Polyatomic molecules, vibronic structures

Baranov, V. L, Gribov, L. A. Djenjer, V. O. (1996). Semiempitical Parametric Method in the Theory of Vibronic Spectra of Polyatomic Molecules, J Structural Chemistry, 37(3), 367. 

Till recently, computations of vibronic spectra have been limited to small systems or approximated approaches, mainly as a consequence of the difficulties to obtain accurate descriptions of excited electronic states of polyatomic molecules and to computational cost of full dimensional vibronic treatment. Recent developments in electronic structure theory for excited states within the time-dependent density functional theory (TD-DFT) and resolution-of-the-identity approximation of coupled cluster theory (R1-CC2) and in effective approaches to simulate electronic spectra have paved the route toward the simulation of spectra for significantly larger systems. 

From a dynamical (and/or spectroscopic) perspective, we may ask ourselves how to describe and predict the vibronic structures which are superimposed on many low resolution Abs. Cross Sections. These vibronic structures are deeply linked to the time evolution of the wavepacket, after the initial excitation, over typical times of a few hundreds of femtoseconds as discussed by Grebenshchikov et al. [31]. In ID, for a diafomic molecule, fhe fime evolufion is rafher simple when only one upper electronic state is involved. In contrast, for friafomic molecules fhe 3D character of the PESs makes the wavepacket dynamics intrinsically complex. So, for most of the polyatomic molecules, the quantitative interpretation of fhe vibronic structures superimposed to the absorption cross section envelope remains a hard task for two main reasons first because it requires high accuracy PESs in a wide range of nuclear coordinates and, second, it is not easy to follow fhe ND N = 3 for triafomic molecules) wavepackef over several hundred femtoseconds,... 

The combination of modem electronic-structure theory with efficient numerical techniques for the solution of the vibronic Schrddinger equation represents a powerful tool for the analysis of complex electronic spectra as well as ultrafast electronic decay processes in polyatomic molecules. In view of the recent surge of interest in the femtosecond real-time spectroscopy of photophysical and photochemical phenomena (see, for example. Ref. 14), the computational study of ultrafast processes In polyatomic systems has become a topic of particular actuality. 

In this section we discuss very briefly a few other examples of vibronic coupling in polyatomic molecules. The pyrazine cation has been treated along similar lines as the 5i -S2 excited manifold of the neutral species. Its X Ag ground state and the A Big first excited state contain a vacancy in the n and n molecular orbitals, respectively. Therefore, they differ from the S and S2 states basically in that the electron is ionized out of these two orbitals rather than promoted to the n virtual orbital. The analysis shows that the conical intersection occurring between the X-A potential-energy surfaces is essentially a replica of that in the S1-S2 electronic manifold of neutral pyrazine. This interesting structural relationship shows that the shape of the surfaces is governed by the occupied rather than the virtual molecular orbitals in this system. 

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