New assignments, line intensities, and HITRAN database for CH3OH at 10 μm☆
Introduction
New assignments for CH3OH are reported in the 10 μm infrared region, together with intensity measurements and exploratory modeling for the strong ν8 CO-stretching band at 1033 cm−1. The assignments are based on the analysis of high-resolution Fourier transform infrared (FTIR) spectra recorded at greater optical densities in order to identify weaker transitions. The investigation is part of a program to compile a comprehensive database of line positions and intensities for methanol infrared absorption bands, in response to advances in high-resolution infrared spectroscopy of astronomical sources [1], [2] as well as observations in the terrrestrial atmosphere [3]. These applications require reliable simulation of the absorption band profiles at any prescribed conditions of temperature and density. Achieving reliable calculations in turn requires detailed understanding of the torsion–rotation structures of the bands, in terms of both the line positions and intensities. The present work was therefore undertaken to expand the dataset of assigned transition line centers for CH3OH in the 10-μm region, and to build a base of intensity measurements for modeling of the ν8 CO-stretch band. These results have been submitted for inclusion in the upcoming edition of the HITRAN compilation of molecular line parameters [4].
The strong ν8 fundamental of CH3OH dominates the 10 μm spectral region, and has been intensively studied since the original work of Borden and Barker [5]. The basic pattern for the hindered internal rotation energies was set out in 1940 by Koehler and Dennison [6], but torsional substructure in the ν8 band was not seen clearly until the study by Woods [7] in 1970 using a high-resolution grating instrument. With the advent of tunable diode laser and Fourier transform spectrometers, the band has since been investigated in considerable detail. Sattler et al. [8], [9] achieved full rotational resolution in their diode laser spectra in 1978 and assigned substantial structure for the ν12=0 torsional ground state, primarily in the R branch. Further analysis of the diode laser results by Henningsen extended transition identifications to the ν12=1 excited torsional state [10], [11]. Subsequently, FTIR studies covering the full CO-stretching band permitted extensive assignment and analysis [12], [13], [14], [15], culminating in the major spectral atlas published by Moruzzi et al. [16] in 1995.
Recently, precise measurements employing CO2 laser sideband techniques were reported by Sun et al. [17], [18], [19], [20] for a substantial group of CO-stretching transitions. Their first study utilized IR-microwave double resonance to measure rotational frequencies directly in the ν8=1 excited state, and also provided IR frequencies for about 50 transitions in ν8 [17]. In their following studies, nearly 250 IR transition frequencies for ν12=0, 1, and 2 torsional states (representing ν8, ν8+ν12−ν12, and ν8+2ν12−2ν12, respectively) were measured with sub-Doppler resolution, with many previously blended features now being clearly resolved in the spectrum [18], [19], [20].
The present paper describes our FTIR results for CH3OH subbands with origins from 950 to 1100 cm−1. For the ν8 fundamental, assignments are reported for a number of new subbands, notably for ν12=1 and 2 excited torsional states. Upper-state term values have been obtained by adding known ground-state energies [16], [21] to the experimental transition line centers, and have been fitted to J(J+1) power-series expansions to obtain excited ν8 substate origins and effective B values. A variety of weaker subbands has been observed as well, arising from the ν7 in-plane rock centered around 1071 cm−1 and from the higher-lying ν6 OH bend and ν5 symmetric CH3 bend in ν12=0←1 and 0←2 torsional combination bands made visible through intensity borrowing induced by torsion-mediated vibrational coupling. An underlying background of even weaker transitions arising from the 2ν8←ν8 CO-stretching hot band centered around 1021 cm−1 was discussed earlier [22].
Absorption intensities have been previously measured by Dang-Nhu et al. [23] for 46 ν12=0 and 1 transitions in the CO-stretching band. Here, we have carried out systematic intensity measurements for over 1000 of the stronger P and R branch features from 985 to 1076 cm−1 observed in FTIR spectra recorded at different optical densities. Exploratory modeling of the ν8 line positions and intensities has been performed with a multi-parameter effective Hamiltonian in order to obtain a database prediction that includes the Q-branch transitions, which are heavily overlapped in the spectrum.
Section snippets
Experimental details
Two different FTIR spectrometers (Bomem at NRC Ottawa and McMath at Kitt Peak) were utilized in this work. For positions and assignments of weak lines, the methanol spectrum was recorded from 930 to 1300 cm−1 at 0.002 cm−1 resolution with the modified Bomem DA3.002 spectrometer at the National Research Council of Canada. A pressure of 0.75 Torr at room temperature was employed, and an optical path length of 2.0 m was achieved with four transits of an 0.5-m multipass cell. In all, 94 scans were
Notation and energy level structure
In this work, we will adopt a notation similar to that of [16] for convenient comparison. Energy levels are labeled as E(σ,v,ν12,K;J), where σ is the torsional symmetry (A or E), v is the vibrational state, ν12 is the torsional quantum number, and K is the component of rotational angular momentum J projected along the molecular a-axis. Resolved K-doublets of A symmetry have an additional +/− superscript on K to distinguish the A+ or A− component of the doublet. For E levels, K is a signed
Intensities and composite database in HITRAN format
In order to produce a database in HITRAN format for remote sensing applications, we also examined the question of line intensities. A detailed discussion of intensity calculations for a Cs asymmetric top molecule containing a methyl-group internal rotor, such as CH3OH, was given previously in [32]. Briefly, the program used for the present intensity analysis employs the same RAM (Rho axis method) as used in the energy (line position) analysis of methanol [21]. The Rho axis system is related to
Discussion and conclusions
In this work, we have extended measurements and line assignments in the 10 μm FTIR spectrum of CH3OH to bring in a substantial number of weaker subbands, including high-K and torsionally excited subbands of the ν8 CO-stretching mode as well as torsional combination subbands arising from the ν7 in-plane CH3-rocking, ν6 OH-bending, and ν5 symmetric CH3-deformation modes. This extends our spectroscopic understanding to many of the weak features seen in the “grass” in the spectral atlas of Moruzzi
Acknowledgements
It is a pleasure to dedicate this paper to Dr. Jon T. Hougen, in recognition of his many contributions to the understanding of internal rotation in methanol. His enthusiastic interest in the research, embodied in innumerable informative discussions, has added greatly to our enjoyment of the work and has been of great help in pushing it forward. RML and LHX gratefully acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada. We thank Z.-F. Lu for
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2015, Journal of Molecular SpectroscopyCitation Excerpt :However, all attempts at modeling the line positions or intensities of the high resolution data set remain far from satisfactory [13,14]. Intensities were addressed by empirically augmenting the model calculations with experimental measurements [13]. The nearby ν7 = 1 CH3 in-plane-rock, ν6 = 1 OH stretch, ν11 = 1 CH3 out-of-plane rock and the ν5 = 1 CH3-deformation “umbrella bend” have also been studied with high resolution FTIR spectroscopy [12,15–17].
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Supplementary data for this article are available on ScienceDirect (www.sciencedirect.com) and as part of the Ohio State University Molecular Spectroscopy Archives (http://msa.lib.ohio-state.edu/jmsa_hp.htm).