• carbon contamination;
  • in situ cleaning;
  • optics;
  • vacuum ultraviolet;
  • soft X-ray

Carbon contamination of optics is a serious issue in all soft X-ray beamlines because it decreases the quality of experimental data, such as near-edge X-ray absorption fine structure, resonant photoemission and resonant soft X-ray emission spectra in the carbon K-edge region. Here an in situ method involving the use of oxygen activated by zeroth-order synchrotron radiation was used to clean the optics in a vacuum ultraviolet and soft X-ray undulator beamline, BL-13A at the Photon Factory in Tsukuba, Japan. The carbon contamination of the optics was removed by exposing them to oxygen at a pressure of 10−1–10−4 Pa for 17–20 h and simultaneously irradiating them with zeroth-order synchrotron radiation. After the cleaning, the decrease in the photon intensity in the carbon K-edge region reduced to 2–5%. The base pressure of the beamline recovered to 10−7–10−8 Pa in one day without baking. The beamline can be used without additional commissioning.