V.B.4.a.1  Experimental

A typical spectrometer used in this work is based on a synthesized x n( n=9 -> n=144) frequency multiplied probe, with a heterodyne receiver or InSb detector, coupled to a temperature controlled 6 m long cell. The temperature of the cell [1] is slowly ramped from ~250 K to ~400 K over a period of ~ 6 hours.  During this period 400 - 1200 spectral scans are recorded.  Each scan measured the spectral intensity at typically 25 kHz intervals, with an integration time of ~10 ms/bin and a total scan time of 15 - 40 s.  The temperature variation over each scan was small enough that the assumption of a constant temperature did not significantly impact the error budget of the experiment.  The microwave spectrometer is described in [2] and more generally in Section III above.

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[1] S.M. Fortman, I.R. Medvedev, C.F. Neese, F.C. De Lucia, Astrophys. J., 714 (2010) 476-486.

[2] I.R. Medvedev, C.F. Neese, G.M. Plummer, F.C. De Lucia, Opt. Lett., 35 (2010) 1533-1535.