An isotope selective technique has been used to determine line polarisations and excitation cross sections in the range 7.5 to 14.0 eV for the electron impact excited 185 nm transition of atomic mercury. A stepwise excitation technique was used involving electron excitation followed by single-mode laser excitation. The role of nuclear spin in the collision process was investigated by making polarisation measurements on the I=0 and I=¹/₂ isotopes.