Abstract
Understanding light-induced processes in biological and human-made molecular systems is one of the main goals of physical chemistry. It has been known for years that the photoinduced dynamics of atomic nuclei can be studied by looking at the vibrational substructure of electronic absorption and emission spectra. However, theoretical simulation is needed to understand how dynamics translates into the spectral features. Here, we review several recent developments in the computation of vibrationally resolved electronic spectra (sometimes simply called 'vibronic' spectra). We present a theoretical approach for computing such spectra beyond the commonly used zero-temperature, Condon, and harmonic approximations. More specifically, we show how the on-the-fly ab initio thawed Gaussian approximation, which partially includes anharmonicity effects, can be combined with the thermo-field dynamics to treat non-zero temperature and with the Herzberg-Teller correction to include non-Condon effects. The combined method, which can treat all three effects, is applied to compute the S1 ← S0 and S₂ ← S0 absorption spectra of azulene.
Highlights
Electronic spectroscopy is the most widely used experimental technique for studying excited electronic states of molecules.[1]
For the vibrationally resolved electronic, or vibronic, spectroscopy, this is known as the Franck-Condon principle, which describes the spectrum as a sum over all possible transitions between the vibrational states of the ground and excited electronic states.[7]
The lowest-energy electronic band extends from the red to the green part of the visible spectrum and is responsible for the characteristic blue color of azulene. Such a broad band is a direct indication of highly rich excited-state dynamics, where the initial wavepacket is strongly displaced from the excited-state minimum and forms a superposition of many vibrational wavefunctions of the excited electronic state
Summary
Electronic spectroscopy is the most widely used experimental technique for studying excited electronic states of molecules.[1]. An example that perfectly illustrates this issue is the benchmarking of electronic structure methods for modeling excited electronic states This is often performed by comparing the vertical excitation energy,[3,4,5] i.e. the energy gap between the ground and excited states computed at a single molecular geometry, with a perceived ‘experimental value’. Laureates: Junior Prizes of the SCS Fall Meeting 2020 would be based on computing full electronic spectra and comparing these directly to the experimental result.[6] as already mentioned above, the simulation of spectra is a challenge of its own and requires approximations to the involved potential energy surfaces and to the photoexcitation process In this brief overview, we discuss a recently developed framework that allows us to avoid some of these approximations and to improve upon the existing computational approaches for simulating electronic spectra
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