Diffraction signals of aligned molecules in the gas phase: Tetrazine in intense laser fields

We performed a theoretical study of the electron diffraction patterns that arise from aligning a molecule, s-tetrazine (C2N4H2), in a high intensity laser field using the Friedrich-Herschbach approach. The molecule is modeled using geometries and zero-point vibrations from a coupled-cluster level ab initio calculation. The alignment is achieved by a circularly polarized, quasicontinuous high-intensity laser pulse that intersects the molecular sample at selected geometries. The molecular ensemble is taken to be at a rotational temperature of 5 K. We find that even at 1 TW/cm(2) there is a very noticeable effect on the diffraction pattern stemming from the alignment. Moreover, using different laser geometries, it is possible to observe different diffraction patterns corresponding to specific projections of the molecular geometry onto the Fourier plane of the detector, making the laser a kind of optical goniometer. Predictably, higher alignment laser intensities lead to narrower orientational distributions, which in turn provide for diffraction patterns with larger modulation depths. We find, however, that for small diffraction angles (s less than or equal to 15 Angstrom(-1)) the effect appears to saturate at an alignment laser intensity of about 4 TW/cm(2). This suggests that there is a window of opportunity where alignment is very beneficial for extracting information from gas-phase diffraction patterns and where detrimental multiphoton processes do not yet compromise the interpretation of the molecular structure.