Dipole saturated absorption modeling in frequency modulation spectroscopy: Dealing with a Gaussian beam, resonance narrowing

Quantitative, nonlinear sub-Doppler sensitive spectroscopy requires reliable formalism for optimizing the detection of weak molecular absorption patterns. Phase modulation spectroscopy (FMS) is a common technique used to challenge the ultimate photon-shot-noise limit. In addition, nonlinear spectroscopy can exhibit surprising effects like resonance narrowing, i.e., resonance width below the transit-time rate, like it has been pointed out in the seventies. Here, by extending our previous work, we propose an analytical development associated with numerical integration to simulate the equivalent Lamb-dip under Gaussian beam conditions. Simulations of nonlinear profiles of two transitions belonging to the polyad Delta P11 of acetylene are discussed, and analyzed by fitting with standard functions. The respective roles of the transit-time, of the collision rate, and of the Rabi frequency (including the power broadening) are carefully discussed and reviewed. It shows "super-narrowing" effects under specific low pressure and low power conditions. A reviewing introduction states previous experimental reports as well about previous modeling. (C) 2019 Elsevier Ltd. All rights reserved.