A SPECTROPHOTOMETRIC APPARATUS WITH LASER SOURCES FOR MEASUREMENTS OFFAST TRANSIENT SIGNALS

A double beam spectrophotometric apparatus, based on laser light sourc es and on photodiode detectors, was built in order to perform accurate measurements of small concentration changes as those produced in chem ical relaxation experiments. Relaxation techniques, where small and fa stly decaying signals have to be monitored, call for high analytical s ensitivity, To improve this figure of merit, the choice of a laser sou rce proved to be particularly advantageous, since the very high light intensity, providing a remarkable reduction of the shot noise, allows signals of good quality to be obtained without amplification problems. The measured noise was found to be close to the calculated noise, whi ch represents the theoretical lower limiting value. Under the best con ditions the signal-to-noise ratio was improved by a factor 40 with res pect to a commercial apparatus equipped with conventional light source , monochromator and photomultipliers. It is shown that the circumstanc es where the use of the present apparatus is specially suitable are th ose where the dynamic techniques become superior to the static methods in the determination of thermodynamic parameters of reacting systems.