In this review we describe the development and the applications of the picosecond-jet
technique, which utilizes a picosecond laser and a supersonically-cold jet beam of large
molecules. The applications include studies of coherence (quantum beats), photodissociation,
isomerization, and partial solvation. The results emphasize the role of
intramolecular energy redistribution, and provide evidence for intramolecular threshold effect
for rates vs. excess molecular energy. Simple theory for this redistribution of
energy among certain modes in molecules is given, and comparison with RRKM
calculation is also made to assess the nature of the statistical behavior for the redistribution.