Photoinduced Charge Transfer on sodium clusters and

Rydberg H-atom Time of Flight spectroscopy of the products

Laser irradiation induces charge transfer between sodium and HBr molecules (or HCl, HF).

The newly formed HBr - anion is unstable and undergoes dissociation to Br and H-atoms. 

The Br‑atom remains on the surface and forms a Na-Br bond, while the H-atom is ejected into the gas phase.

In the Rydberg H-atom Time of Flight spectroscopy experiment, the desorbed H atoms are detected through the high lying Rydberg states. This detection method is more sensitive than the usual quadrupole mass spectrometry and allows one to detect very small hydrogen atom fluxes from the surface.

The detection is done in the following way: after the hydrogen atom desorbs from the surface, it passes through two laser beams. One laser beam (121.6 nm) excites the H atom to the first electronically excited state. The 121.6 nm wavelength is formed by the tripling of 364.8 nm radiation of a dye laser in the Ar/Kr gas cell. A second laser (around 365 nm) beam excites the hydrogen atom to the high-lying Rydberg state. Then, the Rydberg H-atom comes to the microchannel plate located ~50 cm far from the surface. In front of the microchannel plate, the Rydberg H-atom passes through a 100 V biased grid, where it is ionized by an electric field and the resulted cation H⁺ is detected by the microchannel plate.

The time of flight spectrum of desorbed H‑atoms shows a structure in the translational energy distribution. This structure is due to the vibrational excitation of the NaBr particles residing on the surface.

Published in:  S. Dobrin, J. B. Giorgi, H. He, F. Y. Naumkin, J.C. Polanyi, and S.A. Raspopov. J. Chem. Phys., 119, 9795 (2003);

S. Dobrin, J. B, F.Y. Naumkin, and J.C. Polanyi, J. Chem. Phys., 122, paper #014705 (2005)