Picosecond Spectroscopy Using a Photoacoustic Detector.

Picosecond spectroscopy has now become an effective tool for the measurement of ultrafast relaxation rates in many different systems. At the same time photoacoustic spectroscopy has become a similarly successful tool for measuring weak absorption lines in a variety of physical systems. We demonstrated the possibility of combining the time resolution of picosecond spectroscopy with the simplicity and sensitivity of photoacoustic detection. By observing the integrated photoacoustic response to two picosecond pulses with varying time delay, one can measure fast relaxation processes with time resolution limited only by the optimal pulse widths, even though the photoacoustic detection mechanism is much slower. We first developed a new photoacoustic cell with an elliptical cross section to image the acoustic pulse produced by the laser pulse absorption onto the piezoelectric receiver, and improved the cell sensitivity by roughly an order of magnitude. We then carefully analyzed the pressure pulse generation processes and showed that no problem should develop when picosecond pulses are used as long as the beam spot size is kept large enough. With this sensitive apparatus, we studied the short-lived excited states of some organic dyes. We measured their lifetime, their absorption cross-section at one micron, and observed the rotational diffusion effects on the signal. Adequate measurements of excited-state spectra simply do not exist at the present time and we believe picosecond photoacoustics should be a well-adapted tool for these studies. Measurements on weakly absorbing species, systems where excited state fluorescene is not available as a detection mechanism and observations of weakly absorbing excited states in liquids or in solids are three topics that will profit from this new picosecond spectroscopy using photoacoustic detection.