Temperature dependence of line parameters of 12C16O2 near 2.004 μm studied by tunable diode laser spectroscopy

The CO2 molecular spectra in the 2.004 μm region are recorded with a high resolution tunable diode laser absorption spectrometer. A commercial distributed feedback (DFB) diode laser purchased from Nanoplus has been used as light source. The side mode suppression ratio is higher than 40 dB. The typical output power is about 7.8 mW, with a relatively narrow linewidth of about 3 MHz, which can be neglected comparing to Doppler limited molecular absorption linewidth. The continuous tuning range (at constant temperature) is within 5 cm-1. This point is of particular interest to reconstruct properly the baseline, as the laser sweeps the molecular transition over a spectral range that is large enough to yield zero-absorption signals at the end and at the beginning of the scanning interval. The laser wavelength is temperature-stabilized by means of a Peltier thermo-element and is driven by a low noise current supply (ILX Lightwave LDC-3724C). The controller was linked to a computer via a GPIB interface while its current was scanned in a step-by-step mode with typical size of 0.1 mA in order to scan the laser over the selected absorption lines by modulation of the driving current. The laser beam is usually collected by a fixed focus collimator and is separated into three parts via a wedged CaF2 beam splitter. The main beam was passed through a sample cell for the absorption signal. The second beam was introduced onto a wavemeter (Bristol-621A) to record frequency of the absorption signal, and the third one was used as a background signal. Two InGaAs amplified detectors (Thorlabs, PDA10DT-EC) were used to record the sample and the background spectra. A three-channel acquisition system ensures simultaneous recording of the gas sample absorption spectrum and the other two signals (wavemeter and background signal).A home-made cooling cell was developed for the purpose of atmospheric applications, and more precisely described in the paper of Ma HL et al.. Briefly, the absorption path length is 17.4 cm and the cooling system is realized by a dry nitrogen flow, circulating around the absorption cell. Two PT100 sensors mounted at different locations on the wall of the sample cell were used to measure the gas temperature during the recordings. The gas pressure is continuously measured by two capacitance gauges (Shanghai Zhentai CPCA-200, full range 20 kPa; MKS 626C, full range 67 kPa). The gas samples (carbon dioxide, air, nitrogen) were bought from Nanjing Special Gas Co., with a nominal purity of 99.99%.During the measurements, the sampling cell was first evacuated to the order of 10-4 Torr using a mechanical together with a molecular pump (Leybold, TW300), then filled with the natural abundance CO2 gas of 99.99% purity or the mixed gas (dry air＋CO2 or N2＋CO2). In order to mix the gases fully, absorption signals were always recorded after at least 3 hours of preparing the gas mixtures. All measurements were achieved with a temperature varying less than 0.3 K during a spectrum acquisition. For a given couple of pressure and temperature, 3-5 spectra were recorded to reduce systematic errors. To retrieve the Lorentz half-width coefficients of these transitions of CO2, the local baseline of the observed spectra was assumed to be a third order polynomial of the wavenumber and spectral lines were fitted with a Voigt function having four parameters, (line position, integrated absorbance, FWHM of the Gaussian and Lorentz components) that were determined using a multi-peak fitting program (http://www.unipress.waw.pl/fityk/). This software is an interactive nonlinear least-squares fitting program based on the Levenberg-Marquardt algorithm that could be simultaneous fitting few lines for one spectrum at a time. During the fitting procedure, the FWHM of the Gaussian component of the lines were fixed to their theoretical values. The integrated absorbance were also fixed to the values calculated from line intensities (note that these line intensities were taken from HITRAN).