Deep ultraviolet spectral photometry investigation of optical components under vacuum and nitrogen purge conditions

For future development of components for the DUV/VUV spectral range, the precise knowledge of the optical transfer functions such as spectral transmission or reflection are essential. Due to the strong atmospheric absorption of radiation in this range, spectral measurements must be carried out in the absence of oxygen. Tests are usually performed in vacuum or by purging the spectrometer with an inert gas. In vacuum environment, often a radiation induced formation of hydrocarbon contaminants on the surface of the optic is observed, that deteriorates the optical functioning and leads to erroneous measurement results. We report in this article on spectrometry tests under vacuum and purge conditions and compare the results for both environmental conditions. To avoid the contamination of the optics tests were carried out in the DUV spectral range under nitrogen purging. N2 itself shows vibrational and rotational molecular absorption bands in the wavelength range from 110 nm to 150 nm, which are present as peaks with arbitrary amplitudes in the spectrometry results. Several time domain parameters such as integration time, integration range, power monitoring aspects, and frequency bandwidth of the data acquisition modules of the spectrometer are studied to investigate the origin of the disturbing peaks. Also, different polarization states of the radiation, angle resolved scattering and pressure variation of nitrogen gas are considered to adapt the test procedures for managing the presence of these absorption lines in the measured data and to obtain reliable results.