Patent ID: 7935218

Claim:
An optical apparatus, comprising: an illumination system configured to form a pulsed radiation beam; an auxiliary radiation source configured to generate an auxiliary radiation beam, wherein the auxiliary radiation source is arranged with respect to the illumination system so that the auxiliary radiation source does not intercept the pulsed radiation beam; an optical element with a surface on which the pulsed radiation beam and the auxiliary radiation beam are incident in operation; and a gas source arranged and configured to supply a mixture of a first type of gas and a second type of gas to a space adjacent the surface, particles of the first and second types of gas arranged to react with the surface, when activated by the pulsed radiation beam, and with particles of the second and first types of gas, respectively, which have reacted with the surface, the gas source being configured to generate a combination of surface occupation numbers of molecules of the first and second types of gas on the surface under operating conditions, at least prior to pulses of the pulsed radiation beam, the combination of surface occupation numbers lying in a range in which reactions of particles with the surface during pulses of the pulsed radiation beam are in majority reversed, at least as long as the combination of surface occupation numbers is not changed due to radiation in a pulse of the pulsed radiation beam.