Coupling Pulse Radiolysis with Nanosecond Time-Resolved Step-Scan Fourier Transform Infrared Spectroscopy: Broadband Mid-Infrared Detection of Radiolytically Generated Transients

Abstract

We describe the first implementation of broadband, nanosecond time-resolved step-scan FTIR spectroscopy at a pulse radiolysis facility. This new technique allows the rapid acquisition of nano- to microsecond time-resolved infrared (TRIR) spectra of transient species generated by pulse radiolysis of liquid samples at a pulsed electron accelerator. Wide regions of the mid-infrared can be probed in a single experiment, which often takes < 20–30 min to complete. It is therefore a powerful method for rapidly locating the IR absorptions of short-lived, radiation-induced species in solution, and for directly monitoring their subsequent reactions. Time-resolved step-scan FTIR detection for pulse radiolysis thus complements our existing narrowband quantum cascade laser-based pulse radiolysis-TRIR detection system, which is more suitable for acquiring single-shot kinetics and narrowband TRIR spectra on small-volume samples and in strongly absorbing solvents, such as water. We have demonstrated the application of time-resolved step-scan FTIR spectroscopy to pulse radiolysis by probing the metal carbonyl and organic carbonyl vibrations of the one-electron-reduced forms of two Re-based CO<sub>2</sub> reduction catalysts in acetonitrile solution. Transient IR absorption bands with amplitudes on the order of 1 × 10<sup>-3</sup> are easily detected on the sub-microsecond timescale using electron pulses as short as 250 ns.

Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups

“…Time-resolved step-scan FTIR spectra were measured with a Bruker VERTEX 80v step-scan FTIR spectrometer, with 510 nm sample excitation at a 5 Hz repetition rate being provided by a pulsed OPO laser source [Spectra Physics versaScan, pumped by the third harmonic (355 nm) of a Spectra Physics Quanta-Ray Lab-170 Nd:YAG laser; 2−3 nm pulse width, 3 mJ/pulse at 510 nm, measured immediately before the FTIR sample compartment]. A detailed procedure for the step-scan FTIR data acquisition method is provided in the Experimental section of a previous publication, 24 where step-scan FTIR was coupled with pulse radiolysis. For the experiments described in this publication, we used a spectral resolution of 4 cm −1 and an 1815−600 cm −1 optical bandpass filter, resulting in 999 mirror positions.…”