  We study how the combination of long and short laser pulses, can be used to
induce torsion in an axially chiral biphenyl derivative
(3,5-difluoro-3',5'-dibromo-4'-cyanobiphenyl). A long, with respect to the
molecular rotational periods, elliptically polarized laser pulse produces 3D
alignment of the molecules, and a linearly polarized short pulse initiates
torsion about the stereogenic axis. The torsional motion is monitored in
real-time by measuring the dihedral angle using femtosecond time-resolved
Coulomb explosion imaging. Within the first 4 picoseconds, torsion occurs with
a period of 1.25 picoseconds and an amplitude of 3 degrees in excellent
agreement with theoretical calculations. At larger times the quantum states of
the molecules describing the torsional motion dephase and an almost isotropic
distribution of the dihedral angle is measured. We demonstrate an original
application of covariance analysis of two-dimensional ion images to reveal
strong correlations between specific ejected ionic fragments from Coulomb
explosion. This technique strengthens our interpretation of the experimental
data.
