Far Infrared Spectra of Gases

The envelope of absorption in the far infrared is determined for 14 gases by the reststrahlen technique. The pure rotation envelopes of HCl and NH₃ are compared with the envelopes of the R-branch of their oscillation-rotation bands. The formula ${{I}ⱼ(vib.{-}rot.)}{{I}ⱼ(rot.)}={const.}{{{ν}}ᵣ(1{-}{e}^{{-}{h{{ν}}ᵣ}{kT}})}$ is used to modify the shape of the $R$-branch for comparison with the pure rotation envelopes. A thermopile is described for use when the receiver area is square. The novel feature of this thermopile lies in the use of only one receiver for four junctions. Also the cold junctions stay at the temperature of the receiver for all temperature fluctuations which require a time greater than one minute. Drifting is consequently greatly reduced. Absorption cells are described which are useful for wave-lengths greater than 20{μ}. The windows for these cells are made from 1{μ} lacquer films covered with a 50{μ} layer of Kahlbaum paraffin. Interference effects are reduced to a minimum by the use of copper ribs across the windows, a device to prevent pressure changes in the cell so the windows will not bulge and mechanically accurate mounting of the cells. Interference effects are also reduced because the beam passing through the cells has a large angular divergence.