Combination of a scanning tunneling microscope with optical excitation

The study of optical phenomena at the atomic scale is expected to provide new understanding of molecules and their chemical dynamics. The combination of lasers with a scanning tunneling microscope (STM) provides new opportunities by tapping into the unique capabilities of both techniques: spectral and temporal information from lasers and ultrahigh spatial resolution in real space from the STM. One big problem, however, is the difficulty of delivering the electromagnetic energy from lasers with atomic precision and initiating the excitations of individual molecules on the surface one by one.; This dissertation describes experiments that coupled light into the tunneling process by irradiating lasers, either a continuous wave (CW) laser or a femtosecond pulsed laser, to the STM junction directly in the tunneling regime. The coupling mechanism involves a two-step process of photo-induced hot electron resonant tunneling, in which an electron is photoexcited to a higher level in the tip and then tunnels resonantly to a molecular state. Because of the ultrahigh spatial confinement of tunneling electrons, the limitation of spatial resolution in either far-field or near-field optical techniques was overcome and atomic resolution with optical excitation was achieved.; The observation of this novel process for the first time was much aided by the charge bistability of a single magnesium porphine molecule adsorbed on a thin insulating oxide film. Specifically, the tunneling of photo-induced hot electrons from the STM tip to a neutral molecule results in the charging of the molecule with an extra electron, which is stabilized by ionic relaxation in the polar alumina film underneath the molecule. Besides the demonstration of atomic resolution, this photo-induced hot electron tunneling was further proved by controlling the laser wavelength, power and polarization. The use of femtosecond laser pulses to irradiate the STM junction leaded to two-photon absorption in the tip, instead of one-photon excitation by CW lasers. The analysis of individual photo-induced charging events recorded in time revealed the kinetics behind the phenomena.; All the experiments described in this dissertation were performed on the oxidized NiAl(110) surface with a home-made STM at a base temperature of 9 K in ultrahigh vacuum.