| People involved: | Tõnu Pullerits |
| Former members: | Julia Burkhardt, Niklas Christensson, Sergey Polyutov |
This project is related to the following Fields, Subjects and Techniques:
| Fields: | Computational Chemistry |
| Subjects: | |
| Techniques: | Quantum Chemical Calculations |
The aim of this project is to extract Huang-Rhys factors for all vibrational modes of an arbitrary molecule. The focus is especially on large chromophores. The Huang-Rhys factors give information about the vibrational transition probabilities in the course of an electronic transition. This type of vibrational excitations may be of importance in many processes, for example they may play a part in the mechanism of electron transfer from a chromophore like RuN3 towards a semiconductor in a dye-sensitized solar cell.(Brüggemann etc., Physical Review Letters, 97, 2006, p.208301-2.) Beyond the detailed understanding of such processes the Huang-Rhys factors can even be used to obtain the spectral density, important for the work with electron transfer dynamics, or in a more general way for the reconstruction of the vibrational structure of vibronic spectra.
First a model for the calculations is developed, this is implemented in Matlab and the results are compared to experimental data.
There can be chosen different parameters but as the default calculation path the following is used: After obtaining the relevant data from quantum chemical calculations (in this thesis the Gaussian03 package was used), the molecular structures of the ground and the excited state are brought into translationally and rotationally equivalent positions in the 3-dimensional space by alignment of the mass-centers and the principle axes of inertia. This removal of translations and rotations between the ground and the excited state structures of the molecule is done in order to be able to express the structural shifts between the two states along the normal mode coordinates. Assuming that this is possible and starting with molecular structures given by points in 3N-dimensional space where N is the number of atoms within the molecule, the distances between the ground and excited state structures along the normal mode coordinates can be calculated. The distances along each of the normal mode coordinates are numbers in the length unit Å. Those distances are converted into unitless shifts and in a next step expressed as Huang-Rhys factors. The Huang-Rhys factors can then be used to obtain probabilities of vibrational transitions in the course of an electronic transition. According to the model the transitions 0-n (where n is the final vibrational level in the electronic excited state and 0 represents the ground vibrational level in the electronic ground state) are Poisson distributed where the Huang-Rhys factor is the expectation value. Those vibrational transition probabilities are equivalent to the squared wavefunction overlap of the ground and the excited state vibrational wavefunctions (Franck-Condon factors) and are used to construct theoretical spectra which then are compared to corresponding experiments.
The theoretical spectra modelled on the basis of the calculated Huang-Rhys factors were in very good conformance with the modelled parts of gas phase spectra for ethylene and perylene. The comparision to vibronic spectra of rhodamine800 in an ethanol matrix gave less good but still acceptable results. The Matlab program can extract Huang-Rhys factors from Gaussian data of arbitrary molecules but a general statement about the accuracy of the results can only be done after testing the model for a larger number of molecules.
Additional information can be obtained from the finished Master thesis:
Julia Burkhardt: Vibronic coupling in large chromophores (thesis, appendix, literature in zip).