As a postdoc, Eisenthal worked at UCLA (University of California, Los Angeles), where he gained experience in molecular spectroscopy in the research group of Mostafa El-Sayed.
The development of the picosecond laser in the late 1960s created new possibilities for measuring molecular relaxation processes.
Eisenthal made important contributions to the then new field of picosecond-laser spectroscopy.
In 1975 he moved to Columbia University, as the holder of a professorship ("Mark Hyman Professor of Chemistry").
At Columbia University he did fundamental research on free electrons in water, the photochemistry of carbenes, and other photochemical processes (photoisomerization), before his research activities focused on the application of lasers to investigation of the equilibrium and dynamic properties of molecules at liquid interfaces and at solid interfaces.