Imaging conical intersection dynamics during azobenzene photoisomerization by ultrafast X-ray diffraction

Abstract

X-ray diffraction is routinely used for structure determination of stationary molecular samples. Modern X-ray photon sources, e.g., from free-electron lasers, enable us to add temporal resolution to these scattering events, thereby providing a movie of atomic motions. We simulate and decipher the various contributions to the X-ray diffraction pattern for the femtosecond isomerization of azobenzene, a textbook photochemical process. A wealth of information is encoded besides real-time monitoring of the molecular charge density for the cis to trans isomerization. In particular, vibronic coherences emerge at the conical intersection, contributing to the total diffraction signal by mixed elastic and inelastic photon scattering. They cause distinct phase modulations in momentum space, which directly reflect the real-space phase modulation of the electronic transition density during the nonadiabatic passage. To overcome the masking by the intense elastic scattering contributions from the electronic populations in the total diffraction signal, we discuss how this information can be retrieved, e.g., by employing very hard X-rays to record large scattering momentum transfers.

Imaging charge-migration in chiral molecules using time-resolved x-ray diffraction

“…The elastic contribution is the dominant part of the total diffraction pattern by far, as it is three orders of magnitude stronger than the inelastic contribution. Similar observations have been reported earlier in the case of TRXD for different systems [45,78]. In spite of having low magnitude, the information about ultrafast dynamics of charge migration is imprinted in inelastic diffraction patterns, as the elastic part is static in nature.…”

Non-Born–Oppenheimer effects in molecular photochemistry: an experimental perspective

“…Ultrafast TR-PES methods are also finding increased use in tracking the evolution of photoexcited molecules through CIs—as illustrated here for the case of thiophenone [41]—and (weak) vibronic coherences identified in recent time-resolved X-ray diffraction (TR-XRD) experiments have been interpreted as providing insights into the timing of, and the confined spatial distributions of the valence electrons in, the CI-enabled cis → trans isomerization of photoexcited azobenzene molecules [112,113]. In all cases, however, interpretation of the experimental data is heavily reliant on complementary high level theoretical modelling, and the unambiguous interpretation of such ultrafast data measured for more complex, higher dimensionality systems, in the absence of supporting high level theory, is likely to remain a substantial challenge.…”

Unveiling the spatial distribution of molecular coherences at conical intersections by covariance X-ray diffraction signals

“…(28)(29)(30)(31)(32)(33)(34)(35)(36) with state-of-the-art stochastic FEL pulses, we separate the inelastic signatures due to CoIn coherences from the dominant, less distinctive elastic features. This is exemplified for powder diffraction off randomly oriented nanocrystals of azobenzene molecules undergoing photoisomerization through a CoIn (27). The signal allows us to monitor the molecular coherences emerging during the nonadiabatic dynamics, revealing a confined distribution of transition charge densities representing small distances in the molecule.…”

“…2A, where we display the evolution of the charge densities in real and in momentum space. Ab initio nuclear wavepacket simulations (see Materials and Methods) of ultrafast azobenzene photoisomerization involving the CoIn passage (27), with the full inclusion of coupled nuclear and electronic degrees of freedom, were performed on the basis of accurate two-dimensional potential energy surfaces (46) containing the ground state g and the electronically excited nπ * state e. The system is prepared at t = 0 fs in the e state by a vertical excitation, and the passage through the CoIn, taking place at approximately 100 fs, moves part of the populations to the g state, thereby generating a coherence between g and e. While our two-mode effective Hamiltonian captures the reactive pathway through the CoIn, additional effects like vibrational relaxation to other modes might come into play in a multimode treatment. It was shown recently that the coherence emerging at the CoIn can be appreciably large and long lived even in full-dimensional simulations of a much larger molecule with more decoherence pathways (47).…”

“…This represents a challenge when monitoring CoIns, where most core and valence electrons are inactive, and only a few valence electrons are directly involved. Key temporal and spatial information about CoIn dynamics is encoded in the additional UXD inelastic patterns (19,26,27). These signatures directly stem from the localized quantum coherences generated at CoIns.…”