the M axis { ) = 8) for a total of 24 rotations. To check the accuracy of this method, we tried alternate integrations in which the molecular axis was oriented initially in a different position by rotating it about the z axis by a rad and then about the x axis by ß rad. The rotations ( 9 = 8, = 3) were then generated as before.In case B, the rotation was about the z axis with = 3, 4, or 5. The M axis was then moved to sample ne angles about the y axis and rotated about the z axis to sample different angles. The values of and for the different cases are given in Table V.Supplementary Material Available: Complete Table II with results for all of the cases that were investigated (8 pages).
A double beam spectrophotometric apparatus, based on laser light sources and on photodiode detectors, was built in order to perform accurate measurements of small concentration changes as those produced in chemical relaxation experiments. Relaxation techniques, where small and fastly decaying signals have to be monitored, call for high analytical sensitivity. To improve this figure of merit, the choice of a laser source 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, which represents the theoretical lower limiting value. Under the best conditions the signal-to-noise ratio was improved by a factor 40 with respect to a commercial apparatus equipped with conventional light source, monochromator and photomultipliers. It is shown that the circumstances where the use of the present apparatus is specially suitable are those where the dynamic techniques become superior to the static methods in the determination of thermodynamic parameters of reacting systems.
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