Observation of ultrafast NH3 (Ã) state relaxation dynamics using a combination of time-resolved photoelectron spectroscopy and photoproduct detection

Abstract

The ultrafast excited state relaxation of ammonia is investigated by resonantly exciting specific vibrational modes of the electronically excited NH(3) (Ã) state using three complementary femtosecond (fs) pump-probe techniques: time-resolved photoelectron, ion-yield and photofragment translational spectroscopy. Ammonia can be seen as a prototypical system for studying non-adiabatic dynamics and therefore offers a benchmark species for demonstrating the advantages of combining the aforementioned techniques to probe excited state dynamics, whilst simultaneously illuminating new aspects of ammonia's photochemistry. Time-resolved photoelectron spectroscopy (TRPES) provides direct spectroscopic evidence of σ* mediated relaxation of the NH(3) (Ã) state which manifests itself as coupling of the umbrella (ν(2)) and symmetric N-H stretch (ν(1)) modes in the photoelectron spectra. Time-resolved ion yield (TRIY) and time-resolved photofragment translation spectroscopy (TRPTS) grant a measure of the dissociation dynamics through analysis of the H and NH(2) photodissociation co-fragments. Initial vibrational level dependent TRIY measurements reveal photoproduct formation times of between 190 and 230 fs. Measurement of H-atom photoproduct kinetic energies enables investigation into the competition between adiabatic and non-adiabatic dissociation channels at the NH(3) (Ã)/NH(3) (X̃) conical intersection and has shown that upon non-adiabatic dissociation into NH(2) (X̃) + H, the NH(2) (X[combining tilde]) fragment is predominantly generated with significant fractions of internal vibrational energy.

Simulated and Experimental Time-Resolved Photoelectron Spectra of the Intersystem Crossing Dynamics in 2-Thiouracil

“…The TRPES experiments use a femtosecond laser system with UV conversion capabilities, magnetic bottle photoelectron spectrometer, and a gaseous molecular beam source. More details about the apparatus have been described previously [ 42 , 43 , 44 , 45 , 46 , 47 ], while here only a short description is given. A Coherent Inc. (Santa Clara, CA, USA) amplified Ti:Sa laser system (Mira Optima 900F, Legend Elite) was used to pump two optical parametric amplifiers (OPerA and TOPAS-C) to generate pump and probe pulses.…”

Intersystem crossing dynamics in singly substituted thiouracil studied by time-resolved photoelectron spectroscopy: Micro-environmental effects due to sulfur position

“…[2], [36], [37], [38], [39]. Briefly, the set-up is based on a femtosecond laser system with UV conversion capabilities and a magnetic bottle photoelectron spectrometer.…”

Resonant multiphoton ionisation probe of the photodissociation dynamics of ammonia

“…The kinetic energy release measurements have used resonant ionisation of the hydrogen fragment to highlighted the different lifetimes associated with different levels of internal energy in the molecular fragments produced. [1,3,18,23] The different lifetimes have been correlated with various decay pathways, with some invoking complex dynamics around the conical intersection as a possible intermediate where population can be trapped for several hundred femtoseconds before dissociation occurs. [23] The effectiveness of photoelectron spectroscopy has by comparison been limited by the often restricted observation window afforded by the available UV probes.…”

“…Recent experiments have used a 240 nm probe which restricted the observation window to the first 75 fs following excitation. [1] The ion kinetic energy release measurements however suggest the lifetime associated with dissociation can be up to 400 fs for some dissociation processes. [23] This means there is a large component of the excited state dynamics not being measured by either technique.…”