Extending the Limits of Rotationally Resolved Absorption Spectroscopy: Pyrene

Abstract

We report the rotationally resolved gas phase spectrum of pyrene (C 16 H 10 ), which is now the largest molecule to be observed with rotational resolution using absorption spectroscopy. This represents a significant advance in the application of absorption spectroscopy to large carbon-containing molecules of fundamental chemical and astronomical importance. Such spectra will facilitate the search for large and highly symmetric molecules in interstellar space, where they may be abundant but cannot be detected without the support of highresolution laboratory spectra. Detailed assignment and analysis of our spectrum indicates that pyrene is the most rigid rotor yet observed, and that a supersonic expansion cools both the rotational and vibrational temperatures of pyrene vapor produced in an oven source.

Inefficient Vibrational Cooling of C60 in a Supersonic Expansion

“…Previous work has shown effective vibrational cooling of large molecules, such as polycyclic aromatic hydrocarbons (PAHs), in supersonic expansions [13,14,20]. In contrast to this previous work, our lack of signal indicates that the vibrational degrees of freedom are not effectively cooled in C 60 .…”

“…Estimating the expected / for C 60 absorption using our experimental apparatus parallels calculations that we carried out to estimate vibrational cooling of pyrene in a previous publication [13]. We begin with the following formula for estimating the / for a single rovibrational transition:…”

“…We have attempted to observe the 8.5 m band due to the availability of quantum cascade laser (QCL) sources at this wavelength and also because this band coincides with an atmospheric window, permitting ground-based astronomical observations. The cavity ring-down spectrometer has already been tested by carrying out high-resolution spectroscopy of methylene bromide [19] and pyrene [13]. Despite multiple spectral searches under a variety of expansion source conditions, no absorption signals from C 60 have yet been detected.…”

“…The oven is described in detail in our previous work on pyrene [13]. The oven is made from stainless steel with attached strip heaters to provide heating and is capable of operating under vacuum at temperatures up to 975 K for many hours.…”

“…We account for the fact that not all of the molecules in the expansion will overlap with the TEM 00 mode of the cavity and be probed by our spectrometer and that only 51.5% of the sample is 12 C 60 . In our previous work with pyrene, we have measured that only ∼20% of the molecules in the expansion overlap with the TEM 00 mode of the ring-down cavity [13]. Because there is little expansion along the axis of the slit, we assume that jet equals the length of the slit (12.7 mm) and can find the number density of molecules probed by our spectrometer:…”