Experimental observation and analysis of the 3ν₁(Σg) stretching vibrational state of acetylene using continuous-wave infrared stimulated emission

We present a sensitive experimental method for molecular spectroscopy that can be used to determine ro-vibrational states using mid-infrared stimulated emission. Our infrared stimulated emission probing (IRSEP) experiment is based on using a narrow-line, continuous-wave Ti:sapphire laser beam (pump) to excite the molecules to an upper vibrational state and a continuous-wave, mid-infrared beam from an optical parametric oscillator (probe) to detect the stimulated emission by the excited molecules. Spectroscopic data are gathered by tuning the wavelengths of the beams. The molecules are probed before their velocity distribution is disturbed by collisions, which leads to a sub-Doppler resolution. The full width at half maximum of the emission peaks is below 10 MHz. The stimulated emission lines are measured with an accuracy of at least 0.005 cm⁻¹. We use the IRSEP experiment to observe and analyze the symmetric ro-vibrational state [21+] (3ν₁(Σg)) of acetylene (C₂H₂). This state is not accessible via one photon transitions from the ground vibrational state. We use the least-squares method to determine that the band center is at 9991.9725 (12) cm⁻¹ and the rotational parameters are B = 1.156145 (22) and D = 1.608 (87) × 10⁻⁶ cm⁻¹, where the uncertainties in parentheses are one-standard errors in the least significant digit.