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Here, we would like to discuss about the steeper increase of 510 line intensity than 324 with increase in discharge pressure and with power. We suggest that it originates from the increase in density of sputtered Cu atom in the plasma environment.

As shown before and in this picture, these two lines come share the same excited state at 3.817 eV. The 324 line is emitted by a transition to a ground state, so the photon of 324 nm can again excite another Cu atom at ground state in the plasma.

The higher discharge power produces more Cu atoms. And the higher gas pressure and shorter mean free path also results in the higher density of Cu atom. As a result, the plasma with higher pressure and higher discharge power becomes more opaque, not transparent for the light of 324 nm, On the other hand, 510 nm photon is not absorbed so much.

When the Cu density is low, 324 nm emission is produced more than 510 nm, and has stronger intensity. At higher Cu densities, 324 nm emission would be caught by ambient Cu atoms at ground state, and the 510 nm emission, which can easily escape the plasma, becomes relatively stronger.

This mechanism can also explain why the Cu intensities at 10~mm above the target becomes stronger at higher pressures. At higher pressures, Cu atoms at 10 mm above the target can be excited by the 324 nm photons originated from the dense plasma near the target.