Storing Molecular Hydrogen - One Molecule at a Time

Can hydrogen be stored in solids?
Answering this question, which has a bearing on the viability of the hydrogen economy, requires an understanding of the way in which ions interact with neutral molecules. Experimentally, we approach the question by creating small charged complexes such as Na+-H2 in the gas phase and obtaining their infrared spectrum, which, in turn provides crucial information on the length and strength of the bond joining the sodium ion and the hydrogen molecule. This is just the sort of information required to develop and optimise hydrogen storage materials.
The strategy
We expose charged complexes, such as Na+-H2, to tunable infrared radiation, which promotes their fragmentation. By monitoring the Na+ fragmentation products as a function of the infrared wavelength we can obtain the Na+-H2 infrared spectrum. Examples of spectra for several simple complexes are shown in the figure on the right.
The rotational structure in the spectra proves that the H2 molecule prefers to bind side-on to the metal cation, and provides information on on the length and strength of the bond joining the sodium ion and the hydrogen molecule. We are one of few groups world-wide able to record high resolution spectra of charged complexes.
The Apparatus
We probe the ions in a tandem mass spectrometer. Charged complexes, formed through either laser ablation or electron impact, are mass selected from the plasma expansion (the red 'cone') in the first quadrupole mass filter, then travel through the bender to the octopole ion guide where they are irradiated with infrared light.
We've ranged about the periodic table, creating and probing complexes of molecular hydrogen with the metal cations shown in pink and halide anions shown in green. See publications for details.