Christopher J. Manning

Application of Step-Scan Interferometry to Two-Dimensional Fourier Transform Infrared (2D FT-IR) Correlation Spectroscopy

The application of step-scan interferometry to two-dimensional infrared (2D IR) spectroscopy is described. In this 2D FT-IR experiment, a step-scan interferometer is used to study a system undergoing dynamic changes induced by an external perturbation. Because step-scanning removes the spectral multiplexing from the temporal domain, the time dependence of the sample response to the perturbation can be retrieved more conveniently, in comparison to conventional rapid-scan techniques. Time-resolved IR data are then converted to 2D IR correlation spectra. Peaks located on a 2D spectral plane provide information about interactions among various functional groups associated with the IR bands. In the step-scan mode, the FT-IR multiplex advantage is retained; thus, spectral regions far removed from each other can be correlated with the use of 2D analysis from a single scan. 2D FT-IR spectra for a composite film of isotactic polypropylene and poly(γ-benzyl-L-glutamate) subjected to a small-amplitude sinusoidal strain are presented. The 2D FT-IR spectra clearly differentiate bands arising from the polyolefin and polypeptide. Overlapped bands are deconvoluted into individual components on the 2D spectral plane due to their different dynamic behavior. The applicability of step-scan 2D FT-IR to a variety of dynamic experiments is discussed.

Step Scan Interferometry in the Mid-Infrared with Photothermal Detection

A medium resolution mid-infrared FT-IR instrument (IBM Instruments IR 44) has been modified to do step scanning; this has been done with the use of concepts previously applied to both near- and far-infrared instruments. In this paper we illustrate the method used for driving the mirror in the step scan mode and present some preliminary results from using the instrument with photothermal detection. At the current state of development, results obtained with the use of phase modulation indicate that this method produces significantly higher signal-to-noise ratios than does the use of amplitude (chopper) modulation to generate the photothermal signal.

Ultra-rapid-scanning Fourier transform infrared spectrometry

Abstract Several applications of Fourier transform infrared (FT-IR) spectrometry are discussed where both conventional rapid-scanning interferometers and step-scan interferometers are unable to provide the needed information. A new interferometer is described by which it is possible to acquire interferograms with 0.25 cm optical path difference (4 cm−1 spectral resolution) in less than 1 ms. The moving element of this interferometer is a tilted mirror that continuously rotates at a speed of up to 500 Hz. The performance of this spectrometer is demonstrated by examining the spark-initiated combustion of ethane and methane in air. The formation of vibrationally excited CO2 and H2O, with lifetimes approaching 50 ms, was observed.

Step-Scan Fourier Transform Infrared Study on the Effect of Dynamic Strain on Isotactic Polypropylene

Dynamic rheo-optical spectra of two isotactic polypropylene samples of different states of order were measured by step-scan FT-IR spectrometry. Experimental details and data collection considerations are discussed. The features in the dynamic spectra are found to be due to small wave-number shifts and absorption changes. The dynamic rheo-optical spectra depend strongly on the pretreatment of the samples, as well as on the polarization state of the infrared radiation used for analysis.

Step‐scan Fourier‐transform infrared spectrometer

This article describes the modification of a commercial Fourier‐transform infrared (FT‐IR) spectrometer for step‐scan operation. Step‐scan operation decouples the FT‐IR spectral multiplexing from time and is therefore applicable to a variety of time‐dependent spectroscopic experiments, including, particularly, photoacoustic and photothermal spectroscopy. The step‐scan instrument described controls the retardation (moving mirror position) with a feedback loop. The loop uses path difference, or phase, modulation of the reference laser intensity, together with lock‐in amplifiers to detect the mirror position. Since the interferogram can be sampled at intervals as small as 1/4 λHeNe, the maximum free‐spectral range is 31 600 cm−1. For initial positioning of the mirror, stepping can be as rapid as 100 Hz. The current software will allow data collection at ∼1.6 Hz, although the mirror settling time of ≤20 ms would allow data to be collected at 20–30 Hz stepping frequency with more efficient software. The mirror...

Step-Scanning Interferometer with Digital Signal Processing

A novel step-scan FT-IR spectrometer incorporating a digital signal processor for demodulation of the detector signal is described. The potential advantages of this method of signal processing are discussed and illustrated. The instrument is based on a commercial cube-corner interferometer which has been modified by replacement of the drive motor with a stepper motor-micrometer and piezoelectric transducer combination. The interferometer retardation is feedback controlled by a 486–50 personal computer, which also controls the digital signal processor and collects spectral data. More than one phase modulation frequency can be imposed simultaneously, allowing for a multiplex advantage in photoacoustic depth profiling. Digital signal processing allows for simultaneous demodulation of multiple frequencies which would normally require several lock-in amplifiers. Data that illustrate the feasibility of these concepts are presented. The suitability of this instrument for double-modulation step-scan FT-IR measurements such as polymer stretching and electrochemically modulated step-scan FT-IR is also discussed.

Time-resolved spectroscopy using step-scan Fourier transform interferometry

The capabilities of a step-scan Fourier transform spectrometer of obtaining time-resolved spectra are reported. As a demonstration of the method, time-resolved spectra from a pulsed fluorescent lamp are presented. The potential of step-scan interferometry for time-resolved infrared measurements of a variety of transient phenomena is discussed.

Study of Impulse Polymer Rheo-Optics by Step-Scan FT-IR Time-Resolved Spectroscopy

The applicability of step-scan impulse/response FT-IR spectroscopy to the rheo-optical study of polymer films is demonstrated by spectral measurements with isotactic polypropylene. A novel piezoelectrically driven microrheometer is employed to apply repetitive impulses to a polymer sample while time-domain spectra are recorded by step-scan FT-IR spectroscopy. The traditional advantages of Fourier transform spectroscopy are retained while providing a second multiplex advantage for the characterization of the time-dependence of the sample response. Reproducible results, consistent with the frequency-domain literature data and having good signal-to-noise ratio, are obtained. The spectral changes due to molecular reorientation are found to be essentially as fast as the mechanical stretching, also consistent with frequency-domain results. To our knowledge, this is the first reported step-scan FT-IR time-domain rheo-optical measurement. This technique appears to be applicable to a variety of polymer samples. The advantages of time-domain measurements over frequency-domain measurements are briefly discussed.

Noise Sources in Step-Scan FT-IR Spectrometry

Step-scan Fourier transform infrared (FT-IR) spectrometry has been accepted as a useful tool for obtaining vibrational spectra of a variety of time-dependent systems. Unfortunately, a significant signal-to-noise ratio (SNR) disadvantage has been associated with the step-scan mode of data collection relative to the same data collection time with conventional rapid-scan FT-IR spectrometry. The key difference between the two methods is the average mirror velocity, which alters the dynamic range of the detector signal, as well as the frequencies of its components. The SNR disadvantage is shown to be related to low-frequency multiplicative fluctuations, caused in part by temperature variations, which convolve noise with measured spectra. Refractive index variations of air or purge gas in the paths of the infrared- and reference-laser radiation can be a particularly serious temperature-induced problem. The various noise sources are described, and experiments confirming that some are related to temperature variations are reported.

Comparison of Two-Dimensional Power and Phase Spectra Generated from Sample Modulation Step-Scan FT-IR Experiments

In the past fifteen years, sample modulation infrared spectroscopy has been introduced for the analysis of dynamic processes. Sample modulation measurements were initially restricted to a few custom-built dispersive instruments, but the recent introduction of commercial step-scan Fourier transform infrared (FT-IR) spectrometers has made dynamic infrared spectrometry widely practical.