Extreme Ultraviolet Series in Cr VI, Mn VII and Fe VIII

Lines in the extreme ultraviolet region involving the $3d{}_{}{}^2{}_{}{}^{}D,4p{}_{}{}^2{}_{}{}^{}P,5s{}_{}{}^2{}_{}{}^{}S,6s{}_{}{}^2{}_{}{}^{}S$, and several $\mathrm{nf}{}_{}{}^2{}_{}{}^{}F$ terms have been identified in the spectra of Mn VII and Fe VIII. In Cr VI the first member of the $3d{}_{}{}^2{}_{}{}^{}D-nf{}_{}{}^2{}_{}{}^{}F$ series has been found. These spectra have been photographed with a twenty-one foot grazing incidence vacuum spectrograph. Identification was facilitated by the use of constant second difference displaced frequency diagrams.