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

TL;DR

The paper demonstrates a new implementation of the Automated Spectral Assignment Procedure (ASAP) and its extension ASAP^2 for rovibrational analysis, applied to cyclopentadiene with high-resolution infrared spectra recorded at the SOLEIL AILES beamline. By combining ASAP analysis of the ν21 band with ASAP^2 analyses of eight lower-frequency bands, the authors determine precise vibrational energies and band centers, achieving excellent agreement with prior pure rotational data and CCSD(T) calculations. The work shows that ASAP and ASAP^2 can rapidly yield accurate vibrational energies even in the presence of perturbations and hot bands, enabling streamlined construction of vibrational energy level diagrams and improved spectral catalogs for astrochemical contexts. This approach has broad implications for interpreting vibrationally excited molecules in astronomical environments and can be extended to other molecules and spectral regimes.

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 $ν_{21}$ 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.

Figures (5)

• Figure 1: The recorded IR spectra (left) and the energy level diagram of the bands analyzed here (right). One far-infrared and two mid-infrared spectra were recorded. Only the prominent bands in the IR spectra are indicated by the labels at the top (bands not analyzed here are indicated by gray labels). The vibrational energies of the nine modes shown on the right hand side were determined via the eight bands analyzed here (indicated by arrows in the energy level diagram; light arrows for ASAP$^2$ analyses, the dark arrow for the ASAP analysis of $\nu_{21}$), and for $\nu_{13}$ via the interactions known from the pure-rotational analysis Bonah2025a. Interaction partners are indicated in the energy level diagram in blue and red color, respectively.
• Figure 2: Working principle of ASAP. The energies of the $v=0$ levels are known while the energy of the target state $J_{K_a,K_c} = 25_{15, 10}$ of $v_{21}=1$ shall be determined (left side of the plot). The six allowed transitions from $v=0$ into $v_{21}=1$ all have the same offset from their predicted position which equals the energy offset between the actual and predicted energy of the target level: $\Delta \nu_i = \Delta \nu =\Delta E/hc$ (in this example $\Delta\nu \approx -0.031cm^{-1}$). When plotting the six transitions in Loomis-Wood fashion Loomis1928 (with the predicted positions as the reference frequencies), the six experimental lines are aligned vertically at $\Delta \nu$ (see middle and right column; the latter being a zoom into the gray shaded area of the middle column). Identifying the correct lines is simplified by cross-correlating (multiplying) the six spectra which yields the cross-correlation peaks in red on the bottom. Only a single strong peak at $\Delta \nu$ remains, drastically simplifying the analysis process.
• Figure 3: Loomis-Wood plot of cross-correlation plots for the $J_{15, J-15}$ series of energy levels. The row on the bottom is equivalent to the cross-correlation spectrum shown in \ref{['fig:ASAPPrinciple']}. The other rows are obtained equivalently by applying the procedure described in \ref{['fig:ASAPPrinciple']} for the respective target levels. Accidental cross-correlation signals (as for $J=31$) are easily identified as the true cross-correlation signals form an easy-to-follow trend.
• Figure 4: Mid-infrared spectrum of the $\nu_{21}$ band of cyclopentadiene at 5μ bar (black) and the final predicted spectrum (red; see \ref{['tab:v21']}). The top row shows an overview whereas the bottom shows the zoom into the gray marked area. Prominent transitions with $\Delta J_{\Delta K_a, \Delta K_c} = 1_{0, 1}$ are labeled by their respective $J'_{K'_a}$ values. For these transitions, the two asymmetry components are collapsed. Additionally, weaker $Q$-branch patterns are visible.
• Figure 5: ASAP$^2$ cross-correlation peak for the $\nu_{26}$ band of cyclopentadiene (top) and the $n=3115$ contributing lines (bottom). $w$ is the FWHM of a Gaussian fitted to the ASAP$^2$ cross-correlation peak. Saturated lines were excluded from this figure for visual clarity (compare Fig. S2a of the Supporting Information).