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Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized¹B_2u(ππ*) (S₂) and¹B_3u(nπ*) (S₁) states, the participation of the optically dark¹A_u(nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computations. Despite¹A_u(nπ*) and¹B_3u(nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The¹A_u(nπ*) state is populated in 200 ± 50 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.

 

This perspective article highlights the challenges in the theoretical description of photoreceptor proteins using multiscale modeling, as discussed at the CECAM workshop in Tel Aviv, Israel. The participants have identified grand challenges and discussed the development of new tools to address them. Recent progress in understanding representative proteins such as green fluorescent protein, photoactive yellow protein, phytochrome, and rhodopsin is presented, along with methodological developments.

 

We present an ab initio computational study of the Auger electron spectrum of benzene. Auger electron spectroscopy exploits the Auger–Meitner effect, and although it is established as an analytic technique, the theoretical modeling of molecular Auger spectra from first principles remains challenging. Here, we use coupled-cluster theory and equation-of-motion coupled-cluster theory combined with two approaches to describe the decaying nature of core-ionized states: (i) Feshbach–Fano resonance theory and (ii) the method of complex basis functions. The spectra computed with these two approaches are in excellent agreement with each other and also agree well with experimental Auger spectra of benzene. The Auger spectrum of benzene features two well-resolved peaks at Auger electron energies above 260 eV, which correspond to final states with two electrons removed from the 1 e 1 g and 3 e 2 g highest occupied molecular orbitals. At lower Auger electron energies, the spectrum is less well resolved, and the peaks comprise multiple final states of the benzene dication. In line with theoretical considerations, singlet decay channels contribute more to the total Auger intensity than the corresponding triplet decay channels. 

Photoelectron–photofragment coincidence spectroscopy was used to study the dissociation dynamics of the conjugate bases of benzoic acid and p -coumaric acid. Upon photodetachment at 266 nm (4.66 eV) both aromatic carboxylates undergo decarboxylation, as well as the formation of stable carboxyl radicals. The key energetics are computed using high-level electronic structure methods. The dissociation dynamics of benzoate were dominated by a two-body DPD channel resulting in CO 2 + C 6 H 5 + e − , with a very small amount of stable C 6 H 5 CO 2 showing that the radical ground state is stable and the excited states are dissociative. For p -coumarate ( p -CA − ) the dominant channel is photodetachment resulting in a stable radical and a photoelectron with electron kinetic energy (eKE) <2 eV. We also observed a minor two-body dissociative photodetachment (DPD) channel resulting in CO 2 + HOC 6 H 4 CHCH + e − , characterized by eKE <0.8 eV. Evidence was also found for a three-body ionic photodissociation channel producing HOC 6 H 5 + HCC − + CO 2 . The ion beam contained both the phenolate and carboxylate isomers of p -CA − , but DPD only occurred from the carboxylate form. For both species DPD is seen from the first and second excited states of the radical, where vibrational excitation is required for decarboxylation from the first excited radical state.