Heavy Element Photophysics and Photochemistry

Solid State Photophysics and Spectroscopy
Recent Program Accomplishments
Exploring the nature of inhomogeneous line broadening

Our work is providing evidence that studies on the nature of inhomogeneous line broadening, often regarded as a nuisance to be overcome, can provide information about the character and distribution of defects in crystals. The fundamental significance of understanding the relation between structural and electronic properties of f elements in solids has prompted us to extend our modeling capabilities in this area.

Our current work on YPO₄ and LuPO₄ doped with ²⁴⁴Cm (t_1/2 = 18.1 y) has provided evidence for inhomogeneous line broadening arising from a different source, namely alpha-decay-induced radiation damage. The image shows an example of this work. The sample crystals were grown 17 years ago and, in consequence, have undergone extensive alpha radiolysis. The relevant energy levels of Cm³⁺ ions are shown as solid black lines at the left of the figure and the vertical arrows denote light absorption or emission processes that are color coded to the emission spectra that shown at the right of the figure. For example, when the third highest lying component of the ⁶D_7/2 electronic state of Cm³⁺ was excited by absorbing laser light (green arrow at left side of the figure) tuned to that transition, the resulting emission spectrum (green curve at right) was quite broad. This breadth reflects the dominance of inhomogeneous broadening for these experimental conditions. In all cases, emission (yellow arrow) from the lowest lying component of the ⁶D_7/2 electronic state to the ground electronic state multiplet of Cm³⁺ was monitored. The spectral data are presented in terms of the traditional spectroscopy unit of energy, the wave number (cm^-1). Note that 1 cm^-1 = 1.9862x10^-23 joule.

Because of radiation damage, the observed inhomogeneous lines of 5f electron transitions of Cm³⁺ ions in these hosts were between 30 to 50 cm^-1 wide. As shown in the above figure, fluorescence line narrowing was achieved only when the lowest component of the ⁶D_7/2 state of Cm³⁺ was excited (red curve). When we tuned the laser energy to the upper crystal-field components of this J=7/2 multiplet, the fluorescence emission spectra (blue curve and green curve) from the lowest component of the same multiplet became as broad as the total inhomogeneous line width. This result indicates that although a subset of Cm³⁺ ions in these phosphates may accidentally have the same energy in a given excited state with respect to their ground states, other excited states of the same set of ions have differing energies and thus the ions within the subset have differing local environments.This type of line broadening reflects diverse local environments for individual Cm³⁺ ions and is termed microscopic inhomogeneous line broadening. In effect, Cm³⁺ ions in these radiation-damaged phosphates have glass-like local environments.

In contrast, in our earlier work, selectively excited tetravalent actinide ions in the CeF₄ lattice had nearly identical local environments, or, effectively, existed in macroscopic "crystalline domains." Based on laser excitation spectra of Cm⁴⁺, Bk⁴⁺, and Cf⁴⁺ doped into CeF₄, we showed that inhomogeneous optical broadening is correlated in these systems. The hallmark of this unusual effect is that the peak position of emission lines shifts linearly with variation in excitation wavelength as a narrow bandwidth pump laser is tuned across an inhomogeneous line and emission from a lower lying excited state is selectively monitored. In the studied systems, actinide ions selectively excited by a laser to one excited state have the same (or similar) energies in another excited state. This rarely observed effect is attributed to the differing size of the tetravalent actinide ions in comparison with that of the host metal ion, Ce⁴⁺.We extended a published model of inhomogeneous line broadening and characterized this correlation effect in terms of macroscopic inhomogeneous line broadening. Based on this model, ions with the same transition energies have the same local environments.

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