LASER INVESTIGATION OF NA ATOMS DEPOSITED VIA INERT SPACER LAYERS CLOSE TO METAL-SURFACES

Na atoms have been observed on top of inert spacer layer/metal substrates at room temperature in an ultrahigh-vacuum environment via two-photon laser-induced fluorescence. As spacers, 1, 5, and 11 cadmium-arachidate (LB) and single dodecanthiol (SAM) layers were used, thereby changing the distance between atom and metal between 26 and 290 angstrom. A low sticking probability (much less than 10(-6)) was observed, resulting in a coverage of the spacer layers with less than 10(-3) of a monolayer Na. This low coverage is due to the low binding energy of Na at the CH3 end groups of the spacers (millielectron volts). The Na intensity showed a biexponential decay as a function of time with a small time constant (40 s) due to direct evaporation of Na adsorbed on top of the layers and a large time constant (650 s) due to diffusion of the Na atoms through the fatty acid layers. The two-photon detection scheme allowed a Doppler-free (high-resolution) measurement of line broadening and frequency shift of the spectral lines due to the presence of the metal surface. A moderate broadening was observed for Na on top of LB multilayers on Pt/mica substrates, denoting that the Na atoms diffuse through the fatty acid chains with their mean distance from the surface decreasing from 290 to 28 angstrom. In addition, we found a red shift of the spectral lines by a few hundred Megahertz. The results are compared with previous data of sticking and line broadening of Na on a SAM/Au/Si substrate.