A New Scaling Law for Double-photoionization Processes

We have measured the double-to-single photoionization ratio of atomic lithium and beryllium using the ion time-of-flight method. The photon beam of the PGM undulator beam line ionizes the vaporized target atoms. The ions created are extracted by a pulsed electric field across the interaction region and are detected by a Z-stack of micro-channel plates [1]. In the figure shown above, we compare the double-to-single photoionization ratios of He[2], Li[3-5], H2[6], and Be[7]. Besides lining up the respective double-ionization thresholds, we have chosen an energy axis in units of the energy difference between the corresponding double and single ionization thresholds. For the photon energies considered here, both ejected electrons originate from the same shell in the case of He and Be. Therefore, only one single-ionization threshold is of relevance, and ∆E can be unambiguously calculated. However, the lowest double-ionization process in Li is an intershell process that involves a 1s and a 2s electron. In this case, we have used the 1s S ionization threshold, because the 2s photoionization contributes only marginally to the total photoionization cross-section above the double-ionization threshold. In the case of H2 we have used the lowest single-ionization threshold H2 X Σg (15.4eV). While the double-photoionization threshold for a vertical transition is 51.1eV, the data of [6] show that the threshold is at a slightly lower energy. Here, we have used an energy of 50eV. After dividing the He, Li, Be and H2 ratios with a suitable “correlation factor” Cr, all data line up very well and show surprisingly the same energy dependence. This work is supported by NSF Grant No. 9987638. The SRC is operated under Grant No. DMR-0084402.