James L. Chao

Investigation of Time-Dependent Phenomena by Use of Step-Scan FT-IR

The development, during the last decade, of modern step-scan interferometry instrumentation has allowed FT-IR to be applied to the study of time-dependent phenomena in ways not previously possible, because of the problems of uncoupling the spectral multiplexing from the temporal domain in the continuous-scan FT-IR mode. Specifically, the time regime from tens of nanoseconds to tens of milliseconds has been accessible to time-domain measurements to only a very limited degree with continuous-scan instrumentation and not at all for modulation-demodulation (frequency-domain) experiments in this time range. The step-scan technique not only works very well in this time regime and for slower phenomena, but is only prevented from application to faster processes by the signal strength, the speed of available detectors, the intensity of sources, and the speed and sophistication of the electronics. This paper surveys the various types of experiments which are either enhanced by use of step-scan FT-IR methods or are only possible by use of these techniques. The principles of step-scan instrumentation are reviewed, particularly those of retardation control, signal generation, and data acquisition, as well as the place of step-scan FT-IR relative to other techniques of dynamic vibrational spectroscopy. The importance of path difference (phase) modulation, particularly in frequency-domain measurements, the extraction and use of the signal phase, the creation of 2D FT-IR spectra, and the strategies for acquisition of both time- and frequency-domain data in the step-scan mode are discussed and illustrated.

Time-resolved vibrational spectroscopy of an electric field-induced transition in a nematic liquid crystal by use of step-scan 2D FT-IR

Abstract A study of the kinetics of reorientation of a uniaxially aligned nematic liquid crystal (4-pentyl-4-′-cyanobiphenyl) under the influence of an external ac electric field is reported. Step-scan FT-IR was employed for the acquisition of the dynamic linear dichroism spectra. 2D FT-IR frequency correlation maps indicate that the rigid core of the nematic molecule reorients as a unit and suggest that the phenyl chain may reorient more rapidly than the core.

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.

Photoacoustic Depth Profiling of Polymer Laminates by Step-Scan Fourier Transform Infrared Spectroscopy

The use of step-scan Fourier transform infrared (FT-IR) spectroscopy with photoacoustic (PA) detection for depth profiling studies of polymer laminates is demonstrated. Step-scan FT-IR simplifies the extraction of depth profile information due to the single modulation frequency that can be applied over the entire spectral range. Because a single modulation frequency is generally used in step-scan FT-IR, the thermal diffusion length, μ, is constant for all wavelengths in a single scan. In addition, lock-in detection allows for easy extraction of the signal phase. Two methods of depth profiling are discussed and illustrated. The first is the conventional method of varying the probe depth by changing the modulation frequency. The other method depends on the direct use of the signal phase. The phase analysis technique is particularly useful for cleanly separating the signal due to a thin (<5 μm) surface layer from that of the bulk or substrate.

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.

Development and applications of a photoacoustic phase theory for multilayer materials: The phase difference approach

A quantitative photoacoustic (PA) phase theory for multilayer materials has been developed by using the surface temperature approach, and experimentally confirmed with the step‐scan Fourier transform infrared PA phase data of multilayer polymer samples. This theory unifies several previous PA phase theories for specific cases and provides a clear picture of the relationship between the PA phase response of each layer and the thermal, optical, and geometric properties of a multilayer sample. Within the context of this model, it is shown how the PA phase difference is easily obtainable and instrument independent, and thus widely applicable to spectral depth profiling of multilayer materials. The theory can be readily applied to either qualitative discrimination of PA signals from different layers, or quantitative determination of the thermal diffusivities, optical absorption coefficients, or thickness of different layers.

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...

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.

Time-Resolved Step-Scan FT-IR Spectroscopy of the Photodynamics of Carbonmonoxymyoglobin

The kinetics of protein response and of CO recombination after photolysis of the Fe-CO bond in carbonmonoxymyoglobin have been monitored via time-resolved step-scan FT-IR absorption difference spectroscopy in D2O solution. Although the initial photodissociation is too fast to observe with currently available FT-IR instrumentation, we have been able to correlate the CO recombination kinetics with protein secondary structural changes via changes in the amide I band of the polypeptide chain with microsecond time resolution. The spectral and kinetic data corroborate and confirm previously published single-frequency infrared studies. This is the first application of time-resolved step-scan FT-IR spectroscopy in the absorbance difference mode to study the photodynamics of an aqueous protein solution at room temperature. This work also demonstrates the potential of the technique for the sub-microsecond kinetic analysis of other biological molecules of interest.

Corrosive gas environmental testing for electrical contacts

Corrosive gas environmental tests are used to predict the reliability of electrical connectors used in data system products under conditions found in business office environments. A corrosive gaseous environmental acceleration test called G1(T) is evaluated through variations in testing conditions, in particular for those which differ from outside testing standards used in the connector industry. An investigation of those features of various acceleration tests which could be recommended for adoption into an industry-wide testing standard are reported. These studies could be important for gaining a consensus for support of an acceleration testing practice to predict corrosion reliability during the life of a product. Copper, nickel, and porous gold coupons (electroplated gold over nickel over copper) are used to determine the effects of each corrosive gas test environment. The corrosion of copper is quantified by coulometric reduction, while nickel is evaluated by energy dispersive X-ray fluorescence analysis. The porous gold coupons are visually examined for the presence of pore and creep corrosion, which makes it possible to determine the degree of environmental exposure quickly.<<ETX>>