Femtosecond interferometric photoacoustic spectroscopy using incoherent light

An interferometric phase-modulated femtosecond wave packet technique has been combined with photoacoustic detection to study rovibrational wave packets, using 30 ps (full-width-at-half-maximum) incoherent light pulses. The phase-modulated technique is particularly suited to be combined with photoacoustic detection, thanks to the adaptability of the modulation frequency of the light pulses to correspond to a resonance frequency of the photoacoustic cell. As an example to demonstrate the principle, the B-state of iodine has been chosen. The average 330 fs wavepacket period of the vibrational levels 12-15 and wave packet dephasing have been observed, identical to those which would be expected if transform-limited, 100 fs fwhm pulses had been used. It is shown that the combination of these detection techniques results into a very sensitive method for performing time-resolved experiments.