We have measured the drift velocity v of negative ions through superfluid 4He for temperatures T < 0.45 K, under a pressure of 25 bar, for electric fields 20 < E< 2.6 x 10^4 V/cm. From the observed form of v(E), we deduce that roton emission from an ion traveling faster than the Landau critical velocity is governed by hitherto unrecognized selection rules whereby the rotons can be created only in pairs.