The excitation profiles for the ν1 and ν2 vibrations of trans- β-carotene have been studied by means of coherent anti-Stokes Raman spectroscopy (CARS). The experimental profiles are compared to those calculated using the Albrecht theory modified for CARS. In order to obtain meaningful results it was necessary to drastically lower the laser power, specifically the power of ω1, the beam in resonance with the electronic absorption. If the laser power was too high, the intensity of the β-carotene lines did not scale with the square of the power in the pump beam due to saturation effects and, in fact, if the power was high enough the CARS signal was lost in the background emission. Due to the necessity of using low powers, photomultiplier detection was employed. The methods developed made it possible to detect β-carotene in benzene solution at concentrations of ∼5×10−7M, which is considerably lower in concentration than previously observed by CARS.
Modern laser and molecular beam techniques make feasible experimental studies of transition states in chemical reactions. Rough estimates of the signals to be expected under different circumstances are described. Several recent studies are discussed with comments on the type of information which can be obtained.
Emission at the Na D lines has been observed from the intersection of crossed molecular beams of K and NaCI irradiated by a cw dye laser at wavelengths from 590 and 735 nm. The three-beam signal exhibits a threshold near 735 om and is linearly dependent on the laser and molecular beam intensities. This three-beam signal has been observed under experimental conditions in which all two-beam signals are accounted for, and is attributed to the formation ofNa* by photoexcitation of the KNaCI reaction complex. The effective two-body cross section for the process is approximately 10-21 cm 2 in a laser field of 1.5 kW /cm 2 •
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