Resonances in the Photo-ionization Continuum of Ar i (20-150 eV)

The absorption spectrum of argon in the region 600-80 Å (20-150 eV) has been studied photographically and photoelectrically, using synchrotron light as a background source and a resolution of 0.06 Å. Many resonances have been observed in the photo-ionization continuum, which are caused by electronic excitation to auto-ionizing states of Ar i. In the region between 27 and 40 eV the observed structure is due to two types of electronic excitation: (i) the excitation of a single $3s$ electron, resulting in the configuration $3s3p^6\mathrm{np}$ and (ii) the excitation of two of the outer $3p$ electrons simultaneously, resulting in configurations of the type $3s^{2}3p^4\mathrm{nl}{n}^{\prime }{l}^{\prime }$. Between 44 and 59 eV, weak structures have been observed in the absorption continuum because of the simultaneous excitation of a $3s$ and a $3p$ electron, resulting in configurations of the type $3s3p^5\mathrm{nl}{n}^{\prime }{l}^{\prime }$. All of the observed resonances have been tabulated, and a partial classification has been given. The profiles of the resonances involving states $3s3p^6\mathrm{np}{{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}}^{\circ }$, for $n=4, 5, \mathrm{and} 6$, and the two-electron excitation states $3s^{2}3p^4\left({}_{}{}^3{}_{}{}^{}P\right)4s\left({}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)4p$ and $3s^{2}3p^4\left({}_{}{}^3{}_{}{}^{}P\right)3d\left({}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)4p$, have been studied quantitatively. The values of $q$, ${\rho }^2$, and $\Gamma$ have been determined for each of these resonances, and the continuum cross section has been measured between 24 and 36 eV.