Influence of pulse duration on time-resolved sum-frequency generation of surface vibrations

The influence of the pulse duration on time-resolved sum-frequency generati on (SFG) is studied on the basis of the density matrix formulation for non- linear optical processes. A general expression is derived for a sum-frequen cy polarization whose decay as a function of the time delay between the pum p (infrared) and probe (visible) pulses provides a direct measure of the vi brational dephasing time of adsorbates. It is clearly demonstrated how the transient SFG intensity is influenced by the pulse duration and under what conditions the temporal resolution is estimated from the rising edge of the SFG signal, and its decay is described by a single exponential with a time constant of the dephasing. The coherent mixing effect between sequent puls es in the transient SFG signal near zero time delay is also discussed. (C) 1999 Elsevier Science B.V. All rights reserved.