Probing single molecules with the STM in the frequency and time domains

We have constructed a scanning tunneling microscope (STM) and combined it with a tunable femtosecond laser (210 nm to 1040 nm) to probe single molecules with simultaneous spatial and temporal resolutions. Employing the RF lock-in amplifier to measure the laser-induced tunneling current that is directly synchronized with the high repetition rate of the laser ($\sim$80 MHz), time resolved measurement of single molecules with atomic scale resolution can be achieved by varying the time delay between pairs of laser pulses in the two-pulse correlation or two-color pump-probe configuration. A femtosecond laser system with widely tunable wavelength enables resonant excitation of single molecules that are partially decoupled electronically from the underlying metallic substrate by a thin oxide or additionally atomic or molecular layers. The experimental arrangement allows measurement of molecular lifetimes by two-photon photoemission spectroscopy and microscopy.