We have retrieved high-resolution UV spectra of 69 hot stars from the HST archive and determined the strengths of the interstellar Ni II absorption features at 1317.217 Å arising from the ground 3 d ^ { 9 } ~ { } ^ { 2 } D _ { 5 / 2 } electronic state to the 3 d ^ { 8 } ( ^ { 1 } G ) 4 p~ { } ^ { 2 } F ^ { o } _ { 5 / 2 } excited level .
We then compared them to absorptions to either the 3 d ^ { 8 } ( ^ { 3 } F ) 4 p~ { } ^ { 2 } D ^ { o } _ { 5 / 2 } or 3 d ^ { 8 } ( ^ { 3 } P ) 4 p~ { } ^ { 2 } P ^ { o } _ { 3 / 2 } upper levels occurring at , respectively , \lambda = 1741.553 Å ( covered in the spectra of 21 of the stars ) and 1370.132 Å ( seen for the remaining 48 stars ) .
All spectra were recorded by the either the E140M , E140H , or E230H gratings of the Space Telescope Imaging Spectrograph .
By comparing the strengths of the two lines in each spectrum and evaluating a weighted average of all such comparisons , we have found that the f -value of the 1317 Å line is 1.34 \pm 0.019 times the one at 1741 Å and 0.971 \pm 0.014 times that of the one at 1370 Å .
We adopt as a comparison standard an experimentally determined f -value for the 1741 Å line ( known to 10 % accuracy ) , so that f ( 1317 Å ) = 0.0571 \pm 0.006 .
It follows from this f -value and our measured line-strength ratios that f ( 1370 Å ) = 0.0588 \pm 0.006 .
As an exercise to validate our methodology , we compared the 1317 Å transition to another Ni II line at 1454.842 Å to the 3 d ^ { 8 } ( ^ { 1 } D ) 4 p~ { } ^ { 2 } D ^ { o } _ { 5 / 2 } level and arrived at an f -value for the latter that is consistent with a previously measured experimental value to within the expected error .