Observation of the molecular response to light upon photoexcitation

Abstract

When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions. This constitutes the first step of photochemical and photophysical processes that include primary events in human vision and photosynthesis. Here, we report the direct measurement of the initial redistribution of electron density when the molecule 1,3-cyclohexadiene (CHD) is optically excited. Our experiments exploit the intense, ultrashort hard x-ray pulses of the Linac Coherent Light Source (LCLS) to map the change in electron density using ultrafast x-ray scattering. The nature of the excited electronic state is identified with excellent spatial resolution and in good agreement with theoretical predictions. The excited state electron density distributions are thus amenable to direct experimental observation.

Time-resolved x-ray scattering from impulsively aligned or oriented molecules

“…For the present purpose, the standard theory of timeresolved x-ray scattering based on the independent atom model (IAM) suffices. Though deviations from the IAM can reveal important electronic effects such as chemical bonding or electronic excitations [36,37], they do not concern the signatures of rotational alignment or orientation considered here. Thus, the electron density is expressed as a sum of free atomic densities and contributions from coherently populated electronic states [30,[32][33][34][35] are neglected.…”

Features and futures of X-ray free-electron lasers

“…For example, gas-phase time-resolved X-ray scattering has been used recently to observe the molecular electronic response of 1,3-cyclohexadiene to UV light upon photoexcitation. 89 Furthermore, as shown in Figure 7, soft X-ray spectroscopy has been used to reveal the dynamics of the valence holes created by 800 nm strongfield ionization in water and track the primary photon transfer reaction. 90 The rapid development of XFEL has led to two-color X-ray pulses with a tunable delay, thus opening new opportunities for X-ray pumps and X-ray probes, which can be used for studying the ultrafast dissociation and charge migration induced by X-rays.…”

“…For example, gas-phase time-resolved X-ray scattering has been used recently to observe the molecular electronic response of 1,3-cyclohexadiene to UV light upon photoexcitation. 89 Furthermore, as shown in Figure 7 , soft X-ray spectroscopy has been used to reveal the dynamics of the valence holes created by 800 nm strong-field ionization in water and track the primary photon transfer reaction. 90

Figure 7 Schematic of laser pump X-ray probe experiments Water in the jet is ionized by a strong field at a wavelength of 800 nm, with the fluorescence and transmission spectra measured after the time-delayed X-ray probe pulse to reveal the fast-chemical processes that occur in the radiolysis of water.

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Feshbach-Fano Approach for Calculation of Auger Decay Rates Using Equation-of-Motion Coupled-Cluster Wave Functions: Theory and Implementation

“…Advances in laser technology and the development of novel X-ray sources have opened up a new area of applications in which core-level spectroscopies can be used as probes to study electron and nuclear dynamics with unprecedented time and space resolution [4][5][6][7][8][9]. Recently, X-ray spectroscopy was used to reveal the dynamics of liquid H 2 O + [10,11], a photo-induced ring-opening reaction [12], charge-migration and charge-transfer reactions [13][14][15], and to interrogate an interplay between open-shell spin-coupling and Jahn-Teller distortion in the benzene radical cation [16,17].…”