Abstract
Lifetime calculations of Th II J = 1.5 and 2.5 odd states are performed with configuration–interaction many-body perturbation theory (CI-MBPT). For many J = 2.5 states, lifetimes are quite accurate, but two pairs of J = 2.5 odd states and many groups of J = 1.5 states are strongly mixed, making theoretical predictions unreliable. To solve this problem, a method based on intensities is used. To relate experimental intensities to lifetimes, two parameters, one an overall coefficient of proportionality for transition rates and one temperature of the Boltzmann distribution of populations, are introduced and fitted to minimize the deviation between theoretical and intensity-derived lifetimes. For strongly mixed groups of states, the averaged lifetimes obtained from averaged transition rates were used instead of individual lifetimes in the fit. Close agreement is obtained. Then intensity branching ratios are used to extract individual lifetimes for the strongly mixed states. The resulting lifetimes are compared to available directly measured lifetimes and reasonable agreement is found, considering limited accuracy of intensity measurements. The method of intensity-based lifetime calculations with fit to theoretical lifetimes is quite general and can be applied to many complex atoms where strong mixing between multiple states exists.
Highlights
The age of the Galaxy can be estimated using a thorium–uranium cosmochronometer [1].Substantial uncertainty comes from the oscillator strengths of Th II and U II lines used in the abundance calculations
While we focused on specific J = 2.5 and J = 1.5 odd states of Th II, the method is quite general and can be used for many complex atoms where strong mixing is present, including actinides
We have presented configuration–interaction many-body perturbation theory (CI-MBPT) parametric calculations of lifetimes for J = 1.5 and J = 2.5 odd states
Summary
The age of the Galaxy can be estimated using a thorium–uranium cosmochronometer [1]. Substantial uncertainty comes from the oscillator strengths of Th II and U II lines used in the abundance calculations. I, Th II, and Th III, but transition probabilities or lifetimes for Th II were not presented This could be due to complexity and strong mixing of Th II levels that require precision beyond available in ab initio theory. The theoretical framework is parametric relativistic configuration-interaction many-body perturbation theory (CI-MBPT) This theory was applied to similar atoms [12,13], and transition probabilities of Si I [14], La II [15], and La I [16] were found in excellent agreement with experiment, some pairs of levels of La I needed to be adjusted to correct the pair-wise mixing.
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