Molecule-surface scattering with velocity-controlled molecular beams.
Zusammenfassung der Projektergebnisse
In this project, we have designed, constructed and performed first test experiments with a novel apparatus in which molecule-surface interaction can be studied using velocity-controlled molecular beams. The velocity of a beam of CO molecules, laser-prepared in a single rotational level of the first electronically excited (metastable) state, can be precisely controlled by passing it through an array of electric field stages in a so-called Stark decelerator. The speed of the metastable CO molecules can thus be changed at will between 30—1300 m/s. Phase-stability in the Stark decelerator ensures that the metastable CO molecules stay together in a compact packet throughout the deceleration or acceleration process, both in real space and in velocity-space. The latter results in a very narrow velocity distribution of the molecules at the exit of the decelerator, corresponding to a translational temperature in the tens of milli-Kelvin range. The compact size of the packet at the exit of the decelerator enables efficient optical transfer of the metastable molecules to an other state by a pulsed laser; it enables, in particular, an efficient transfer via stimulated emission pumping (SEP) to selected rotational and vibrational levels in the electronic ground-state of the CO molecule. The metastable CO molecules as well as the ground-state molecules can subsequently scatter from a well-defined Au(111) surface. After the scattering process, the CO molecules can be state-selectively detected with another pulsed laser, and information on the internal quantum state distribution as well as on the velocity distribution and the spatial orientation of the scattered molecules can thus be obtained. The use of Stark-deceleration in combination with SEP provides fully state-selected beams of CO molecules with a continuously tunable velocity and with a narrow velocity-distribution. Scattering studies with these beams not only enable an extraordinarily high resolution in the incidence translational energy but also give access to unusually low incidence translational energies.
