Monolayers of MoS2 on Ag(111) as decoupling layers for organic molecules: resolution of electronic and vibronic states of TCNQ

The electronic structure of molecules on metal surfaces is largely determined by hybridization and screening by the substrate electrons. As a result, the energy levels are significantly broadened and molecular properties, such as vibrations are hidden within the spectral line shapes. Insertion of thin decoupling layers reduces the line widths and may give access to the resolution of electronic and vibronic states of an almost isolated molecule. Here, we use scanning tunneling microscopy and spectroscopy to show that a single layer of MoS2 on Ag(111) exhibits a semiconducting bandgap, which may prevent molecular states from strong interactions with the metal substrate. We show that the lowest unoccupied molecular orbital (LUMO) of tetracyanoquinodimethane (TCNQ) molecules is significantly narrower than on the bare substrate and that it is accompanied by a characteristic satellite structure. Employing simple calculations within the Franck–Condon model, we reveal their vibronic origin and identify the modes with strong electron–phonon coupling.

An update to the MARVEL data set and ExoMol line list for 12C2

ABSTRACT The spectrum of dicarbon (C2) is important in astrophysics and for spectroscopic studies of plasmas and flames. The C2 spectrum is characterized by many band systems with new ones still being actively identified; astronomical observations involve eight of these bands. Recently, Furtenbacher et al. presented a set of 5699 empirical energy levels for 12C2, distributed among 11 electronic states and 98 vibronic bands, derived from 42 experimental studies and obtained using the MARVEL (Measured Active Rotational-Vibrational Energy Levels) procedure. Here, we add data from 13 new sources and update data from 5 sources. Many of these data sources characterize high-lying electronic states, including the newly detected 3 3Πg state. Older studies have been included following improvements in the MARVEL procedure that allow their uncertainties to be estimated. These older works in particular determine levels in the C 1Πg state, the upper state of the insufficiently characterized Deslandres–d’Azambuja (C 1Πg–A 1Πu) band. The new compilation considers a total of 31 323 transitions and derives 7047 empirical (marvel) energy levels spanning 20 electronic and 142 vibronic states. These new empirical energy levels are used here to update the 8states C2 ExoMol line list. This updated line list is highly suitable for high-resolution cross-correlation studies in astronomical spectroscopy of, for example, exoplanets, as 99.4 per cent of the transitions with intensities over 10−18 cm molecule−1 at 1000 K have frequencies determined by empirical energy levels.

Vibronic coherence evolution in multidimensional ultrafast photochemical processes

The complex choreography of electronic, vibrational, and vibronic couplings used by photoexcited molecules to transfer energy efficiently is remarkable, but an unambiguous description of the temporally evolving vibronic states governing these processes has proven experimentally elusive. We use multidimensional electronic-vibrational spectroscopy to identify specific time-dependent excited state vibronic couplings involving multiple electronic states, high-frequency vibrations, and low-frequency vibrations which participate in ultrafast intersystem crossing and subsequent relaxation of a photoexcited transition metal complex. We discover an excited state vibronic mechanism driving long-lived charge separation consisting of an initial electronically-localized vibrational wavepacket which triggers delocalization onto two charge transfer states after propagating for ~600 femtoseconds. Electronic delocalization consequently occurs through nonadiabatic internal conversion driven by a 50 cm−1 coupling resulting in vibronic coherence transfer lasting for ~1 picosecond. This study showcases the power of multidimensional electronic-vibrational spectroscopy to elucidate complex, non-equilibrium energy and charge transfer mechanisms involving multiple molecular coordinates.

Study of the vibronic coupling in the ground state of Methylthio radical

Using a methodology based on the crude adiabatic approximation, we study the complete linear and quadratic vibronic coupling in the ground state of SCH3 radical. In order to build the representation of the hamiltonian, we evaluated 30 integrals intervening in the formulation of the vibronic coupling. Diagonalization of this representation gives the vibronic levels. For the lowest vibronic states, the implied modes are Q1 (symmetric C-S stretching) and Q4 (CH3 rocking). Energy gaps A1-A2 and A2-ε resulting from the splitting due to the Jahn-Teller coupling E e = A1 + A2+ ε are evaluated to 250 and 169 cm-1, respectively. Essential coupling parameters are surrounded to simplify the study of highly vibronic states.

Quasi Diabatic Propagation Scheme for Direct Simulation of Proton-Coupled Electron Transfer Reaction

We apply a recently-developed quasi-diabatic (QD) propagation scheme to simulate proton-coupled electron transfer (PCET) reactions. This scheme enables a direct interface between an accurate diabatic dynamics approach and the adiabatic vibronic states. It explicitly avoids theoretical efforts to pre-construct diabatic states for the transferring electron and proton or reformulate diabatic dynamics methods to the adiabatic representation, both of which are non-trivial tasks. Using partial linearized path-integral approach and symmetrical quasi-classical approach as the diabatic dynamics methods, we demonstrate that the QD propagation scheme provides accurate vibronic dynamics of PCET reactions and reliably predict the correct reaction mechanism without any a priori assumptions. This work demonstrates the possibility to directly simulate challenging PCET reactions by using accurate diabatic dynamics approaches and adiabatic vibronic information.

Quasi Diabatic Propagation Scheme for Direct Simulation of Proton-Coupled Electron Transfer Reaction

We apply a recently-developed quasi-diabatic (QD) propagation scheme to simulate proton-coupled electron transfer (PCET) reactions. This scheme enables a direct interface between an accurate diabatic dynamics approach and the adiabatic vibronic states. It explicitly avoids theoretical efforts to pre-construct diabatic states for the transferring electron and proton or reformulate diabatic dynamics methods to the adiabatic representation, both of which are non-trivial tasks. Using partial linearized path-integral approach and symmetrical quasi-classical approach as the diabatic dynamics methods, we demonstrate that the QD propagation scheme provides accurate vibronic dynamics of PCET reactions and reliably predict the correct reaction mechanism without any a priori assumptions. This work demonstrates the possibility to directly simulate challenging PCET reactions by using accurate diabatic dynamics approaches and adiabatic vibronic information.

Elucidation of near-resonance vibronic coherence lifetimes by nonadiabatic electronic-vibrational state character mixing

Recent work suggests that the long-lived coherences observed in both natural and artificial light-harvesting systems (such as the Fenna–Matthews–Olson complex) could be attributed to the mixing of the pigments’ electronic and vibrational degrees of freedom. To investigate the underlying mechanism of these long coherence lifetimes, a sophisticated description of interactions between the molecular aggregates and the nonequilibrium fluctuations in the surrounding environment is necessary. This is done by implementing the hierarchical equations of motion approach on model homodimers, a method used in the intermediate coupling regime for many molecular aggregates wherein the nonequilibrium environment phonons play nontrivial roles in exciton dynamics. Here we report a character change in the vibronic states—reflective of property mixing between the electronic and vibrational states—induced by an interplay between system coupling parameters within the exciton-vibrational near-resonance regime. This mixing dictates vital aspects of coherence lifetime; by tracking the degree of mixing, we are able to elucidate the relationship between coherence lifetime and both the electronic energy fluctuation and the vibrational relaxation dephasing pathways.