Knowledge Bank

We have developed a supersonic jet expansion apparatus that is used with the 6.65-m vacuum spectrometer at the Photon Factory, KEK, Japan. A predispersed synchrotron radiation beam enters a 2-mm diameter hole close to the supersonic jet to define a section of the jet stream to record molecular absorption. Pure nitrogen or a mixture of nitrogen in helium is used. The absorption spectrum of $N2$ at $\sim 25$ K in the wavelength region 95--100 nm has been investigated photographically and photoelectrically with this apparatus. We have obtained the oscillator strengths of the $b1\Pi u-X1\sum g+$ bands from absorption cross section measurements at very low temperature. These band oscillator strengths have been investigated by many authors, but only Lawrence $etal.1$ pointed out that the measured cross sections can be severely affected by line saturation in these bands. Our results will be compared with previous work. This work is supported by NASA grant NAGW 1596 to Harvard College Observatory.