Anthony E. Dowrey

Recent Developments in Two-Dimensional Infrared (2D IR) Correlation Spectroscopy

Recent developments in two-dimensional infrared (2D IR) correlation spectroscopy are reviewed. Since the initial introduction of the basic concept seven years ago, the field of 2D IR spectroscopy has evolved considerably. The method for generating 2D IR spectra from perturbation-induced time-dependent fluctuations of IR intensities and the properties of such 2D spectra are summarized first. Applications of 2D IR spectroscopy are then surveyed, and improvements in the instrumentation are reviewed. Different types of external perturbation schemes capable of inducing dynamic fluctuations of IR spectra are listed. Finally, a new 2D correlation method for dynamic spectral data with arbitrary time-dependence is discussed.

A Spectrometer for Measuring Time-Resolved Infrared Linear Dichroism Induced by a Small-Amplitude Oscillatory Strain

A spectrometer for detecting dynamic infrared linear dichroism (DIRLD) induced by a small-amplitude oscillatory strain is described. The strain-induced dynamic variations of absorbance and linear dichroism, as well as the normal static absorbance and dichroism, are measured simultaneously as functions of IR wavenumber, temperature, and strain frequency. The phase (temporal) relationships between the dynamic optical signals and the applied strain are also obtained. The instrument is sensitive enough to detect dynamic optical signals on the order of 10−4 absorbance units with a time resolution of about 14 μs. The dynamic dichroism signals arise from the strain-induced temporary reorientation of dipole-transition moments associated with the molecular vibrations of chemical functional groups. Both the rate and extent of reorientations are strongly influenced by the local molecular environment of the functional groups. Because of the specificity of IR absorbance bands to the individual submolecular structures, DIRLD spectroscopy is especially suited for the study of the intra- and intermolecular interactions and the detection of subtle changes in the local molecular environment. Example DIRLD spectra of atactic polystyrene are presented to demonstrate the potential utility of this spectroscopic technique.

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.

A Double-Modulation Fourier Transform Infrared Approach to Studying Adsorbates on Metal Surfaces

Polarization modulation has been combined with Fourier transform infrared (FT-IR) spectroscopy to yield a sensitive method for obtaining infrared reflection-absorption spectra of monolayer films adsorbed on low-area metal surfaces. A 10-Å film of cellulose acetate adsorbed on a polished copper coupon was examined and the results compared with spectra obtained on the same instrument without polarization modulation. The signal-to-noise ratio is significantly better with the polarization modulation approach. The new method has the additional advantage of being sensitive only to molecules adsorbed on the surface. IR absorption by background gases is not detected.

Enhancing the information content of vibrational spectra through sample perturbation

Abstract Infrared and Raman band frequencies, intensities and line shapes are often sensitive to the local molecular environment determined by molecular conformation, surrounding matrix, temperature, pressure, etc. The variety of local environments experienced by a condensed-phase molecule can lead to vibrational spectra with broad bands containing many overlapped spectral features. The spectral resolution of these overlapped features can be enhanced by making perturbations to the sample environment. Examples of perturbations which can be applied to the sample to enhance the information content of infrared spectra are changes in temperature, concentration and mechanical strain. In each instance, the spectra obtained as a function of the perturbation can be cross-correlated to produce a two-dimensional correlation map defined by two independent wavenumber axes. in this representation, infrared bands which respond to the perturbation in a similar or different manner can be clearly identified. This information can be used to help resolve overlapped bands and make unambiguous band assignments.

Instrumental Aspects of Dynamic Two-Dimensional Infrared Spectroscopy

Dynamic two-dimensional infrared (2D IR) correlation maps are a convenient means of examining the information contained in time-resolved IR spectra. Dynamic 2D IR spectra can be collected with the use of either dispersive or Fourier transform (FT) IR spectrometers. Use of a step-scanning FT-IR spectrometer has advantages over conventional rapid-scan FT-IR spectrometry when one is acquiring time-resolved IR data on time scales faster than about 0.1 s, because the spectral multiplexing is removed from the time domain. Dynamic IR spectra of atactic polystyrene (undergoing a small-amplitude oscillatory strain) collected on both dispersive and FT instrumentation are compared. Although the dispersive approach produces higher signal-to-noise ratios over small spectral regions, the multiplex advantage makes the FT approach attractive when broader spectral coverages are required. The first vibrational circular dichroism (VCD) spectrum [of (–)-α;-pinene] collected on a step-scanning interferometer is also presented.

Two-dimensional infrared (2D IR) spectroscopy. A new tool for interpreting infrared spectra

Two-dimensional infrared (2D IR) spectroscopy is used to study atactic polystyrene. 2D IR is a technique based on time-resolved detection of IR signals in response to an external perturbation, such as mechanical strain. Since different chemical functional groups respond to the applied perturbation at unique and often different rates, characteristic time-dependent variations of the IR-band intensities are observed. Correlation analysis of the dynamic variation of the IR signals yields a new spectrum defined by two independent wave numbers. Peaks located on a 2D IR spectral plane imply interactions and connectivities among chemical functional groups. By spreading convoluted IR bands over two dimensions, the spectral resolution is also greatly enhanced.

Characterization of Polymers Using Polarization-Modulation Infrared Techniques: Dynamic Infrared Linear Dichroism (DIRLD) Spectroscopy

High-frequency polarization-modulation infrared techniques [1–8] can be successfully applied to the characterization of polymers using either Fourier transform or dispersive instrumentation. These techniques include infrared reflection-absorption spectroscopy (IRRAS) [2–4], infrared linear dichroism under both static [5] and dynamic [6–8] conditions, and vibrational circular dichroism (VCD) [1] for chiral systems. Some intrinsic advantages of polarization-modulation make it especially attractive for high-sensitivity IR spectroscopy. Since VCD and IRRAS are covered thoroughly in other papers of this symposium book, our discussion will concentrate on linear dichroism of polymer films; in particular, the application of polarization modulation to dynamic infrared linear dichroism (DIRLD) spectroscopy.

Dynamic infrared linear dichroism study of high-density and low-density polyethylene near the β-transition temperature

Abstract High-density polyethylene (HDPE) and low-density polyethylene (LDPE) films under small-amplitude oscillatory strain were examined using time-resolved infrared (IR) linear dichroism spectroscopy. A negative dynamic dichroism at 1473 cm −1 and a positive dynamic dichroism at 1463 cm −1 were observed at +32°C (above the β-transition temperature, T β . This observation is interpreted to mean that above T β the orthorhombic crystallographic b axis reorients parallel to the direction of applied strain, while the crystallographic a axis reorients perpendicular to the strain direction for both HDPE and LDPE. Interestingly, the dynamic dichroism of both bands changes sign at −50°C (below T β ), indicating that the dynamic reorientation directions for the crystalline a and b axes are shifted below T β . This result clearly shows that different deformation mechanisms operate above and below T β . Even though the IR peaks in this region are made up of overlapping bands from both crystalline and amorphous components, they are easily resolved in this experiment. The high spectral resolution was achieved because the electric dipole transition moments associated with individual bands reorient in different directions, at different rates, and to different extents under dynamic strain.

Prism-Based Infrared Spectrographs Using Modern-Day Detectors

A comparison of prism-based spectrographs to grating-based spectrographs is made when each of the systems is coupled to a modern-day liquid-nitrogen-cooled photovoltaic array detector. A comparison of the systems is also made using a room-temperature microbolometer array detector. Finally, infrared microspectroscopy of samples whose size is ∼10 micrometers will be demonstrated using a prism spectrograph outfitted with both types of detectors. The results of the study show that prism-based spectrographs offer an economical alternative to grating-based systems when spectral coverage is more critical than spectral resolution. The results also demonstrate that spectra with good signal-to-noise ratios can be collected on any of the systems with a total integration time of 10 seconds or less.