Download Modelling of radiative and non-radiative decay for Eu2+ and various

VUV Spectroscopy Calculations
Supervisor: Dr Mike Reid, Room 606, [email protected]
New luminescent materials for the conversion of vacuum ultraviolet (VUV) radiation from noble
gas (e.g. xenon) discharges to visible radiation are needed for flat-panel (plasma) displays and
mercury-free fluorescent tubes [1]. Recently we have developed a basic theoretical model for
transitions between the ground-state 4fN configuration and the excited 4fN-15d configuration [2,
3]. We are now in a unique position to investigate the dynamical processes of competing
radiative and non-radiative decay and the details of the electronic and vibronic energy levels.
Several possible projects are presented here. The final project could be a combination of both, or
something slightly different.
These projects would give a student experience with some or all of the following:
1. Computer modelling of experimental data.
2. Understanding of electronic structure of complex condensed matter systems, and hence a
good understanding of applied quantum mechanics.
3. For part of one possible project, some experience with modern ab-initio densityfunctional computer codes.
A student taking one of these projects would have to be comfortable with computer
programming, able to write simple programs in a language such as Matlab, and have a good
knowledge of quantum mechanics, i.e. be taking the Phys411 course.
Modelling of radiative and non-radiative decay for Eu2+ and various 3+ lanthanide ions in
CaF2.
A surprising result from our recent work was the observation of highly-excited energy levels of
3+ lanthanide ions, e.g. Tb3+, with unusually long lifetimes and therefore sharp spectroscopic
features (see Ref. [3]). Theoretical modelling of the excited states gives a tentative explanation.
Usually, highly-excited 4fN-15d states decay rapidly by ejecting the 5d electron from the
lanthanide ion into the conduction band of the crystal. In the case of the highly-excited states
with long lifetimes the energy is not available to the 5d electron, but is localized in the
arrangement of the 4f electrons, so the energy is not quickly lost to the surrounding crystal. This
observation has obvious implications to the efficiency of phosphors.
We have now begun detailed modelling of these relaxation rates, or the competition between
radiative and non-radiative relaxation in other situations, and a recent 480 student made
considerable progress on this problem [4,5], modelling the linewidths of certain two-photon
absorptions in Eu2+, which are determined by non-radiative transitions. However, there is a lot
more to do before we properly understand the non-radiative processes. This is a challenging
problem, which will require some programming as well as development of the theoretical
techniques.
There are two improvements that can be made here, the first reasonably simple, the second more
difficult:
1. Instead of fitting parameters for each of the possible vibrations, to represent their
efficiency at non-radiative decay, calculate the effect of motion of the atoms for each
vibrational mode. This calculation could initially use simple symmetry principles to
estimate the form of the vibrations.
2. If time permits, use a density-functional computer program to calculate the exact form of
the vibrations and to calculate the electronic structure of the conduction band. This would
enable us to draw some much more interesting conclusions about the non-radiative
processes in both Eu2+ and Tb3+.
Investigation of the radiative and non-radiative transitions of Sm2+ ions in various host
crystals.
In the excited states of Sm2+ in some crystals it is possible to observe states arising from the 4fN
configuration and the 4fN-15d configuration very close together. Often there are interesting
dependencies of the non-radiative relaxation rates as a function of temperature. The aim of this
project would be to apply some of the computational techniques developed for Eu2+ [4,5] and
apply them to Sm2+.
References
[1] R. G. Denning. New optics - new materials. J. Materials Chem., 11:19-28, 2001.
[2] M. F. Reid, L. van Pieterson, R. T. Wegh, and A. Meijerink. Spectroscopy and calculations
for 4fN  4fN-15d transitions of lanthanide ions in LiYF4. Phys. Rev. B, 62:14744-14749, 2000.
[3] L. van Pieterson, M. F. Reid, and A. Meijerink. Reappearance of fine structure as a probe of
lifetime broadening mechanisms in the 4fN  4fN-15d excitation spectra of Tb3+, Er3+, and Tm3+
in CaF2 and LiYF4. Phys. Rev. Lett., 88:067405-1-4, 2002.
[4] V. Deev, Phys480 Project, 2003, Phys493 Project 2004.
[5] G. W. Burdick, A. Burdick, V. Deev, C.-K. Duan, and M. F. Reid, Simulation of two-photon
absorption spectra of Eu2+:CaF2 by direct calculation. Submitted to Journal of Luminescence.