Title:	Computational XAS for Organic Donor Molecules
DNr:	SNIC 2021/22-2
Project Type:	SNIC Small Compute
Principal Investigator:	Iulia Brumboiu <iubr@umk.pl>
Affiliation:	Kungliga Tekniska högskolan
Duration:	2021-01-04 – 2022-02-01
Classification:	10407
Keywords:

Abstract

With the advent of next generation synchrotron radiation facilities (e.g. MAX IV) and X-ray free electron lasers (e.g. European XFEL), which can provide well-defined femtosecond X-ray pulses, it is becoming possible to record ultrafast processes with good temporal resolution. This is achieved by first bringing the system of interest in a valence excited state via a visible pump pulse and then probing the excited state dynamics using different variants of X-ray spectroscopy (absorption, emission, pthotoemission and inelastic scattering). These techniques allow the study of ultrafast chemical processes that take place in the excited state, such as the processes involved in photosynthesis, light-harvesting, or photodegration. However, the interpretation of such time-resolved (TR) measurements is strongly linked to the theoretical description of the underlying ultrafast processes and the X-ray spectroscopy. The aim of the current project is to develop a methodology to describe time-resolved X-ray spectroscopy and apply it to organic molecules used as electron donors in organic photovoltaics. The project will make use of the core-excited state molecular gradients we have recently implemented at the algebraic diagrammatic construction (ADC) level of theory. These gradients enable us to estimate the Hessian matrix, perform geometry optimizations and transition state searches for excited states. Additionally, by interpolation between local points, the gradient and Hessian are used to construct an approximate excited-state potential energy surface and perform excited state dynamics, allowing the inclusion of vibrational effects in X-ray absorption and modelling of resonant inelastic X-ray Scattering (RIXS). RIXS has been shown in recent years to be able to provide information about weak bonds (e.g. hydrogen bonds) in the vicinity of the core-excited atom, bringing extra information compared to X-ray absorption (XAS) and photoemission (XPS) which are sensitive only to the strong chemical bonds at the core-excited site. We will apply the methodology to compute time-resolved X-ray spectra of organic molecules at the ADC and density functional theory (DFT) levels. In particular, we are interested in electron donor molecules such as triphenylamine (TPA) and poly-3-hexylthiophene (P3HT) which are used in organic photovoltaics. The long-term goal of this project is to elucidate the photodegradation mechanisms which limit the lifetime and performance of organic solar cells.