Understanding why the chemistry of Beryllium is different

In this project we investigate the unusual bonding mechanisms of beryllium. This element and its compounds exhibit unique properties. For example, beryllium alloys are used as lightweight structural materials due to their exceptional strength to weight ratios. The remarkable durability of the metal is reflected by the fact that it is used as a plasma facing material in fusion reactors. The chemistry of beryllium is underexplored due to its toxicity. To circumvent this problem there have been many theoretical studies of beryllium and its compounds. However, beryllium poses a severe challenge for many computational techniques. The difficulties encountered are directly related to the unusual bonding characteristics.

At present, the experimental data needed to evaluate quantum chemical models for beryllium are lacking. This validation is needed to establish confidence in the computational methods used to identify compounds with valuable physical and chemical properties. Experimental studies of prototypical beryllium compounds are the primary objectives of this program. Spectroscopic techniques, applied to gas phase molecules and ions, are being used to obtain structural and thermodynamic properties. The species that are being studied, oxides, hydroxides and carbides, are chosen because they have been the subjects of high-level theoretical investigations.

 

Probing actinide bonding by electronic and photoelectron spectroscopy

The role of the 5f electrons in actinide bond formation is one of the central issues of actinide chemistry. This is a subtle question that can be investigated using a combination of strategic experimental measurements and state-of-the-art theoretical calculations. The primary objective of our ongoing program is to obtain high-resolution spectroscopic data for prototypical actinide molecules in the gas phase. There is a critical need for such data as the results obtained from condensed phase measurements are complicated by solvent-solute or lattice interactions. For the current stage of theoretical method development, the presence of these perturbations in the test data set can often obscure the relationship between measured and observed properties. Our studies are focused on the characterization of the ground and low-lying electronic states of prototypical actinide compounds. These are the states that determine both the physical and chemical properties of the molecules, and they are also the most amenable for detailed theoretical investigations.

Experiments carried out to date show that photoionization techniques are well suited for studies of the low-lying electronic state structures of actinide cations. In our current program we are extending this approach to studies of the low energy states of neutral molecules through the application of negative ion photodetachment spectroscopy. We also examine the limitations imposed on optical spectroscopy by the electronic complexity (density of vibronic states) of open-shell polyatomic actinide species.

 

Precision Chemical Dynamics and Quantum Control of Ultracold Molecular Ion Reactions

A grand challenge of chemistry is to understand and control chemical reactions with single quantum state precision. It is anticipated that this full quantum control of chemical reactions will lead to a revolution in physics and chemistry with impacts reaching from material science to drug design.

To fully realize this goal, however, requires knowledge and control of the quantum states of both the reactants and the products of the chemical reaction – i.e. so-called state-to-state chemistry. As part of a multi-institutional team, we develop the spectroscopic techniques that will enable a new generation in precision chemical sensing and quantum control of chemical reactions. With these techniques, full chemical reaction characterization and control will be possible, providing fundamental insight into chemical reaction dynamics in a fully quantum regime, leading to a myriad of technological benefits.

Spectroscopic data are needed for the photo-manipulation of ultra-cold molecular ions. We use resonantly enhanced multi-photon ionization (REMPI) techniques to characterize a variety of diatomic and triatomic molecular ions. Gas phase samples of refractory materials (e.g., metal containing molecules) are produced by pulsed laser ablation. The products in the ablation plume are cooled to temperatures around 10-20 K by supersonic expansion in an inert carrier gas (typically He or Ar).

 

Optically pumped atomic lasers

Hybrid gas phase / solid state lasers show promise for the construction of efficient high-powered lasers that have high beam quality and long-range propagation characteristics. The best-known example is the diode-pumped alkali vapor laser (DPAL), which is now developed to the point of being a kW class device. Scaling studies indicate that far higher powers are achievable. However, technical challenges are encountered in that stem from the chemically aggressive nature of the alkali metal vapors.

It is well known that rare gas atoms (Rg=Ne, Ar, Kr, Xe), excited to the metastable np5(n+1)s ³P₂ states, have spectroscopic properties that are closely similar to those of the alkali atoms. These metastables can be generated using low power electrical discharges. Lasing of the metastable atoms is achieved by optical pumping of the np5(n+1)p¬np5(n+1)s transitions. The energy transfer step needed to create a population inversion can be accomplished using He as the collision partner. Hence, the entire lasing medium is chemically inert.

Diode pumped rare gas laser systems may outperform the classic DPAL system. The primary optical and kinetic performance is very similar. The rare gas system removes alkali vapor/melt pool control issues, reduces sensitivity to contaminants, and eliminates the need for hydrocarbon energy transfer agents. The multi-level structure and availability of various rare gas mixtures, offers wavelength diversity, and even spectral agility within a single system. Our current research in this area is focused on the development of rare gas lasers and studies of the reactions of excited alkali atoms in the gas phase.