Ultrafast Electron Diffraction: Oriented Molecular Structures in Space and Time

Abstract

The technique of ultrafast electron diffraction allows direct measurement of changes which occur in the molecular structures of isolated molecules upon excitation by femtosecond laser pulses. The vectorial nature of the molecule-radiation interaction also ensures that the orientation of the transient populations created by the laser excitation is not isotropic. Here, we examine the influence on electron diffraction measurements--on the femtosecond and picosecond timescales--of this induced initial anisotropy and subsequent inertial (collision-free) molecular reorientation, accounting for the geometry and dynamics of a laser-induced reaction (dissociation). The orientations of both the residual ground-state population and the excited- or product-state populations evolve in time, with different characteristic rotational dephasing and recurrence times due to differing moments of inertia. This purely orientational evolution imposes a corresponding evolution on the electron scattering pattern, which we show may be similar to evolution due to intrinsic structural changes in the molecule, and thus potentially subject to misinterpretation. The contribution of each internuclear separation is shown to depend on its orientation in the molecular frame relative to the transition dipole for the photoexcitation; thus not only bond lengths, but also bond angles leave a characteristic imprint on the diffraction. Of particular note is the fact that the influence of anisotropy persists at all times, producing distinct differences between the asymptotic "static" diffraction image and the predictions of isotropic diffraction theory.

4D electron imaging: principles and perspectives

“…11). In a recent series of publications, 37 which generalized the previous results, 38 the anisotropic scattering was found to be distinct from that of isotropic patterns. We reexamined their time-dependent manifestations in UED, and worked out a theoretical treatment to include arbitrary polarization directions and laser-induced sample distributions.…”

4d Ultrafast Electron Diffraction, Crystallography, and Microscopy

“…Also, using a new apparatus we plan to study structural dynamics of macromolecular systems with the aid of new theoretical formulation, including charge density maps (60), of diffraction from isolated proteins. Oriented reactions (73,74) are also part of the new effort in UED.…”

Time-Resolved Electron Diffraction from Selectively Aligned Molecules

“…It has been shown theoretically that electron diffraction from aligned molecules in the gas phase reveals not only the interatomic distances but also the corresponding angles [6,7]. Aligning molecules in helium droplets using powerful cw lasers has been proposed [8], and electron diffraction from adiabatically aligned molecules has been demonstrated experimentally [9].…”