Beside experimental uncertainties, the small difference between the angles determined by this method and those of single crystals also results from the fact that the thin film samples are polycrystalline where some of the molecules are amorphous. Using this method, the relative orientation angle in DEAC film is determined to be 53.40, close to 600 of its single crystalline structure, and that of the perylene film is determined to be 6.20, also close to -0.20 of its single crystalline structure. The signal anisotropy changes caused by intermolecular energy/electron transfers are utilized to calculate the cross angles between the electronic transition dipole moment of the donor and the vibrational transition dipole moments of the acceptor, yielding the relative orientation between two adjacent molecules. Herein, the polarization-selective ultraviolet/infrared (UV/IR) mixed frequency ultrafast spectroscopy is applied to investigate the relative molecular orientations in two organic thin films of 7-(diethylamino)coumarin-3-carboxylic acid (DEAC) and perylene. Molecular packing patterns are crucial factors determining electron/energy transfer processes that are critical for the optoelectronic properties of organic thin film devices. The theoretical results here agree reasonably well with published experiment results and pave way for realizing even higher enhancement at nearer-Brewster angle. It cannot improve the signal to noise ratio when the dominate noise is from the signal itself. Nevertheless, near-Brewster angle reflection will enhance both the signal and the noise caused by the signal itself, therefore this method only works if the noise is unrelated to the signal, particularly if the noise is caused by the fluctuation in the probe. Considering all the above factors, the enhancement on the order of 1,000 is possible in a current experiment. Moreover, traditional energy ratio of pump and probe pulses, which is 9:1, may not be ideal and could be changed to 2:1 in the reflection geometry. In current experiments, the enhancement factor is mainly limited by the precision of incident angle, for which ordinary rotation stages are probably not adequate enough. Leaking s-polarized light due to imperfectness of IR polarizers in the reflection geometry may limit the enhancement factor, but such limit is above what a typical experiment can reach. Our simulation shows that the dispersion in reflection will only alter the 2D IR lineshape slightly and can be corrected. Light dispersion and the leakage of s-polarized light are considered in simulating the enhancement factor of the reflection mode. In this work, we simulated 2D IR spectroscopy in both transmission geometry and Brewster-angle reflection geometry. The barrier reduction by deuterium substitution was partly attributed to the difference between the wave functions of H and D atomic nuclei. We performed ab initio theoretical calculations of the multi-component molecular orbital method.
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