Application of External-Cavity Quantum Cascade Infrared Lasers to Nanosecond Time-Resolved Infrared Spectroscopy of Condensed-Phase Samples Following Pulse Radiolysis

The development of flash photolysis in the late 1940s by Norrish and Porter revolutionized the application of fast...

Application of External-Cavity Quantum Cascade Infrared Lasers to Nanosecond Time-Resolved Infrared Spectroscopy of Condensed-Phase Samples Following Pulse Radiolysis

By DRS Daylight Solutions

Pulse radiolysis, utilizing short pulses of high-energy electrons from accelerators, is a powerful method for rapidly generating reduced or oxidized species and other free radicals in solution. Combined with fast
time-resolved spectroscopic detection (typically in the ultraviolet/visible/near-infrared), it is invaluable for monitoring the reactivity of species subjected to radiolysis on timescales ranging from picoseconds to seconds.
However, it is often difficult to identify the transient intermediates definitively due to a lack of structural information in the spectral bands. Time-resolved vibrational spectroscopy offers the structural specificity
necessary for mechanistic investigations but has received only limited application in pulse radiolysis experiments. For example, time-resolved infrared (TRIR) spectroscopy has only been applied to a handful of gasphase studies, limited mainly by several technical challenges. We have exploited recent developments in commercial external-cavity quantum cascade laser (EC-QCL) technology to construct a nanosecond TRIR apparatus that has allowed, for the first time, TRIR spectra to be recorded following pulse radiolysis of condensed-phase samples. Near single-shot sensitivity of DOD ,1 6 103 has been achieved, with a response time of ,20 ns. Using two continuous-wave EC-QCLs, the current apparatus covers a probe region from 1890–2084 cm1, and TRIR spectra are acquired on a point-by-point basis by recording transient absorption decay traces at specific IR wavelengths and combining these to generate spectral time slices. The utility of the apparatus has been demonstrated by monitoring the formation and decay of the one-electron reduced form of the CO2 reduction catalyst, [ReI(bpy)(CO)3(CH3CN)]þ, in acetonitrile with nanosecond time resolution following pulse radiolysis. Characteristic red-shifting of the m(CO) IR bands confirmed that one-electron reduction of the complex took place. The availability of TRIR detection with high sensitivity opens up a wide range of mechanistic pulse radiolysis investigations that were previously difficult or impossible to perform with transient UV/visible detection.
Index Headings: Nanosecond time-resolved infrared spectroscopy; TRIR; Pulse radiolysis; External-cavity quantum cascade laser; EC-QCL.

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