Excitation profiles of the coherent anti-Stokes resonance Raman spectrum of β-carotene

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Abstract
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.