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Optics of Organic Nanomaterials
Published in Vladimir I. Gavrilenko, Optics of Nanomaterials, 2019
The optical spectra of PPV or oligomers and their derivatives present clear signs of strong vibronic coupling. A very good theoretical simulation of the absorption and emission spectra of oligomers containing from two to five rings is obtained (Fig. 9.15) by considering the coupling of the S0 to S1 or S1 to S0 electronic transition with two Raman modes (at 0.16 and 0.21 eV) corresponding to the C−C bond stretching and phenylene breathing modes, respectively (Bredas et al., 1999).
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Published in Splinter Robert, Illustrated Encyclopedia of Applied and Engineering Physics, 2017
[atomic, solid-state, thermodynamics] A process in theoretical chemistry that involves the interaction in a molecule that hold the exchange between electronic and nuclear vibrational motion, referred to as “vibronic coupling.” Vibronic coupling pertains to the mixing of electronic states of the molecule as a result of small vibrations and is tied to the derivative of the wave function for the configuration. Nonadiabatic chemical processes occur at the conical interactions. In a molecular energy configuration when two potential energy surfaces are degenerate and intersect the energy state in conical intersect. At this point the nonadiabatic coupling between these two energy states is also nonvanishing. The energetic boundary layer surrounding the conical intersect no longer obeys the Born–Oppenheimer approximation, supporting the nonadiabatic conditions. Nonadiabatic effects, specifically involve the splitting and scattering of the wave packet (quantum effect dominated solution to the time-dependent Schrödinger equation) at potential energy surface crossings, such as associated with tunneling. Chemical interactions that can be described by the conical intersection are photoelectric reactions, combustion, and explosion, for instance. One specific example of the conical intersect is the stability of dna (DNA molecule encoded with a range of biological properties: genetic structure) under irradiation by ultraviolet light, where the “excited” electrovibrational state of the molecular wave packet will “roll-back” to the electronic ground state on the conically curve potential energy surface of the molecular binding. The number of vibrational degrees of freedom is directly related to the number of atoms making up the molecule; for instance, a diatomic molecule has two degrees and consequently progressively upward. On a nuclear level, similar principles will apply; only in this case, the energy potentials are outlined by the proton and neutron interactions (see Figure N.47).
Vibronic mean-field and perturbation theory for Jahn-Teller and pseudo-Jahn-Teller molecules
Published in Molecular Physics, 2021
Non-adiabatic vibronic coupling plays a central role in the spectroscopy and dynamics of open-shell and electronically excited molecules. Frequency-domain spectroscopy of the low-energy vibronic excitations in such systems is often shaped by symmetry-enforced conical intersections between degenerate electronic states – the Jahn-Teller effect [1–3] – or more generic interactions between non-degenerate states, often termed pseudo-Jahn-Teller coupling [4]. A practical and conceptually appealing framework for describing these types of vibronically coupled systems is the quasi-diabatic model of Köppel, Domcke, and Cederbaum [5]. In this approach, the electronic states of interest are represented in a quasi-diabatic basis such that nuclear coordinate derivative coupling can be safely neglected, while introducing nuclear coordinate-dependent (non-derivative) coupling between the diabatic electronic states. When parameterised with high-level quantum chemical input, quasi-diabatic models can accurately simulate the complex vibronic spectra and dynamics of strongly non-adiabatic systems [6–9].
UV absorption spectrum and photodissociation dynamics of CH2OO following excitation to the B 1 A′ state
Published in Molecular Physics, 2021
Behnam Nikoobakht, Horst Köppel
The UV absorption spectrum has been simulated and found to be in good agreement with the corresponding experimental ones [2,4]. We revealed the role of the coupling between the B and C states by considering its dependence on the C–O–O bond angle, compared with a constant value 0.045 eV and zero coupling between the B and C states. The quantum dynamics leads to a spectrum close to the available experimental results. The vibronic coupling leads to broader and more diffuse spectral structures and a higher background on the high-energy side of the transition. For comparison, we also carried out a propagation calculation using the uncoupled B state Hamiltonian retaining only the O–O stretching (or alternatively the C–O–O bending mode). The resulting spectrum consists of the discrete (below barrier) and continuous (above barrier) contribution. The discrete part of the spectrum was understood in terms of oscillations of the WP in the shallow well of the diabatic B state, while the continuous part of the spectrum indicates prompt photodissociation of CHOO. The dynamics including inter-surface or vibronic coupling is considerably more complicated and deserves further investigation. Possible scenarios for the remaining deviations from experiment have been delineated above, regarding possible improvements of the theoretical treatment. Regarding the experimental side, ambiguities concerning the background subtraction e.g. the strong IO contribution to the raw spectral data [5], may deserve further attention.