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A molecular beam is formed when a gas is expanded from a region of higher pressure into a region of lower pressure, through a small orifice. Molecular beams have advantages over the application of gases by background dosing. Surface coverage of the adsorbate of interest can be buit up rapidly to keep co-adsorption of contaminants to a minimum.

The versatility of the molecular beam technique was considerably enhanced by development of the supersonic molecular beam, which is generated by a free jet expansion source. Gas expands isentropically with molecular velocities greater than the speed of sound. The translational and vibrational energies of the beam can be independantly controlled, and hence the energies of molecules that impinge on the surface. A surface can be exposed to a very high flux of gas-phase molecules at low background pressures.

Supersonic molecular beams are ideally suited to probing a fundamental process in surface science: the adsorption of molecules onto a surface. Sticking probablilty is measured using a King and Wells experiment, to quantify the reactivity of the molecule toward the surface. Surface reactions can be followed if a gas is beamed onto a surface that has been precovered with another reactive species. The sticking curve for the species in the beam can be obtained while the desorption of other species that are either displaced or formed by surface reaction is monitored at the same time. STM is the only other technique that offers the possibility of observing the progress of a surface reaction, and it can rarely do so at moderate to high temperatures.

Our SMB Apparatus

The apparatus used in the group consists of a dual supersonic molecular beam system attached to a UHV sample analysis chamber. The two molecular beams converge on the crystal, and are separated by 30 degrees. The nozzles for the molecular beams can be resistively heated to 1800K. The highest gas pressure that can be placed in the lines behind the nozzles is 2 bar. The sample currently being studied is a Pt{110}-(1x2) crystal. It can be moved in the three translational directions, rotated fully about the vertical axis and tilted slightly. The sample itself can be resistively heated to 1250K and cooled below 160K.