Pulsed and cw photothermal phase shift spectroscopy in a fluid medium: theory.

A theoretical treatment of photothermal phase shift spectroscopy in a fluid medium for the most general conditions is given. The medium is assumed to be flowing. Results for a stationary medium appear as a special case. Both pulsed and cw excitation are considered. For pulsed excitation, the results are valid for excitation pulses of arbitrary length. For the cw case, modulated excitation is explicitly considered, and the results for unmodulated excitation appear as a special case.