Now that we have our lasers ready to go (which is feat in of itself), we need some way to allow our lasers to interact with our molecules. In our experiments, we make use supersonic expansions. A supersonic expansion is achieved when a high pressure (1 to 6 atm backing pressure) gas is expanded through a small orifice (400-1200 micron) into a vacuum. Since the gas is actually a mixture of buffer gas (like helium or a 70/30 neon/helium mixture) with a small percentage of the molecule we want to study, the molecules undergo many, many collisions with the buffer gas. In these collisions the molecule cools, losing most of its vibrational and rotational energy. This situation is ideal for doing spectroscopy because we've greatly reduced the number of transitions the molecule can undergo. In addition, it is ideal for studying clusters because if the molecule has too much energy the clusters will fall apart.