Electron diffraction of molecules in specific quantum states:: A theoretical study of vibronically excited s-tetrazine

The electron diffraction patterns of s-tetrazine (C2N4H2) in specific electronic and vibrational states have been calculated for isotropic samples, for molecules that are aligned in high-intensity laser fields, and for orientationally clamped molecules. Using structures and normal mode displacement coordinates from a coupled-cluster ab initio calculation, we evaluate diffraction patterns of the molecule in the vibrationless level of So, and the 0(0), 1(2), 6a(3), and 16a(8) vibrational levels of the S, state. It was found that both the electronic and the vibrational excitations lead to changes in the diffraction patterns of isotropic samples in the range of +/-1%. Effects of the respective orientation of the excitation laser polarization and the detector direction are clearly observed. Alignment of the molecule in intense laser fields, with intensities ranging from 1.06 to 9.56 TW/cm(2), provides for an increase in the modulation depths of all diffraction patterns and can be used to further enhance the observability of vibrational diffraction patterns. This study demonstrates that vibrational probability density distributions should be observable using electron diffraction and that skillful orientation of the excitation laser polarization, the polarization of the alignment laser, and the direction of signal detection can be exploited to map the wave functions from different sides. The differences in diffraction patterns of nominally isoenergetic vibrational states provide an opportunity for experimentally observing the flow of vibrational energy through the phase space by time-dependent three-dimensional mapping of the wave packet.