Multiphoton ionization of cesium through resonant dissociative states ofCs2

The multiphoton excitation of cesium through potentially resonant continuum molecular states is investigated over the 6200-5000-Å wavelength region with a tunable dye-laser source having a 0.06-0.08-Å linewidth and a space-charge ionization detector sensitive to a few ions per second. Conditions are established under which the lifetime of the resonant intermediate state against photoexcitation exceeds the lifetime against dissociation. Single- and double-photon resonances occur for the same wavelength and give absorption maxima corresponding to the line spectrum from the intermediate state following dissociation. These lines are modulated in amplitude as a function of wavelength by the more slowly varying absorption resonance from the initial to the intermediate continuum state. As a consequence, the resulting dispersion curve for two-photon absorption in cesium shows what appear to be resonant intermediate $d\pi \left({}_{}{}^3{}_{}{}^{}\Pi _g^{}{}_{}{}^{}\right)$ terms dissociating to give a $5^{2}D$ atom and resulting in the strong development of features corresponding to the fundamental series ($5^{2}D\to n^{2}F$) of atomic cesium in absorption. Components to $n=50\mathrm{}$ were observed and recorded to a precision sufficient to determine the average quantum defect for $F$ states to be $〈n-n^{*}〉=0.033\mathrm{}$ in the limit of large $n$. More complex structure is attributed to $p\pi \left({}_{}{}^3{}_{}{}^{}\Pi _g^{}{}_{}{}^{}\right)$ and $p\sigma \left({}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}\right)$ terms dissociating to give a $6{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ atom and resulting in the development of the $n=8\mathrm{}$ to 32 components of the $6{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\to n{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2},\frac{5}{2}}^{}{}_{}{}^{}$ part of the diffuse series in absorption. Terms dissociating into $6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ were found only at higher photon energies (∼2.27 eV) corresponding to the $n\ge 10$ members of the $6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\to n{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}$ series.