses at 400 and 266nm. The multiphoton excitation produces both NO2+ parent ions and NO+ fragment ions. Here we mainly present the time dependent photoelectron images in coincidence with NO2+ or NO+ and the (NO+,e) photoelectron versus fragment ion kinetic energy correlations. The coincidence photoelectron spectra and the correlated energy distributions make it possible to assign the different dissociation pathways involved. Nonadiabatic dynamics between the ground state and the AB22 state after absorption of a 400nm photon is reflected in the transient photoelectron spectrum of the NO2+ parent ion. Furthermore, Rydberg states are believed to be used as “stepping” states responsible for the rather narrow and well-separated photoelectron spectra in the NO2+ parent ion. Slow statistical and fast direct fragmentation of NO2+ after prompt photoelectron ejection is observed leading to formation of NO++O. Fragmentation from both the ground state and the electronically excited aB23 and bA23 states of NO2+ is observed. At short pump probe delay times, the dominant multiphoton pathway for NO+ formation is a 3×400nm+1×266nm excitation. At long delay times (>500fs) two multiphoton pathways are observed. The dominant pathway is a 1×400nm+2×266nm photon excitation giving rise to very slow electrons and ions. A second pathway is a 3×400nm photon absorption to NO2 Rydberg states followed by dissociation toward neutral electronically and vibrationally excited NO(AΣ2,v=1) fragments, ionized by one 266nm photon absorption. As is shown in the present study, even though the pump-probe transients are rather featureless the photoelectron-photoion coincidence images show a complex time varying dynamics in NO2. We present the potential of our novel coincidence imaging machine to unravel in unprecedented detail the various competing pathways in femtosecond time-resolved multichannel multiphoton dynamics of molecules.