Handbook of Vibrational Spectroscopy: 5 Volume Set

Fast Time-resolved Mid-infrared Spectroscopy Using an Interferometer

Gregory D. Smith and Richard A. Palmer – Duke University, Durham, NC, USA

1 INTRODUCTION The development of fast time-resolved mid-infrared (MIR) spectroscopy since the 1980s has provided an effective tool for the study of transient chemical species in dynamic systems. Interest in this particular spectroscopy stems from the virtual universality of infrared (IR) absorption, the rich molecular structural information contained in the MIR spectrum, and the fast time scale of vibrational motions [compared, for example, with the nuclear magnetic resonance (NMR) time scale]. The incorporation of an interferometric spectrometer in these time-resolved studies allows multiple vibrational frequencies to be monitored simultaneously across a wide spectral window. With this tool, the processes of ligand binding and dissociation, protein folding, photocycles, reaction kinetics, and excited-state electronic structures, as well as a host of other dynamic chemical phenomena, can all be measured and characterized. Over the years, efforts in this area have centered on increasing the ! speed at which broadband, high-resolution spectra can be collected.

Time-resolved MIR spectroscopic studies of transient species have been conducted over a temporal range stretching from minutes to femtoseconds. Figure 1 shows that each time regime along that scale has come to be dominated by a specific time-resolved infrared (TRIR) technique, that a considerable overlap exists at the boundaries, and that interferometric techniques dominate a large portion of the dynamic spectrum. The superiority of each technique in a particular time regime can be ascribed to that technique’s maximum time resolution, sensitivity, spectral range, cost, and ease of use.

Although several laser-based approaches to TRIR, both single-frequency and continuum-generation methods, offer extremely high time resolution, Fourier transform interfero-metric time-resolved infrared (FT-TRIR) techniques have distinct advantages over both these methods in terms of spectral bandwidth, resolution, and sensitivity. The use of an interferometer to encode the spectral characteristics also brings a wide array of added benefits to TRIR experiments. Fourier transform infrared (FT-IR) pectroscopy’s well-known multiplex (Fellgett), throughput (Jacquinot), and registration (Connes) advantages, which have led to the almost total replacement of dispersive instruments for static measurements, also persist in time-resolved spectroscopy (TRS) applications. The rise time of practical broadband MIR detectors is currently limited to ~1 ns, and therefore, the fastest FT-TRIR technique as currently demonstrated has its upper limit in that time regime. However, the pulsed-asynchro! nous sampling technique (see below and Volume 1, Part 2) promises to extend that limit by several orders of magnitude, thus pressing laser-based techniques even further in their most advantageous area, namely time resolution.

Despite the present temporal limitation of FT-IR, the advantages provided by interferometric spectral encoding make it superior to dispersive and laser-based methods for producing fast time-resolved, high spectral resolution measurements in which the resulting spectra cover the entire MIR region. The chief drawbacks of the latter techniques, and consequently the areas where Fourier transform (FT)-based methods excel, include the limited tunability of probe lasers (both in range and resolution), the complexity of multicomponent laser systems, and the expense of the equipment. For many TRIR experiments, the modest cost and standard availability of time-resolving features on commercial FT-IR spectrometers make their implementation in time-resolved studies throughout the second to nanosecond time scale relatively routine.

This article deals with each of the FT-TRIR techniques indicated in Figure 1. The sections cover the technique’s theory of operation, instrumental requirements, specific caveats, advantages and limitations, current applications, and the state of the art. Although every attempt is made to keep the discussion of each technique general, occasionally it is necessary to mention specific instrumentation, and in these cases, the authors have chosen to comment on instrumentation with which they are personally familiar. These instances should not be taken as an indication of the superiority of a particular instrument or piece of equipment, but can be considered at least one proven option for collecting TRIR data. Specific references are intended to direct the reader to more thorough coverage of certain topics in the literature and to applications of each FT-TRIR technique to real chemical problems. Although FT-IR instruments are also used for dynamic measurements in the frequency domai! n (modulation/demodulation method), these applications will not be covered in this article, and readers are referred to other parts of this work (see Volume 1, Part 2; Volume 2, Part 7; and Volume 4, Parts 1 and 2).

2 RAPID AND ULTRARAPID SCANNING FOURIER TRANSFORM INFRARED The operation of the classic Michelson interferometer for continuous, or constant-velocity, scanning has been dealt with fully in the literature and in this volume (see Volume 1, Part 2). In addition to its use for static spectroscopy, it also represents the simplest and most convenient FT-TRIR technique. In rapid-scanning FT-TRIR, the moving mirror’s velocity is chosen so that on the time scale of the dynamic event, each scan is effectively instantaneous. Figure 2 shows the data-collection scheme for generating a stack of time-resolved interferograms using rapid-scanning interferometry. For clarity, the interferograms are shown as the change in the intensity during the course of a time-dependent perturbation, and the numbered circles underneath the stack plot display the order in which data are collected by the spectrometer’s analog-to-digital converter (ADC). Successive scans of the interferometer collect entire interferograms instantaneously with respect to the temporal evo! lution of the dynamic event so that t-t 2.Thefirst interferogram (points 1-10) corresponds to the steady state of the sample just before being excited, and in the second interferogram (points 11-20), the state just after excitation.

Typical mirror velocities for standard commercial instruments range from a few hundredths to ~6 cm, providing optical velocities of twice those values. The better, more expensive models support a wide range of variability, often extending even beyond these two limits, whereas cheaper instruments offer only a limited number of scan speeds, usually tending toward the middle of the scale where the mirror is more easily controlled. Thus, for the generation of a 4 spectrum, the spectrometer requires between 1 s and 40 ms, depending on the scan velocity selected, for the completion of…