  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 A interval. We aimed to assign energy levels and log gf values to
these lines. 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 3050-10000 A region was also
examined. 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
assignement 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 A interval, is the by-product of this study on Mn II.
