Imaging fast relaxation dynamics of NO₂

Abstract

Time-resolved spectroscopy combined with velocity map imaging technique have been used to investigate the multiphoton dynamics of NO 2 molecule. Two different pump-probe excitation schemes were used to explore different potential energy surfaces (PESs) located in the first dissociation region and in the Rydberg region around 9.2 eV. Integrated and energy resolved signals of NO + 2 , NO + and photoelectrons were recorded as a function of time. When exciting with 403 nm photons, the NO + signal exhibits an intriguing oscillatory behaviour with a period of 512 fs. The NO + and photoelectron kinetic energy distributions produced by this pump wavelength were cold, while those produced when employing 269 nm photons as pump were very rich, evidencing the presence of multiple excitation channels. A couple of sharp long lived photoion-photoelectron peaks represents the most salient feature of latter. They were assigned to an excitation by two 269 nm photons to a Rydberg state dissociating into NO(A 2 Σ + )+O( 3 P ). This NO + peak as well as another one located at 0 eV display very complex time dependencies including the signatures of two dissociation dynamics on timescales of 400 and 600 fs. The different pathways responsible of this temporal behaviour are discussed in view of shedding light onto the underlying multichannel multiphoton dynamics.

Spectral dependence of photoemission in multiphoton ionization of NO2 by femtosecond pulses in the 375–430 nm range

“…Moreover, we remark that the reaction pathway proposed here for  = 375 nm does not contradict the interpretation by Davies et al 7 in their timeresolved study: at t=0, the dominant contribution is the ionization of the Rydberg state (which may include a nascent dissociation of the NO2 excited state) while at longer pumpprobe delays, dissociation through the Rydberg state may occur prior ionization probing the dissociation process as proposed in Davies et al 7,9 . Their determined dissociation time 9 around 500 fs is indeed coherent with the lifetimes around 600 fs for [R*(6a1) -1 ] Rydberg states lying around 9.3 nm previously reported by both López-Martens et al 25 and Blanchet and coworkers 26 . Such lifetimes support our proposed reaction pathways: photodissociation after three-photon absorption within the shorter duration of our laser pulse is not possible.…”

Aurore: A platform for ultrafast sciences

“…The Aurore beamline 5 is dedicated to investigation of relaxation dynamics in small quantum systems: molecules and nano-particles. The long-term stability of the laser pointing and intensity of Aurore allow to record static 80 or time-resolved photoelectron 8,43,[81][82][83][84] or ion imaging [85][86][87] for more than 12 hours without any realignment.…”

High-harmonic transient grating spectroscopy of NO2 electronic relaxation

“…This has made of NO 2 a kind of hellish grail of molecular physicists. Time-resolved photoionization has revealed unassigned regular oscillations with a 500 fs period for a pump centered at 400 nm 8,[25][26][27] .…”