The acetylene spectrum in the 1.45 µm window (6627–7065 cm^–1)

Highlights

• High sensitivity CRDS of acetylene in the 1.45 µm transparency window (6627–7065 cm^–1).

• More than 3600 transitions assigned to 123 bands of ¹²C₂H₂,¹²C¹³CH₂ and ¹²C₂HD.

• Spectroscopic parameters and Herman-Wallis coefficients are derived.

• A recommended empirical line list of 5260 transitions is generated in the region.

• Some important deficiencies are evidenced in the HITRAN2016 and ASD lists.

Abstract

The high-resolution absorption spectrum of acetylene has been recorded at room temperature by high sensitivity cavity ring down spectroscopy (CRDS) in the 6627–7065 cm^–1 region. The studied spectral range corresponds to a spectral interval of very weak absorption (an acetylene transparency window). The positions and intensities of more than 6500 lines are determined in the considered interval where previous intensity information was mostly missing. On the basis of previous studies and on effective Hamiltonian predictions, 3062 ¹²C₂H₂, 462 ¹²C¹³CH₂ and 104 ¹²C₂HD absorption lines belonging to a total of 123 vibrational bands are assigned. For comparison, the HITRAN2016 database provides line parameters of only ten ¹²C₂H₂ bands in the same region. Spectroscopic parameters of 113 upper vibrational levels were derived from standard band-by-band fits of the line positions (typical rms values of the (obs.-calc.) deviations are better than 0.003 cm^–1). Many bands are found to be affected by perturbations. The vibrational transition dipole moment squared and Herman-Wallis coefficients of 47 bands were derived from a fit of the measured intensity values. In order to generate a recommended line list in the region, the derived spectroscopic parameters and Herman-Wallis coefficients were used to compute the line parameters for these 47 bands while experimental position and intensity values were kept for the other bands. Overall, the obtained recommended list including a total of 5260 transitions will help to fill a spectral gap around 1.47 µm where very scarce spectroscopic information is provided in the current spectroscopic databases. When compared to the HITRAN list in the region, some important deviations concerning both line positions and line intensities are evidenced as a result of inaccurate high J extrapolations.

Introduction

The present contribution takes part in a long term project aiming at constructing an empirical spectroscopic database for acetylene in the near infrared using Fourier-transform spectroscopy (FTS) in the region of the strong bands and cavity ring down spectroscopy (CRDS) in the weak absorption intervals between the bands (or transparency windows). Indeed, in spite of being the most studied four-atoms molecule and of increasing needs for planetary applications, the status of the spectroscopic databases of acetylene (C₂H₂) in the near infrared is far to be satisfactory both in terms of completeness and accuracy. For instance, in its last version, the HITRAN database [1] provides only the few strongest bands for each ΔP variation of the polyad quantum number (P = 5V₁ + 3V₂ + 5V₃ + V₄ + V₅, where V_i are the conventional vibrational normal modes quantum numbers, and i = 1–5 correspond to the symmetric CH and CC stretching modes, the antisymmetric CH stretch, and the trans- and cis-degenerate bending modes, respectively).

In 2017, we constructed a spectroscopic database (EDB17 hereafter) for acetylene in the wide 5850–9415 cm^–1 region [2] by gathering results of three FTS studies [3], [4], [5] and three CRDS studies [6], [7], [8]. Compared to the HITRAN2016 [1] and GEISA2015 [9] spectroscopic databases in the region, the number of bands and lines was increased by more than a factor of ten. As a rule, in absence of perturbation, the line positions included in the database are empirical values calculated using spectroscopic parameters of the lower and upper energy vibrational levels derived from a band-by-band fit of the measured line positions. Line intensities are computed using the vibrational transition dipole moment squared and Herman-Wallis coefficients derived from band-by-band fits of measured intensity values. In case of perturbation, experimental values are generally preferred. Note that a recent CRDS study in the 5693–5882 cm^–1 region [10] and a FTS study in the 9280–10740 cm⁻¹ interval [11,12] will allow extending the database to lower and higher energies. Fig. 1 shows an overview of the EDB17 and HITRAN2016 in the 5600–8200 cm^–1 interval.

As illustrated in Fig. 1, the 6341–7000 cm⁻¹ interval was excluded from the empirical database of Ref. [2]. This spectral region including the strongest band in the region, namely the v₁ + v₃  band centered at 6543 cm⁻¹ with line intensities up to 1 × 10⁻²⁰ cm/molecule, was extensively studied by Keppler et al. [13] by FTS. These authors used a 352.5 m absorption pathlength which allowed them to detect lines with intensity smaller than of 10⁻²⁵ cm/molecule. Keppler's measurements are the main source of line positions for the acetylene HITRAN2016 list in the 6250–6850 cm⁻¹ region but no line intensities were provided in Ref. [13]. As detailed in Ref. [14], HITRAN line intensities in the region rely on transition dipole moment and Herman-Wallis coefficients derived from FTS spectra in Refs. [14], [15], [16], [17]. Above 6750 cm⁻¹, the HITRAN2016 database does not provide any data up to 7050 cm⁻¹ except for the relatively weak 2v₁ + v₅¹ − v₄¹ hot band near 6805 cm⁻¹. Indeed, the 6700–7100 cm⁻¹ interval is a region of weak absorption for acetylene which requires a high sensitivity to be characterized. It is formed by weak bands of the high energy range of the ΔP = 10 manifold and of the low energy edge of the ΔP = 11 manifold (see Fig. 1). Some years ago, a high number of transitions was assigned in this region from a series of CRDS spectra recorded in our laboratory between 6667 and 7015 cm⁻¹ [18,19]. The assignments relied on the global effective Hamiltonian model developed at Université Libre de Bruxelles [20], [21], [22], [23]. No intensity information was reported in these CRDS studies.

In the present work, we reconsider these previous CRDS spectra and extend the spectral coverage by new recordings at lower and higher energy (6627–7065 cm⁻¹). Our goal is twofold: (i) to provide intensity information requested to incorporate these data in spectroscopic databases and (ii) to extend the assignments using the global effective model developed at IAO-Tomsk for ¹²C₂H₂ [24,25]. The first step of the analysis is the construction of a global experimental list of about 7000 lines, as described in the following Part 2. The next steps are the rovibrational assignments (a total of 123 vibrational bands were assigned) and the derivation of the spectroscopic parameters of the corresponding upper vibrational levels (Part 3 and 4, respectively). Finally, for 47 bands, the vibrational transition dipole moment squared and Herman-Wallis coefficients could be determined from a fit of the measured intensities (Part 5). Gathering the retrieved position and intensity information, a recommended empirical line list is proposed and compared to the previous data available in the literature for the strongest bands (Part 6).