New Mn II energy levels from the STIS-HST spectrum of the HgMn star HD 175640

Aims. The NIST database lists several Mn ii lines that were observed in the laboratory but not classified. They cannot be used in spectrum synthesis because their atomic line data are unknown. These lines are concentrated in the 2380−2700 Å interval. We aimed to assign energy levels and log gf values to these lines. Methods. Semi-empirical line data for Mn ii computed by Kurucz were used to synthesize the ultraviolet spectrum of the slow-rotating, HgMn star HD 175640. The spectrum was compared with the high-resolution spectrum observed with the HST-STIS equipment. A UVES spectrum covering the 3050−10 000 Å region was also examined. Results. We determined a total of 73 new energy levels, 58 from the STIS spectrum of HD 175640 and another 15 from the UVES spectrum. The new energy levels give rise to numerous new computed lines. We have identified more than 50% of the unclassified lines listed in the NIST database and have changed the assignment of another 24 lines. An abundance analysis of the star HD 175640, based on the comparison of observed and computed ultraviolet spectra in the 1250−3040 Å interval, is the by-product of this study on Mn ii.