Knowledge Bank

While making gain measurements near the 5577 {\AA} $O(1S)$ fluorescence in e-beam excited $Ar/O2$, Powell and $Murray1$ found that during the e-beam pulse the probe laser suffered up to 99.9% absorption. Soon after the termination of the e-beam pulse the absorption disappeared and gain on $ArO(1S)$ was observed. A similar absorption was observed in e-beam excited argon, krypton, or xenon alone; however, recovery was less rapid. The originally suggested absorber was $Ar2+$. We have $measured2$ the photodissociation cross sections far $Ar2+,Kr2+,andXe2+$, over the wavelength range 5650 \AA to 6950 \AA, The cross sections at 5650 \AA were $1\times 10-20,2\times 10-19$ and $2\times 10-19$, respectively. From an investigation of the potential curves for $Ar2+$ it appears clear that the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma u+\leftarrow 2\Pi g$. transition is at too long a wavelength for the molecular ion to be the absorber discovered by Powell and Murray. We have monitored the temporal and spectral behavior of the absorption in e-beam excited rare gases with and without additives. By comparison with the excimer fluorescence under the same conditions, we have reached the tentative conclusion that the rare gas excimer itself is responsible for the absorption. The probable transition is $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma 1u\leftarrow 3\Pi 2g$. Both of these states arise from the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}P2$ levels of the excited rare gas atom.