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Molecule-Surface-Scattering with Velocity-Controlled Molecular Beams


Grätz,  Fabian
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Grätz, F. (2014). Molecule-Surface-Scattering with Velocity-Controlled Molecular Beams. PhD Thesis, Freie Universität, Berlin.

Cite as: https://hdl.handle.net/11858/00-001M-0000-0024-436E-A
This thesis describes the design and construction of a novel molecule-surface scattering apparatus. The setup combines a Stark decelerator with a state-of-the-art surface scattering chamber, and allows background-gas free scattering of packets of velocity-controlled polar molecules in selected quantum states from well-defined surfaces with unprecedented energy resolution. Results for scattering of CO (a3Π) in different vibrational quantum states from a clean Au(111) surface are presented. CO is co-expanded with Xe or Ar to yield a supersonic molecular beam which produces fast, internally cold molecules. Excitation of CO to the long-lived electronically excited a3Π1 state makes it accessible to manipulation with electric fields due to the state’s large dipole moment. The complete removal of both carrier gas and remaining ground-state CO is achieved by focusing and deflecting the metastable CO with a switched electrostatic hexapole state selector. The 131-stage Stark decelerator is used to tune the velocity of a part of the metastable CO pulse, with an initial velocity of 360 m/s, in the range from (512 ± 4) m/s – (50 ± 4) m/s, corresponding to an incident translational energy between (307 ± 5) cm-1 and (3.0 ± 0.5) cm-1, respectively.This range can be extended up to (1300 ± 25) m/s, corresponding to an energy of(1980 ± 80) cm-1, by simple modifications to the setup. Prior to collision, the molecules can be laser-prepared in different vibrational and rotational quantum states. State-selective detection of the scattering products makes it possible to determine the quantum state, the scattering angle, and the kinetic energy of the scattered molecules. Therefore, detailed information about the scattering process can be obtained. The design of the machine allows for flexible laser access near the surface, such that a variety of different preparation and detection schemes can be employed. Ultra-high vacuum conditions in the scattering chamber are achieved by baking the entire apparatus, including the decelerator and the hexapole. Because of the carrier gas removal and a total of three differential pumping stages, no pressure rise is detected in the scattering chamber due to the incident molecular beam. Scattering experiments of CO (a3Π) on Au(111) have been performed. The scattering process leads to electron emission because the metastable excitation energy exceeds the work function of the surface. The electron emission yield, i.e. the number of electrons emitted per incident metastable particle, has been measured to be 0.13 ± 0.05. A laser ionization-depletion method is used which makes it possible to directly compare the intensity of the metastable CO ionization signal to the intensity decrease of the electron emission signal. It was also shown that vibrationally excited metastable CO leads to a higher electron emission yield. By preparing different vibrational distributions of CO in the a3Π electronic state via Franck-Condon pumping, the electron emission yield was determined for groups of vibrational levels. The first three vibrationally excited levels have an increased electron emission yield which on average is a factor of 1.5 larger than that of the vibrational ground state. Averaged over the vibrational levels with quantum number v = 4, 5, 6 , this factor is approximately 2.6 . In the range between 93 m/s and 512 m/s, the electron emission was observed to be independent of the velocity of the incident molecules. When the surface temperature is raised from 250 K to 900 K, the intensity of the electron emission signal increases by 27 %. These results are not in agreement with the models currently used to describe the de-excitation of electronically excited molecules at metal surfaces. This demonstrates the need for more theoretical work to incorporate the role of vibration into the description of the de-excitation mechanism.