  Ultrafast electron diffraction/microscopy technique enables us to investigate
the nonequilibrium dynamics of crystal structures in the femtosecond-nanosecond
time domain. However, the electron diffraction intensities are in general
extremely sensitive to the excitation errors (i.e., deviation from the Bragg
condition) and the dynamical effects, which had prevented us from
quantitatively discussing the crystal structure dynamics. Here, we develop a
four-dimensional precession electron diffraction (4D-PED) system by which time
($t$) and electron-incident-angle ($\phi$) dependences of electron diffraction
patterns ($q_x,q_y$) are recorded. Nonequilibrium crystal structure refinement
on VTe$_{2}$ demonstrates that the ultrafast change in the crystal structure
can be quantitatively determined from 4D-PED. We further perform the analysis
of the $\phi$ dependence, from which we can qualitatively estimate the change
in the reciprocal lattice vector parallel to the optical axis. These results
show the capability of the 4D-PED method for the quantitative investigation of
ultrafast crystal structural dynamics.
