We report a precise experimental determination of the Landau critical velocity v
L
for roton creation in Hen. The technique used was based on measurements of the drift velocity,
v
, of negative ions through isotopically pure liquid
4
He at
ca
. 80 m K , 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
—
v
L
) oc
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
-1
. 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
v
L
(
P
) deduced from the measurements of
v
(
E
) at various pressures for
E
> 500 V m
-1
agree to within 1.5% with theoretical predictions based on Landau’s excitation model of HeII, incorporating accepted numerical values of the roton parameters. The observed pressure dependence of
v
L
(
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 |
V
k0,k0
| 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 |
V
k0,k0
| falls to zero at P « 3 bar (1 bar = 10
5
Pa).
The effect of water vapour on the diffusion coefficients of ions in nitrogen and oxygen