Abstract

Time-domain experiments to establish vibrational energy and vibrational coherence relaxation mechanisms for adsorbates at surfaces are carried out using transient sum-frequency generation (SPG). An excited vibrational level can be populated, or a vibrational coherence created, by an intense, resonant infrared pump pulse. The subsequent relaxation of the excited-state population, or of the vibrational coherence, can be followed by monitoring the SFG signal from delayed probe pulses. In the case of a vibrational coherence, the probe is simply a visible pulse that up-converts the vibrational coherence into an SFG signal. To measure the decay of an excited-state vibrational population, the probe consists of a pair of synchronized picosecond infrared and visible pulses. This technique also has the potential to distinguish homogeneous and inhomogeneous contributions to the coherence decay by measuring a surface photon echo, observed by introducing a symmetric delay between the pump infrared, the probe infrared, and the probe visible pulses.

© 1991 Optical Society of America