Observation of the molecular response to light upon photoexcitation

Haiwang Yong,Jennifer M Ruddock,Sébastien Boutet,Brian Stankus,Sergio Carbajo,Michael P Minitti,Peter M Weber,Andrés Moreno Carrascosa,Adam Kirrander,Mats Simmermacher,Mengning Liang,Nikola Zotev,Darren Bellshaw,Wenpeng Du,Joseph S Robinson,Yu Chang,Nathan Goff,Jason E Koglin

doi:10.1038/s41467-020-15680-4

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.

Highlights

When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions
In the time-resolved x-ray scattering experiment, the ensemble of free CHD molecules is probed using 9.5 keV mean energy x-ray photons generated by the Linac Coherent Light Source (LCLS)[28], both with the excitation laser on and off
The scattering signals are detected on a 2.3-megapixel CornellSLAC Pixel Array Detector (CSPAD) and binned according to the delay time between the laser pump and x-ray probe pulses

Summary

Introduction

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. Photoexcitation is the first step in all photochemical and photophysical processes, which include photovoltaics, photosynthesis, light-emitting diodes, photodynamic therapy, photocatalysis, and the primary events in human vision[14] This first step results in a change in electron density that sets all subsequent dynamics in motion and determines the outcome of the reaction. We use a higher-energy 200 nm pump pulse to excite the molecule to an electronic 3p Rydberg state This carries a number of advantages in terms of the goals of our experiment. We provide the direct evidence of the initial redistribution of electron density in real space upon photoexcitation

Methods

Results

Conclusion