Photoexcitation is achieved by the third harmonic of a Q-switched Nd:YAG laser (Surelite II - 10, Continuum). A small fraction of the output is diverted by a beam splitter and attenuated by a set of neutral density filters, so that the energy of the pulses on the sample can be adjusted between 3 and 30 mJ. No significant changes with laser fluence were observed in any of the measured parameters, except that the S/N ratio degraded at the lower energies. The unfocussed beam was shaped by means of a slit (280 mm width) positioned in front of the quartz cuvette. Part of the energy was splitted to an energy meter (Laser Precision RJ-7620) equipped with a pyroelectric energy probe (Laser Precision RJp-765) for normalization purposes. The pressure wave induced in solution was detected by a PZT piezoelectric transducer (Panametrics V-103). The signal was then amplified (60 db) and recorded by a digitizing oscilloscope (LeCroy 9450A) operated at 2.5 ns/channel. Typically 4000 points were acquired, with a time window of 10 ms. The quartz cuvette was placed inside a temperature controlled sample holder (Quantum Northwest, Inc. TASC 300) which assured a temperature stability of better than 0.02 °C inside the solution. The sample holder allows magnetic stirring of the sample and dry gas purge of the atmosphere, in order to prevent condensation at low temperatures. The N₂ purging mechanism also minimizes CO₂ solubilization effects during the experiment. Data acquisition and analysis were performed by means of dedicated software (Sound Acquisition and Sound Analysis, Quantum Northwest, Inc.).
Generally 100 waveforms were averaged for the reference signal (S/N improvement of ´ 10) whereas only 9 waveforms were averaged for the samples to avoid extensive photodegradation of oNBA (S/N improvement of ´ 3).