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Abstract
The quenching of the Einstein A-coefficients was observed by measuring the branching ratio of visible and XUV line intensities for CIV, CIII, and NV ions. The branching ratio, defined as the intensity ratio of two spectral lines originating from the same upper energy level, is, for optically thin plasmas, the same as the ratio of the coefficients of spontaneous emission, known as the Einstein A-coefficients. These are believed to be constants in free space. A surprising decrease of the branching ratio of the visible to the XUV line intensities with increasing plasma density in laser-produced plasmas as observed using a novel XUV-visible duo-multichannel spectrometer. The plasma was created by the interaction of a pulsed CO$\sb2$ laser (10 J/pulse, 150 ns) or ruby laser (5 J/pulse, 15 ns) with a solid target. Spatially resolved measurements of the line emission as a function of distance from the target surface were made. The experiment with a CO$\sb2$ laser showed an order of magnitude decrease in the branching ratio between CIV 5801-5812 A and CIV 312.41-312.46 A transitions from the ratio at low density, with significant broadening but no observable shift of the lines. Similar measurements on the higher density plasmas produced by a ruby laser showed an even larger decrease. Several possible explanations for this effect were ruled out on the basis of the spectral analysis and results from supplementary testing experiments on the plasma opacity.
