High Resolution Photoelectric VUV Cross Section Measurements of Atmospheric Molecules

Accurate cross sections of the Schumann-Runge (S-R) bands of O2 are needed to determine the opacity of the atmosphere to solar radiation that causes the dissociation of stratospheric constituents such as NO. In particular, the principal photodissociation processes for NO occur in the δ(0,0) and δ(1,0) bands near 191 and 183 nm, respectively; the former is in the range of the S-R (5,0) band, and the latter is in the range of the S-R (9,0) and (10,0) bands. Detailed cross section measurements that resolve the rotational structure of these bands are necessary for the calculation of predissociation rates. However, the determination of accurate cross sections is straight-forward only if the instrumental band width is small compared with the line width being measured. The true cross section of rotationally discrete lines may vary significantly over the instrumental band width, and this can lead to seriously underestimated peak cross sections and distorted line profiles. The two experimental conditions required for cross section measurements of high accuracy for such molecules are the attainment of the highest possible spectroscopic resolution and the use of photoelectric rather than photographic detection.