Although the field of gas phase rotational spectroscopy is mature and millions of rotational spectral lines have been measured in hundreds of molecules with sub-MHz accuracy, it remains a challenge to measure these rotational spectra in excited vibrational modes with the same accuracy. Based on the recently demonstrated quantum cascade laser (QCL) pumped molecular laser (QPML) we demonstrate how an infrared QCL may be used to enhance absorption strength or induce lasing of terahertz rotational transitions in highly excited vibrational modes in order to measure their frequencies more accurately. We measured the spectra of more than 20 individual lines to obtain the rotational constants of the nitrous oxide (laughing gas) molecule and the results reproduced the measured spectra to within the experimental uncertainty of +/-5 kHz. This technique can be applied to hundreds of other molecules for which a QCL can be tuned across their vibrational modes and will allow for a better knowledge of the rotational spectrum of their excited vibrational modes.
The work is featured on the front cover of Volume 76, Issue 12 (2022) of Applied Spectroscopy.