High-Resolution Infrared Spectroscopy and ASAP Analysis of Cyclopentadiene: The Vibrational Modes below 860 cm$^{-1}$ and the ${\nu}_{21}$ Mode at 961 cm$^{-1}$

Abstract

The spectroscopic fingerprints of vibrationally excited states of astronomical molecules are interesting for multiple reasons. They are excellent temperature probes of the corresponding astronomical regions and are thought to be the origin of many unknown lines in astronomical survey spectra. Rovibrational spectra provide accurate vibrational energies and can guide subsequent pure rotational studies. The Automated Spectral Assignment Procedure (ASAP) greatly simplifies the rovibrational analysis when the rotational spectrum of either the upper or lower vibrational state is known with a high degree of accuracy (e.g., from a rotational analysis). Here, we present a new implementation of ASAP for the analysis of cyclopentadiene, a cyclic pure hydrocarbon that has already been detected astronomically toward the cold core of the Taurus Molecular Cloud. Using the synchrotron radiation extracted by the AILES beamline of the SOLEIL facility, we recorded mid- and far-infrared high-resolution spectra of cyclopentadiene. We analyzed the rovibrational spectrum of the $\nu_{21}$ fundamental (961 cm$^{-1}$) with ASAP and used ASAP$^2$ to determine the vibrational energies of the eight vibrational modes below 860 cm$^{-1}$. ASAP$^2$ is an extension of ASAP for rovibrational bands where the rotational structures of the lower and upper states are known with high accuracy, leaving only the vibrational band center to be determined. The presented rovibrational fingerprints agree with the results from pure rotational spectroscopy, demonstrating the efficiency and reliability of our new ASAP implementation.