Four-wave mixing methods such as excited-state absorption (ESA) offer valuable insight into electronic structure and electron dynamics, and use of X-ray pulses extends these to core electrons. Although perturbative methods are often used to model such processes, real-time methods offer an unparalleled ability to visualize and interpret the associated electron dynamics. To model the X-ray ESA (core electron excitations), we use real-time time-dependent density functional theory (RT-TDDFT) due to its attractive tradeoff between speed and accuracy. Since pathologies in the DFT functional do not permit driving full electrons to the excited state directly, we apply a two-step simulation protocol. In essence, the first simulation is used to generate the excited population state wavepacket, and the second performs the excited state absorption. As a test-case, we demonstrate this technique by calculating the multidimensional ESA spectra of the oxygen and nitrogen core electrons in 5-flurothiophene-2-carbonitrile.

Calculating excited-state absorption using RT-TDDFT

ACS Spring 2022

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