Frequency-comb referenced spectroscopy of v4- and v5-excited hot bands in the 1.5 μm spectrum of C2H2

Doppler-free transition frequencies for v(4)- and v(5)-excited hot bands have been measured in the v(1) + v(3) band region of the spectrum of acetylene using saturation dip spectroscopy with an extended cavity diode laser referenced to a frequency comb. The frequency accuracy of the measured transitions, as judged from line shape model fits and comparison to known frequencies in the v(1) + v(3) band itself, is between 3 and 22 kHz. This is some three orders of magnitude improvement on the accuracy and precision of previous line position estimates that were derived from the analysis of high-resolution Fourier transform infrared absorption spectra. Comparison to transition frequencies computed from constants derived from published Fourier transform infrared spectra shows that some upper rotational energy levels suffer specific perturbations causing energy level shifts of up to several hundred MHz. These perturbations are due to energy levels of the same rotational quantum number derived from nearby vibrational levels that become degenerate at specific energies. Future identification of the perturbing levels will provide accurate relative energies of excited vibrational levels of acetylene in the 7100-7600 cm(-1) energy region. (C) 2015 Elsevier Inc. All rights reserved.