Abstract

Difference frequency mixing of amplified femtosecond visible pulses with white light continuum radiation produces relatively broadband infrared pulses (~10 meV), whose bandwidth is determined by the phase matching bandwidth of the difference frequency crystal. This radiation is easily tuned over a broad range (hundreds of meV) by angle tuning of the crystal. One application is the study of photogenerated charge carriers in the quasi-one-dimensional organic semiconductor trans-polyacetylene. Beause of the strong electron-phonon coupling, electrons and holes are dressed by the lattice leading subgap levels (0.45eV). Using transient mid-infra-red spectroscopy, we can observe the subpicosecond lattice deformation as well as measure gemmate recombination dynamics and interchain excitation probabilities. These data have been used to understand the photoconductivity and nonlinear optical properties of polyacetylene. Up-conversion gating of long tunable narrowband infrared pulses by an amplified subpicosecond laser is particularly valuable for subpicosecond vibrational spectroscopy. For example, the behavior of the CO ligands can be tracked during organometallic photochemistry to provide information about their cooling, rebinding, and reorientation. Issues such as the solvation of catalytically active metal carbonyls in solution and the pathway by which CO enters and exits hemoglobin are addressed.

© 1992 Optical Society of America