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Fourier-transform intra-cavity laser absorption spectroscopy has been used to record high-resolution vibration–rotation overtone spectra of carbon-13 substituted acetylenes, H 12 C 13 CH and H 13 C 13 CH, in the third CH stretching overtone region between 12 500 and 12 730 cm −1 . The observed spectra of both species contain only parallel bands for which rovibrational spectroscopic parameters have been determined. Several anharmonic resonances have been identified and rotationally analysed using traditional vibration–rotation theory. Altogether six bands, which include ν 1 + 3 ν 3 and 4 ν 3 for H 12 C 13 CH and ν 1 + 3 ν 3 for H 13 C 13 CH, have been observed for both species. Four of the six bands in each molecule are reported for the first time.

A high-resolution vibration–rotation overtone spectrum of H13C12CH has been recorded with a Fourier transform infrared spectrometer in the wavenumber region 6400 to 6700 cm−1. The main band, assigned as the C–H stretching combination band ν1+ν3, and some overtone and hot bands have been rotationally analyzed. Altogether eight parallel bands have been observed. The vibrational labels have been deduced on the basis of the assignments of the fundamental ν3 antisymmetric C–H stretching band system.

We describe the design, construction, and initial performance evaluation of a high-repetition-rate cavity ringdown spectrometer. The spectrometer is based on the use of the Pound-Drever-Hall technique to lock the laser frequency to the maximum of a transmission fringe of the interferometer used as a sample cell. This results in continuous injection of light into the interferometer. The injection is repetitively interrupted with an acousto-optical modulator to generate ringdowns (exponential decays) at a typical rate of 10 kHz. Averaging of these large numbers of fitted ringdown times allows us to attain a minimum detectable absorption of 1.43×10−11 cm−1 Hz−1/2 short term and 9.0×10−11 cm−1 Hz−1/2 long term. In addition, the spectrometer has a continuous tuning capability of ∼1 cm−1, which allows the use of standard linearization and frequency calibration techniques for the spectrum. To illustrate the operation and sensitivity of the spectrometer, part of the Q-branch of a weak acetylene overtone has been recorded.