We have used a pulsed dye-laser beam at 6010 Å, of 1-psec duration and ≲ I-mJ energy, to produce a line plasma in a Cs metal vapor. This produces a line source of large thermal energy. The heat diffusion after the laser pulse can be studied by transmission monitoring of a weak cw He-Ne laser beam that is parallel to but separated from the pulsed laser beam by an adjustable displacement. This is possible because the He-Ne beam is absorbed by the minority Cs2 molecules but not by the majority Cs atoms; as the heat pulse diffuses through the probe beam, the transient temperature increase causes a corresponding change in the population distribution of the Cs2 molecules and hence a transient increase in transmission of the probe beam. In other words, the Cs2 works like a thermometer. Our work demonstrates a new and simple method to measure heat diffusion in a well-defined geometry, and it is especially suitable for use in a hot corrosive system because it is a noncontact method. © 1982 Optical Society of America.