Jun Masui

Optical spectra of sodium microclusters

Photoabsorption spectra have been measured for free neutral sodium clusters containing fromN=3 to 40 atoms. In the size range ofN≈3 to 5, a transition occurs from molecule-like absorption to collective excitations of the valence electrons. ForN≈6 to 12, the data are well described by an ellipsoidal shell model. In open-shell clusters, the multiple surface plasma resonances expected for spheroidal or ellipsoidal shapes are observed. The experimental resonance positions provide a sensitive measurement of the cluster distortions. ForN≳13, the per atom strength of these collective resonances is reduced; this may be due to peak fragmentation caused by interaction between the surface plasmon and nearby single-electron resonances. In three distinct wavelength regions, one of which corresponds to the position of the Na atom “D-lines”, additional absorption is seen in the spectra of all investigated clusters.

Beam depletion method for measuring velocity distributions of cluster beams

A simple method to investigate the velocity distributions of cluster beams is presented. Following photon absorption, excited clusters are removed from the beam. This results in a reduced signal at the detector. Using time‐resolved detection, all the parameters of the velocity distribution can be extracted from the signal shape. The technique is demonstrated for a neutral sodium cluster beam. An analytical calculation of the signal shape is in excellent agreement with the data.