Shedding Light on Heavy Molecules, One by One

Abstract

One of the major motivations for research employing molecular beams is the capability to work with individual molecules under collision free conditions. However, the production of molecular beams has traditionally been limited to small and stable molecules with sufficiently high vapor pressures to be effectively seeded without extensive heating. While relatively high temperature molecular beam sources have been used, most larger molecules tend to decompose upon heating. New techniques have been developed that produce large molecular ions for analytical purposes, such as laser desorption, matrix assisted laser desorption, and ion spray. Of these, only the first can be used to reliably produce intact neutral molecules, which can be combined with a molecular beam. We capitalize on this capability of laser desorption to bring complex neutral molecules into the gas phase without ionization, for fundamental spectroscopic studies. This becomes particularly powerful when the desorbed material is entrained in a supersonic expansion. The cooling in a supersonic jet (a) stabilizes the molecules by reducing their internal energy, (b) makes it possible to carry out spectroscopy, and (c) allows for the study of clusters. The feasibility of this approach has been demonstrated by previous work and by our own work and on small biomolecules [1-10]. When extending these techniques to larger molecules we need to address the question whether such systems can be photoionized at all [11,12].