The study of molecular complexes in the gas phase provides important information on intermolecular forces and spectroscopy has played a vital role in this field. As an illustration, the complex formed between an aluminium atom and a water molecule is described here.

To obtain Al(H2O), it is necessary to bring together aluminium atoms and water molecules. Getting water into the gas phase is easy, but aluminium poses more of a problem since at ordinary temperatures the solid has a very low vapour pressure. An obvious solution is to heat the aluminium in an oven. However, the high temperature has a concomitant downside; if water is passed through (or near) the oven the high temperature will almost certainly prevent the formation of a weakly bound complex such as Al(H2O). Instead, the heat may allow the activation barriers to be exceeded for other reactions, leading to products such as the insertion species HAlOH.

A solution to this apparent quandary is to make Al(H2O) by the laser ablation–supersonic jet method, which was mentioned briefly in Chapter 8 (see Section 8.2.3). Any involatile solid, including metals, can be vaporized by focussing a high intensity pulsed laser beam onto the surface of the solid.