Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in N O 2

The multiphoton multichannel photodynamics of N O 2 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 N O 2 photodynamics was studied using a two color pump-probe scheme with femtosecond pulses at 400 and 266 nm . The multiphoton excitation produces both N O 2 + parent ions and N O + fragment ions. Here we mainly present the time dependent photoelectron images in coincidence with N O 2 + or N O + and the ( N O + , 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 A B 2 2 state after absorption of a 400 nm photon is reflected in the transient photoelectron spectrum of the N O 2 + 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 N O 2 + parent ion. Slow statistical and fast direct fragmentation of N O 2 + after prompt photoelectron ejection is observed leading to formation of N O + + O . Fragmentation from both the ground state and the electronically excited a B 2 3 and b A 2 3 states of N O 2 + is observed. At short pump probe delay times, the dominant multiphoton pathway for N O + formation is a 3 × 400 nm + 1 × 266 nm excitation. At long delay times ( > 500 fs ) two multiphoton pathways are observed. The dominant pathway is a 1 × 400 nm + 2 × 266 nm photon excitation giving rise to very slow electrons and ions. A second pathway is a 3 × 400 nm photon absorption to N O 2 Rydberg states followed by dissociation toward neutral electronically and vibrationally excited N O ( A Σ 2 , v = 1 ) fragments, ionized by one 266 nm 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 N O 2 . 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.