2-PHOTON LASER OBSERVATION OF DIFFUSION OF NA ATOMS THROUGH SELF-ASSEMBLED MONOLAYERS ON A AU SURFACE

The first direct laser observation of atomic sodium deposited on a self assembled monolayer (SAM) on a Au/Si(100) crystal is reported. The applied two-photon detection scheme allows a Doppler-free measurement of the change in linewidth, spectral peak position and peak intensity as function of time after depositing Na on top of the SAM. A small initial sticking probability of Na on CH3-(CH2)21-SH is determined (<< 10(-6)). From the data the mean distance of the Na atoms from the Au surface is calculated to decrease after deposition within 7 min from 32 to 20 angstrom due to diffusion of the Na atoms through the SAM with a diffusion coefficient of 0.083 angstrom2/s.