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Ultrafast Electron Diffraction: Oriented Molecular Structures in Space and Time
Author(s) -
Baskin J. Spencer,
Zewail Ahmed H.
Publication year - 2005
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200500331
Subject(s) - electron diffraction , ultrafast electron diffraction , dephasing , gas electron diffraction , photoexcitation , diffraction , chemistry , excitation , molecular physics , excited state , femtosecond , atomic physics , electron , dipole , laser , reflection high energy electron diffraction , physics , optics , condensed matter physics , organic chemistry , quantum mechanics
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.