Bieske Laser Spectroscopy Group
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An enduring issue in chemistry is understanding the interactions between atoms and molecules. Weak intermolecular bonds are important in a variety of different situations including solvation, atmospheric chemistry, and interstellar chemistry. We investigate interactions between ions and neutral molecules by forming charged complexes and clusters and spectroscopically probing them using infrared radiation. Over the past 10 years we have spectroscopically characterized a series of positively and negatively charged complexes including Cl--H2, Br--H2, I--H2, Cl--(CH4)n, F--(CH4)n, Cl--(C2H2)n and Cl--(C6H6)n. More recently we have turned to metal-containing clusters such as Li+-H2 and Na+-H2 motivated by their importance in understanding hydrogen storage at ionic sits in solid media such as zeolites.
How does the experiment work?
Infrared spectra of charged complexes such as Li+-H2 are obtained by exposing them to tunable infrared radiation in a tandem mass spectrometer. In the figure above, the green arrow represents an ablation laser beam that generates metal cations such as Li+ from a rotating Li/Al alloy rod. The metal cations are entrained in a gas expansion (eg H2) and form weakly bound complexes and clusters such as Li+-H2, Li+-(H2)2 and Li+-(H2)3. The cation complexes (metal cation with solvent molecules attached) are extracted from the plasma expansion, mass selected in a first quadrupole mass filter, and pass into an octopole ion guide where they are irradiated with tunable infrared light. Complexes absorbing light photodissociate, yielding ion fragments that are mass-selected by a second quadrupole and eventually sensed by an ion detector. Plotting ion yield against frequency of the IR radiation gives the infrared spectrum. As an example the spectrum of Li+-H2 in the H-H stretch region is shown below. Analysis of the rotational structure provides detailed information on the interaction between Li+ and an H2.
