Time-resolved electron diffraction from selectively aligned molecules.

We experimentally demonstrate ultrafast electron diffraction from transiently aligned molecules in the absence of external (aligning) fields. A sample of aligned molecules is generated through photodissociation with femtosecond laser pulses, and the diffraction pattern is captured by probing the sample with picosecond electron pulses shortly after dissociation-before molecular rotation causes the alignment to vanish. In our experiments the alignment decays with a time constant of 2.6+/-1.2 ps.