The evaporation of superfluid 4He by rotons is investigated using a recently developed pulsed source of both positive (R+) and negative (R−) group velocity rotons. The R+ and R− rotons have very different momenta parallel to the free liquid surface and this causes angular dispersion of the two beams of evaporated atoms in the vacuum. On moving a bolometer horizontally through these beams, we find that the maximum flux of atoms from R− rotons occurs at an angle corresponding to an average R− roton energy of ħω/kB ≃ 10.5 K. The signal at this angle is compared with the evaporation signal at the maximum flux caused by R+ rotons. These R+ rotons have an average energy of ≃ 10.7K. The relative sizes of these two signals enables an estimate to be made of the probability of evaporation by R− rotons relative to that for R+ rotons. We find that «P−a»/«P+a» ≃ 4 × 10−3 where the brackets signify averages over the angles and energies allowed by the geometry of the experiment.