We report a precise experimental determination of the Landau critical velocity vL for roton creation in He II. The technique used was based on measurements of the drift velocity, V, of negative ions through isotopically pure liquid 4He at ca. 80 mK, under the influence of weak electric fields, E, for pressures, P, within the range 13 < P < 25 bar. It relied on the use of the equation (v-vL) proportional to E^1/3, which is believed to correspond to the creation of rotons occurring predominantly in pairs and which fitted the experimental data to very high precision for E > 500 V/m. At lower values of E, however, small deviations from this equation were observed. These are tentatively attributed, not to the predicted onset of single-roton emission, but to a novel form of ion-vortex scattering. The values of vL(P) deduced from the measurements of v(E) at various pressures for E > 500 V/m agree to within 1.5% with theoretical predictions based on Landau's excitation model of H II, incorporating accepted numerical values of the roton parameters. The observed pressure dependence of vL(P) is significantly stronger than that predicted; however, a discrepancy that appears to point towards the decreasing accuracy with which the roton parameters are known at high pressures. The modulus of the matrix element |Vk0k0| characterizing roton-pair emission has also been deduced and is found to decrease rapidly with falling pressure. A linear extrapolation of the data suggests that |Vk0k0| falls to zero at P ~ 3 bar (1 bar = 10^5 Pa).