Precision measurement of molecular rotational spectra in excited vibrational modes*

Rotational spectroscopy is an important field in molecular physics with widespread applications in radio astronomy. However majority of rotational spectroscopy measurements have been performed only in the ground vibrational mode of molecules because the thermal population of excited vibrational modes drops exponentially with vibrational energy. Rotational spectra in each excited vibrational mode is uniquely altered by the associated vibrational motions and coupling among neighboring ro-vibrational energy levels. This makes measurements of rotational spectra in excited vibrational modes of high interest for reconstructing the complex intramolecular energy transfer pathways within these molecules. Standard techniques to obtain rotational spectrum in excited vibrational modes typically suffer from complexity or reduced precision compared to that routinely achieved for rotational spectra in the ground vibrational mode. Here, we demonstrate how an infrared QCL may be used to measure rotational transitions in highly excited vibrational modes by enhancing absorption strength or inducing lasing of terahertz rotational transitions in these vibrational modes. We used a tunable QCL to excite v₃ R-branch transitions in nitrous oxide and either enhanced absorption or induced lasing on 20 v₃ rotational transitions, whose frequencies between 299 and 772 GHz were then measured using either heterodyne or modulation spectroscopy. The spectra were fitted to obtain the rotational constants B₃ and D₃, which reproduce the measured spectra to within the experimental uncertainty of ± 5 kHz.