tokens: intensities of the rotational lines at their ground states. For example, for CHOH, the expected peak intensity of its line at 44 069 MHz transition, nm MHz) calculated by MADEX ( Cernicharo 2012 ) is only 0.3 K. For the 46 980 MHz ethanol line (7 transition of the trans conformer, nm MHz), the expected peak intensity is 0.2 K for the conditions of the experiment. The first experimental run was held in October 2015 using the quartz windows. To obtain the spectra we performed ON/OFF scans, according to Eqs. ( 4 ) and ( 7 ), with ON: chamber filled with the gas, and OFF: empty chamber. However, no lines were observed. Fig. 8 Observed lines of methanol and ethanol at 44 069 and 46 980 MHz, respectively. The problem arose from the quartz windows which reduce the sensitivity (see Fig. 6 ) and, in addition, introduce baseline artefacts due to internal reflections. We have also noticed that thermal emission from people around the chamber leads to significant degradation of the baseline. In the following experiments we installed the Upilex windows which have a much better transmission (see Sect. 2.4 ). Figure 8 shows the lines of CHOH and CHCHOH after 10 min of ON-OFF observing time at a total static pressure of 0.3 mbar when the chamber was filled with each of these gases. From just a single measurement of the ethanol line we could estimate a broadening coefficient of 20 MHz/mbar. Figure 9 shows the methanol line observed at different pressures (baseline ripples can be seen in the 0.1 mbar spectrum). As pointed out in the previous section, in the pressure broadened regime, the peak height remains constant while the linewidth changes with pressure. For pressures below 0.02 mbar the line reaches its Doppler width (see Fig. 10 ). Typical integration times were 10 min during these measurements except for the lowest pressure for which we integrated during 40 min. All spectra were fitted to Voigt profiles, with the Doppler width fixed to the expected value at 300 K (97 kHz), and peak intensities, collisional linewidths (FWHM) and integrated intensities where derived from the fit. Figure 10 shows these experimental values versus pressure, and the linear fits that show the linear dependence of collisional width and integrated intensity with pressure. The self-broadening coefficient thus derived is MHz/mbar (HWHM). The upper panel of Fig. 10 also shows the expected evolution of the peak intensity using the derived broadening parameter. Fig. 9 Observed line of methanol at 44 069 MHz (7 transition) for different values of the total gas pressure. The integration time was 40 min for the narrower feature. The spectral resolution is 180 kHz obtained by a five channel smoothing of the original data. A shift by K is introduced in the vertical scale of each spectrum with respect to the previous one, as pressure decreases. The Upilex windows were adopted for all the other observing periods. In the second observing run in April 2016 we used HCOOH and OCS as probe
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