Rotational analysis of the first negative band spectrum of oxygen. II

Abstract

In a previous paper (Nevin 1938)*, the author has given an analysis of the (0, 1) λ 6438, (0, 0) λ 6026, and (1, 0) λ 5632 bands of the first negative band spectrum of oxygen and has shown that the system is due to a transition 4 ∑ - g → 4 Π u . The present paper contains an analysis of the (0, 2) λ 6856 and (2, 0) λ 5295 bands. Using the method described in (I) to excite the spectrum, a discharge through helium containing a small amount of oxygen, the plates of the band λ 6856 taken in the second order of the grating could be measured only as far as λ 6720 on account of the large number of ghosts and the blackening produced below this point by the enormously over-exposed helium line λ 6678. To photograph the remainder of the band it was necessary to use a hollow cathode discharge through commercial oxygen as described by Frerichs (1926). This source is considerably less intense than the discharge through helium the exposures with which, in the case of the present band, were between 12 and 18 hr. with Ilford Astra III plates. To reduce the exposures to a reasonable time a cylindrical lens made by Hilger, 15 cm. long and 5 cm. wide with a focal length of 30 cm., was mounted at right angles to the axis of the spectrograph between the grating and the plate as described by Oldenberg (1932). The lens was mounted so that its axis could be adjusted exactly perpendicular to the slit and the grating rulings, a point of the highest importance if the definition is not to be impaired. As far as could be judged there was no loss of resolving power over the length of plate covered by the lens. Satisfactory exposures of a range of 180A were obtained in about 6 hr. with the hollow cathode discharge taking a current of 0.8 amp. The plates were measured with respect to third order iron standards. The λ 5295 band had already been photographed in the second order at the same time as the bands analysed in (I). However, to complete the analysis it was found necessary to take a third order plate, and to save time the arrangement described above was used, the entire band being obtained on a single plate with an exposure of 5 hr. The resolving power attained on this plate with sharp lines was about 200,000. The theoretical resolving power of the grating in this order is 250,000, so the reduction caused by the lens must have been very small. * Referred to as (I) throughout this paper.