The influence of saturation and Stark shifting on the collisionless coherent anti-Stokes Raman scattering (CARS) spectra of Q-branch vibrational transitions has been studied theoretically and experimentally. These processes can significantly affect the magnitude and the spectral distribution of the CARS signal at the input power levels required to obtain detectable signals in typical low-pressure CARS experiments. The theory is developed with use of the Placzek approximation and rotation-vibration Bloch equations are obtained. The Doppler effect, nonresonant susceptibility, and spatial variation of power density in the probe volume are neglected. The Rabi frequency is found to increase with the square root of the final-state vibrational quantum number while the Stark frequency is nearly independent of this quantity. In order to understand the nature of some of the effects observed in scanning CARS, approximate analytic solutions are given for simple cases and CARS output spectra are obtained as a function of detuning. A Rabi-split response spectrum is found for all J with a Stark-induced asymmetry favoring the lower-frequency peak. Although the orientational degeneracy is lifted by both the Rabi and Stark terms, only a very weak dependence on the rotational quantum number is found in the spectral distribution; this allows use of the unsaturated rotational-quantum-number scaling in the saturated regime. It is apparent that the deviations due to saturation are weak for rotational temperature measurements while they are important for vibrational temperature measurements. Experimental results for low-pressure (2–5 mbar) nitrogen are also discussed. Appreciable deviations due to saturation and Stark effects are found for pump laser and Stokes power density products greater than 30 (GW/${\mathrm{cm}}^2$${)}^2$. Semiquantitative agreement with theory is obtained for the observed dependences on power and on rotational and vibrational quantum numbers. A method allowing quick correction of saturated experimental data is proposed.

• Received 30 October 1987

DOI:https://doi.org/10.1103/PhysRevA.38.1948