Nanosecond time-resolved step-scan FT-IR spectroscopy in conventional and supercritical fluids using a four-window infrared cell

Fast time-resolved step-scan FT-IR (s²-FT-IR) has been used to study excited states and reaction intermediates in conventional and supercritical solvents. We have developed a four-port IR cell for s²-FT-IR measurements. The generation of W(CO)₅(Xe), following photolysis of W(CO)₆ in supercritical Xe, has been used to optimize our s²-FT-IR measurements in supercritical fluids using the four-port IR cell. We have compared a number of different approaches for obtaining transient time-resolved IR (TR-IR) data. The IR diode-laser-based and s²-FT-IR approaches for TR-IR have been compared directly. The kinetic decay of the CpMo(CO)₃ (Cp = η⁵-C₅H₅) radical in supercritical CO₂ has been determined using both TR-IR approaches, and we find no significant difference in signal-to-noise between these techniques for most of our TR-IR kinetic measurements. We have attempted to compare s²-FT-IR to the scanning dispersive TR-IR method by obtaining the infrared spectrum of the triplet excited state of 4-phenylbenzophenone, which has been published previously. The importance of obtaining high spectral resolution s²-FT-IR spectra for reactions in condensed phases is investigated. The IR spectrum of the CpFe(CO)₂ radical in n-heptane shows that important information regarding the structure of the radical can only be obtained by performing time-resolved s²-FT-IR experiments at high spectral resolution.