Collisionally-excited Balmer line emission from H occurs whenever neutral hydrogen enters a sufficiently strong shock. Such emission has many important applications for the study of Herbig-Haro objects and supernova remnants. This paper explores the collisional excitation and ionization of the H-like ion He II in shocks, a process that may prove to be useful in cases where no Balmer emission from H occurs because the preshock H is completely ionized. Like the Balmer lines of H, Balmer lines of He II also radiate owing to collisional excitation at the shock, though the complexities of broad and narrow components present in the H lines should be absent in He II emission.
New narrowband images of the Trapezium region of Orion in He II 4686 and the adjacent continuum near this wavelength fail to show any He II emission signatures from shocks. Upper limits on the He II fluxes in these images, combined with predictions of the brightness of He II 4686 in nonradiative shocks give an upper limit to the mass outflow rate from Theta_1 Ori-C consistent with the mass loss rate estimated from UV observations. The new images show that HH 202 represents the superposition of two bow shocks moving in opposite directions, which explains some anomalies seen previously in emission line images of this source taken with HST.