The emission spectrum of copper in the wavelength range of 200-230 nm was investigated when xenon, instead of argon or krypton, was employed as the plasma gas in glow discharge plasma (GDP). The Cu II emission lines, which were assigned to the 3d94p-3d94s transition of the copper ion, were observed, but their emission intensities were different depending on the plasma gas employed. The Cu II 224.700-nm line was studied intensively in an argon GDP. As was previously reported, the excitation mechanism of this line is an asymmetric charge-transfer collision with the argon ion. Here, a coincidence in the total excitation energy enables the corresponding energy level of the 3d94p3P2 to be selectively populated through the charge-transfer collision. On the other hand, in a xenon GDP, the emission intensities of the Cu II 224.700-nm line as well as the other Cu II lines were not so strong, but a statistical relationship was found in the plots of the reduced emission intensity of these Cu II lines versus the excitation energy. This result implies that these Cu II lines are not emitted by a non-thermal process, such as charge-transfer collision, but by a thermal excitation process, such as electron collision in the xenon GDP. The first ionization potential of xenon is more than 3 eV lower than that of argon; therefore, it is almost impossible to excite the 3d94p levels of the copper ion by the charge-transfer collision with xenon ion. A similar result to the xenon GDP was obtained with the krypton GDP because the ionization potential of krypton was still insufficient to obtain these high-lying excited levels.
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