1. Could you explain the significance of your article to the non-specialist? 

In this article it is shown that gas phase electronic and photoelectron spectra for actinide oxides can be used to explore the role of the 5f orbitals in actinide bond formation. Spectra for the oxides of thorium and uranium are discussed. The 5f-like states are found to be spectators and it is shown that complex patterns of low-lying electronic states associated with partially filled f orbitals are readily understood in terms of a simple ligand field theory model. 

2. What has motivated you to conduct this work? 

This work has been motivated by recent developments in the field of relativistic quantum chemistry. High-level theoretical models that predict the electronic structures and properties of actinide compounds are currently being developed. These models must be evaluated through comparisons with definitive experimental results. Gas phase data are most suitable for this purpose, but there have been very few gas phase studies of actinide compounds. We are addressing this need by carrying out spectroscopic studies of Uranium and Thorium compounds as these elements may be safely handled in a University research environment. In the long-term it is hoped that the interplay of theory and experiment will produce computation models that are capable of reliably predicting the properties of the actinides, and that these models will be used to facilitate the safe use and recovery of actinide compounds. 

3. Where do you see this work developing in the future? 

In the near future this work will be expanded through spectroscopic studies of a wider range of actinide compounds (oxides, sulfides, halides and pnictides). Close collaborations with theoretical groups will be continued, to focus the experimental program on the most fruitful problems. It is likely that many of the IE's for refractory actinide compounds that have been obtained from electron impact measurements have been underestimated. This thermodynamic database can now be corrected through the systematic application of high-resolution optical ionization methods and measurements involving charge exchange reactions. Looking beyond the characterization of gas phase actinide molecules, the binding of solvent molecules and ligands will be addressed in future studies. It is evident from comparisons of gas phase and matrix spectra that the physical interactions of the actinides are unusual. Experimental and theoretical studies of these intermolecular forces are of both fundamental and practical interest. 

4. Are there any particular challenges facing future research in this area? 

There are several technical challenges that must be overcome in future research in this case, but there are no obvious roadblocks. The development of suitable laser ablation sources for the refractory compounds will be an important aspect of the experimental program, along with the push to increase the spectral resolution. Spectroscopic studies of transuranium compounds are obviously desirable, and this work will need to be conducted in a suitable facility (e.g., a National Laboratory). A major challenge for the theoretical program is posed by the sheer size of the computations needed to obtain accurate results. It is anticipated that this problem will be solved through a combination of methods development and computational hardware advances.