Knowledge Bank

We have constructed a high-resolution infrared spectrometer based on a quantum cascade laser (QCL) which operates near 8.4 $\mu$m. The ultimate purpose of this spectrometer is to obtain a rotationally-resolved gas phase spectrum of buckminsterfullerene (C$\mathrm{<mrow\; data-mjx-texclass="ORD">60</mrow>}$). We performed initial testing of the spectrometer with methylene bromide (CH$2$Br$2$), but to test the high-temperature capabilities of our instrument, we have observed a C-H bending mode of pyrene (C$\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">10</mrow>}$) near 1184 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ (near the expected band center of the C$\mathrm{<mrow\; data-mjx-texclass="ORD">60</mrow>}$ vibrational band). The observed spectra were rotationally resolved, and individual features had a linewidth (FWHM) of $\sim$10 MHz. To our knowledge, pyrene is the largest molecule to be observed with rotational resolution by infrared absorption spectroscopy. Gas-phase pyrene was generated in a high-temperature (420 - 440 K) oven and cooled by a continuous supersonic expansion from a 150 $\mu$m $\times$ 1.6 cm slit using argon as a carrier gas. The cooled pyrene was observed by continuous-wave cavity ringdown spectroscopy (cw-CRDS). We have collected 2 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ of the band, which is observed to be a b-type band. The observed spectra were fit to an effective asymmetric top Hamiltonian using PGOPHER. Using this fit and knowledge of the vibrational band strength, we estimate the vibrational temperature of the cooled pyrene to be $\sim$70 K, while the rotational temperature was as low as 13 K.