Heavy Element Photophysics and Photochemistry

First Optical Observation of Actinide Nuclear Quadrupole Splitting in a Solid Phase

Nonlinear laser spectroscopy experiments, such as spectral hole-burning (SHB) and optical detection of nuclear magnetic resonance (ODNMR), we selectively excite and detect f-element ions at specific sites in structurally disordered samples and measure their hyperfine and superhyperfine energy level structures in their ground and excited states. These energy level structures, when theoretically interpreted, provide information about the immediate crystallographic structure (local environment) surrounding heavy element ions in solids.

Using SHB, we have measured hyperfine and nuclear quadrupole levels that give rise to MHz-wide lines whereas, for the same transitions, the inhomogeneous line broadening due to radiation damage and crystalline defects is 10⁴ MHz. An example of such spectra is shown at the right in which we measured nuclear electric quadrupole splitting in the ⁷F₀ground state of ²⁴³Am³⁺ (nuclear spin I=5/2) in a single crystal of LaCl₃.

In this experiment, a single-frequency laser pumped the⁷F₀®⁵D₁transition of Am³⁺at 17173 cm^-1. Because the laser line width was narrower than the nuclear quadrupole splitting, it selectively excited a subset of Am³⁺ions (only one of the three ground state quadrupole levels was pumped). When such an excited ion relaxes back to the ground state, it can end up in a quadrupole level that is different than that from which it was excited. Due to slow nuclear relaxation between the quadrupole levels at low temperature, the population of the laser pumped level decreases while that in the other two levels increases.

After pumping at 17173 cm^-1, the laser power was reduced and its wavelength was scanned across the inhomogeneously broadened optical transition while monitoring fluorescence from ⁵D₁ to ⁷F₂. In the resulting SHB spectrum (upper curve), a hole (decreased optical absorbance) appears at the pump energy and antiholes (enhanced optical absorption) appear symmetrically on both sides of the hole. The energy differences between the hole and antiholes directly measure the quadrupole splitting in the ground state. Also shown (lower curve) is the inhomogeneously broadened ⁷F₀ ® ⁵D₁ transition. Its 8.5 GHz width precludes measuring the observed 150 MHz nuclear quadrupole splitting by conventional optical methods. This is the first actinide system for which nuclear quadrupole splitting has been measured optically in a solid. Analysis of this SHB spectrum provided insight into electron-nucleus coupling mechanisms and the relationship between hyperfine energy levels and crystalline structure. The nuclear quadrupole coupling constant and crystal field antishielding factor were obtained. Our work shows that the crystal field antishielding effect dominates the observed nuclear quadrupole splitting of ²⁴³Am³⁺ in LaCl₃ whereas contributions from 5f electrons and the pseudoquadrupole interaction are negligible.

Combined with a similar experiment on ²⁴³Am³⁺ in CaWO₄, in which we determined the first-order hyperfine and nuclear quadrupole splitting in the ⁵D₁ excited state of ²⁴³Am³⁺ as well as its ground state quadrupole splitting, our work has determined that the lower crystal field state of the ⁵D₁ multiplet of Am³⁺in LaCl₃ is a singlet with no first- order hyperfine splitting. This conclusion is inconsistent with previously published crystal field modeling that predicted that the singlet would occur at higher energy than the doublet of the ⁵D₁ state of Am³⁺ in LaCl₃. Calculating the crystal field interaction with a more advanced theory, we have shown that this discrepancy is due to the sign of the second order crystal field parameter that was incorrectly determined in previous work. Resolving this significant issue in the modeling of f-element electronic energy level structures is an unexpected bonus in our nuclear quadrupole splitting measurements on Am³⁺in LaCl₃.

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