Development of nanosecond time-resolved infrared detection at the LEAF pulse radiolysis facility

Abstract

When coupled with transient absorption spectroscopy, pulse radiolysis, which utilizes high-energy electron pulses from an accelerator, is a powerful tool for investigating the kinetics and thermodynamics of a wide range of radiation-induced redox and electron transfer processes. The majority of these investigations detect transient species in the UV, visible, or near-IR spectral regions. Unfortunately, the often-broad and featureless absorption bands in these regions can make the definitive identification of intermediates difficult. Time-resolved vibrational spectroscopy would offer much improved structural characterization, but has received only limited application in pulse radiolysis. In this paper, we describe in detail the development of a unique nanosecond time-resolved infrared (TRIR) detection capability for condensed-phase pulse radiolysis on a new beam line at the LEAF facility of Brookhaven National Laboratory. The system makes use of a suite of high-power, continuous wave external-cavity quantum cascade lasers as the IR probe source, with coverage from 2330 to 1051 cm(-1). The response time of the TRIR detection setup is ∼40 ns, with a typical sensitivity of ∼100 μOD after 4-8 signal averages using a dual-beam probe/reference normalization detection scheme. This new detection method has enabled mechanistic investigations of a range of radiation-induced chemical processes, some of which are highlighted here.

Electrocatalytic Reduction of Carbon Dioxide with a Well‐Defined PN3−Ru Pincer Complex

“…This study reveals a new ligand platform for the development of high‐performance electrocatalysts of pincer‐based metal complexes with versatile functionality and reactivity for CO 2 conversion. Further studies involving pulse radiolysis with time‐resolved infrared detection to elucidate the detailed mechanism are currently underway …”

“…Further studies involving pulse radiolysis with time-resolved infrared detection to elucidate the detailed mechanism are currently underway. [16]…”

Application of Pulse Radiolysis to Mechanistic Investigations of Catalysis Relevant to Artificial Photosynthesis

“…The proposed catalytic cycle for water oxidation starts with oxidation of the initial clustert op roduce metal oxide particles with all but one iridium atom in oxidation state IV (Scheme 2). This process was pH dependent over pH [4][5][6][7] and was described as an oxidation coupled to the loss of three protonsp er electron, which was rationalizedb yt he acid-base equilibriao ft he initial IrO x ·H 2 Oc luster. An additional oxidation coupled to the loss of one proton (or uptake of OH À )w as necessary to produce the active form of the catalystw ith all iridium atoms in oxidation state IV.I tw as proposed that at least one more oxidation to produce Ir V would turn the catalystover to yield O 2 ,p rotons, and the (Ir IV ,Ir III )s tate;t husc losing the catalytic cycle (Scheme 2).…”

“…The [Co III ÀH)] 2 + intermediate has been produced in PR experiments and was originally observed by meanso fU V/Vis detection ( Figure 10, top, green trace).H owever,t oc onfirm the identityo ft he [Co III ÀH)] 2 + intermediate, we conducted additional experimentsb yu sing our recently developed method of TRIR spectroscopy coupled with PR (PR-TRIR). [4] Thus, we used TRIR spectroscopy to detect the transient speciesp roduced by PR of [Co(OH 2 )] 2 + in aqueous solution.T he n(CoÀH) stretching frequency of [Co III ÀH)] 2 + observed at ñ = 1960 cm À1 (Figure 10, lower left) is in the typical regionf or metal hydride vibrations, and is close to the DFT-predicted value of ñ = 1985 cm À1 .Moreover,t he analogous species produced in D 2 Oe xhibits an IR band at ñ % 1400 cm À1 (Figure 10, lower right), which corresponds to the isotopic shifto ft he n(CoÀH) stretching frequency (n H /n D = 1.4) predicted by ah armonic oscillator model. The resultso btained from TRIR spectroscopy allowed unambiguous assignment of the transientspeciesas[ Co III ÀH)] 2 + .…”

“…However, certain organic solvents produce reducing species upon radiolysis (e.g.,a lcohols, THF,D MF,C H 3 CN), whereas others produce oxidants (e.g.,C H 2 Cl 2 ,C Cl 4 ,C lCH 2 CH 2 Cl);t hese types of solvents have been used in variousP Ra pplicationso ver the years.O ne recent driving force for this is the development of time-resolved infrared (TRIR) detection methodology for PR, which benefits from the higher mid-IR transparency of organic solvents relative to water. [4] More work is required to learn how to fully control free radicalp roduction upon radiolysis of organic solvents, but progress is being made. [5] From the above discussion, it is clear that PR complements LFP-quenching experiments.…”