Photostop

The use of photodissociation to produce cold, slow molecules from a molecular beam of cold, fast molecules is dubbed “photostop”. The essence of the scheme is thus: a pulsed molecular beam of a precursor molecule AB seeded in a noble carrier gas is crossed by a laser beam. The laser light dissociates AB, producing fragments A and B. These recoil from the dissociation site. The speed of the molecular beam and the wavelength of the laser beam are tuned so that the recoil velocity of those A molecules recoiling opposite to the molecular beam direction cancels out their initial velocity. This leaves a certain amount of A close to stationary in the laboratory frame. The photostop technique was first demonstrated with the production of cold, slow NO radicals by the photodissociation of nitrogen dioxide, both at Durham and in South Korea. The work described in this thesis began as a development of the NO photostop experiment, with an attempt made to magnetically trap NO. Latterly, the photostop project became part of the Millikelvin Molecules in a Quantum Array (MMQA) collaboration, the aim of which is the trapping of ten million cold, polar molecules. To this end, the photostop of SH radicals by the photodissociation of hydrogen disulphide was demonstrated.