Non-LTE radiative transfer calculations are performed in atmospheric m
odels appropriate for B star photospheres to demonstrate that the high
-l Rydberg transitions of Mg II are in emission. The emission arises f
rom small deviations in the populations of these Rydberg levels from t
heir thermodynamic equilibrium values. This mechanism is the same as t
he one presented by Chang et al. (1991) and by Carlsson, Rutten, & Shc
hukina (1992) to explain the emission from the Mg I Rydberg transition
s 6g-7h and 6h-7i in the solar spectrum near 12 mu m. Our work predict
s wide-ranging infrared emission from the Mg II Rydberg levels, spanni
ng nearly the entire range of B spectral types. Small divergences betw
een the Rydberg departure coefficients enhance stimulated emission, wh
ich produces rising monochromatic source functions (dS(v)/dh > 0) and
emission. Flux profiles of the Mg II high-l alpha-transitions (Delta n
= +1) from n = 4 and 5 show an emission peak superposed on a wider ab
sorption trough, similar in form to the solar Mg I lines, while for hi
gher n, the profiles are in full emission. The emission strengths incr
ease strongly for lower surface gravities where the rates of thermaliz
ing collisions are lowest. Maximum emission is predicted for the alpha
-transitions from n = 5, 6, and 7, reaching F-lambda/F-c approximate t
o 1.15 at line center (no rotation) and W-lambda approximate to -0.1 A
ngstrom. Transitions from higher n exhibit progressively lower continu
um contrasts because of the steep rise of the continuous opacity with
wavelength in the infrared and because of increased Stark broadening.
The largest source of uncertainty affecting the predicted Mg II infrar
ed emission strengths is the scale of the collisional excitation rates
between the Rydberg levels. However, reasonable variation of these ra
tes does not eliminate the emission.