Ultrafast surface dynamics studied with femtosecond sum frequency generation

We present an investigation into femtosecond time-resolved vibrational sum frequency spectroscopy as a tool for studying ultrafast surface dynamics. I t is demonstrated that the presence of experimentally observed new spectral features does not necessarily mean that a novel chemical species has appea red on the surface. In addition, we present a simple model for calculating the sum frequency intensity as a chemical reaction takes place on the surfa ce via a transition state or reaction intermediate. These calculations show that a transition state can be observed and its lifetime can be determined .