Improved efficiency of selective photoionization of palladium isotopes via autoionizing Rydberg states

Abstract

are only approximately 50 MHz [2], it is not feasible to selectively ionize isotopes using narrow linewidth lasers; however, two effective techniques exist for selective ionization. The first is based on a proposal by Balling and Wright [1] using circularly polarized lasers, in which only odd-mass number isotopes having nonzero nuclear spin and thus having hyperfine structure are able to be excited due to optical transition selection rules. The feasibility of this scheme as applied to platinum group metals is reported by Chen [3], implemented by Yamaguchi and Sasao [4, 5], and further developed in our previous paper [6]. The second technique, resonant ionization mass spectrometry (RIMS) [7, 8], obtains selectivity of ionized isotopes via a narrow slit and accelerating fields, such as the RISIKO separator system as applied to palladium [9]. Comparison between the efficiencies and costs of both techniques are essential before commercial-scale projects can be undertaken. We have developed our apparatus employing the first technique, that is, exploiting transition rules for selective excitation [10]. Resonant excitation to autoionizing states is a promising means to obtain efficient ionization after such mass-selective excitation. Autoionizing Rydberg states come about due a characteristic in the spin-orbit interaction of Pd, where the ion Pd II has two ground states of different ion core configurations, namely 4d 9 (2 D 5/2) at 67241.3(8) cm −1 and 4d 9 (2 D 3/2) at 70779.8(8) cm −1 [11]. Any states existing between the two ground states are autoionizing, and the ion yield is over an order of magnitude higher than excitation to the ionization continuum [12]. The conventional Pd odd-mass selectivity excitation scheme excites via intermediate states having a 2 D 5/2 core [3-6], as shown as configuration (a) in Fig. 1. For this scheme, selectivity is obtained via (0-1-0) transition rules, that is, the first level is J = 0, the first intermediate state is J = 1, and the second intermediate state is J = 0. If Abstract Odd-mass-selective ionization of palladium for purposes of resource recycling and management of long-lived fission products can be achieved by exploiting transition selection rules in a well-established three-step excitation process. In this conventional scheme, circularly polarized lasers of the same handedness excite isotopes via two intermediate 2 D 5/2 core states, and a third laser is then used for ionization via autoionizing Rydberg states. We propose an alternative excitation scheme via intermediate 2 D 3/2 core states before the autoionizing Rydberg state, improving ionization efficiency by over 130 times. We confirm high selectivity and measure odd-mass isotopes of >99.7(3)% of the total ionized product. We have identified and measured the relative ionization efficiency of the series of Rydberg states that converge to upper ionization limit of the 4d 9 (2 D 3/2) level, and identify the most efficient excitation is via the Rydberg state at 67668.18(10) cm −1 .