Time-resolved high-harmonic spectroscopy of valence electron dynamics

Abstract

Time-Resolved High-Harmonic Spectroscopy (TRHHS) is an emerging technique for probing valence electron dynamics in molecules undergoing photochemical reactions. A general description of the technique including experimental and theoretical aspects is given. The relation of TRHHS to other time-resolved techniques is discussed, with particular emphasis on the specificities of TRHHS. Coherence endows TRHHS with two of its original properties: the interference of excited and unexcited molecules makes it highly sensitive to small excitation fractions and to the phase of photorecombination matrix elements. In addition, the technique is also sensitive to the variation of the vertical ionization potential, providing additional insights into the photochemical dynamics. The principles of the technique are discussed in relation to recent results on the photochemistry of Br 2 and NO 2 , revealing its sensitivity to different aspects of the dynamics, most notably electronic dynamics occurring in non-adiabatic dynamics.

High-harmonic transient grating spectroscopy of NO2 electronic relaxation

“…Last, a 3D calculation taking into account the evolution of the molecular wavepackets along the three dimensions was performed, assuming constant harmonic dipole moments and phases except for from the I p τ phase. The results hardly show any modulation in the harmonic signal 53 . Since the model neglecting wavepacket motion and taking into account only the population evolutions gave the best qualitative agreement with their experiments, Kraus et al concluded that the experimental signal reflected the variation of the diabatic state populations, the phase modulations I p τ associated to the vibrational motion being quickly washed out by wavepacket spreading in the three dimensions.…”

Probing Electronic Dynamics during Photochemical Reactions

“…1, provides the intensity of the diffracted radiation as an additional observable that complements the intensity of the undiffracted radiation. While the latter has been shown to be formally identical in collinear and transientgrating pump-probe experiments carried out with identical spatially-averaged excitation fractions, [17] the diffracted radiation has an enhanced contrast. Whereas the undiffracted radiation arises from signals of excited and unexcited molecules weighted by their relative populations, the diffracted radiation results from an interference of emissions with equal weights and thus enhanced contrast.…”

“…[18] Theoretical models of increasing detail have also been developed and are described in refs. [14,16,17]. The experimental results, through comparisons with the theoretical models, have demonstrated the main principles and sensitivities of TRHHS.…”

“…[9][10][11][12] The application of high-harmonic spectroscopy to studying photochemical dynamics, termed time-resolved high-harmonic spectroscopy (TRHHS), has been described in refs. [13][14][15][16][17][18] and will be reviewed here. So far, TRHHS has been applied to studying the photodissociation of Br 2 , [13,14,17] the conical intersection dynamics and subsequent dissociation of NO 2 , [15][16][17] both following photoexcitation at 400 nm, as well as the non-adiabatic photodissociation of CH 3 I and CF 3 I following photoexcitation at 267 nm.…”

“…[13][14][15][16][17][18] and will be reviewed here. So far, TRHHS has been applied to studying the photodissociation of Br 2 , [13,14,17] the conical intersection dynamics and subsequent dissociation of NO 2 , [15][16][17] both following photoexcitation at 400 nm, as well as the non-adiabatic photodissociation of CH 3 I and CF 3 I following photoexcitation at 267 nm. [18] Theoretical models of increasing detail have also been developed and are described in refs.…”