Douglas J. Moffatt

Fourier Self-Deconvolution: A Method for Resolving Intrinsically Overlapped Bands:

The general theory of Fourier self-deconvolution, i.e., spectral deconvolution using Fourier transforms and the intrinsic lineshape, is developed. The method provides a way of computationally resolving overlapped lines that can not be instrumentally resolved due to their intrinsic linewidth. Examples of the application of the technique to synthetic and experimental infrared spectra are presented, and potential applications are discussed. It is shown that lines in spectra having moderate signal/noise ratios (∼1000) can readily be reduced in width by a factor of 3. The method is applicable to a variety of spectroscopic techniques.

Noise in Fourier self-deconvolution.

A general formula for computing changes in the signal-to-noise ratio of a spectrum resulting from the Fourier self-deconvolution procedure is derived. Self-deconvolution reduces the intrinsic halfwidths of lines by a factor K, which is in practice limited by the noise in the spectrum. With the help of the derived formula, the rate of decrease in the SNR as a function of K for eight different smoothing (apodization) functions is studied. With high K values there are significant differences in the SNR as a result of the use of different smoothing functions. With K = 4 a difference of more than 1 order of magnitude between two extreme cases is demonstrated, and with K = 5 a difference of almost 2 orders of magnitude in the SNR is predicted.

Precision in Condensed Phase Vibrational Spectroscopy

Algorithms for the computation of bandwidths, and center of gravity and least-squares frequencies are developed. The sources of error are discussed, and it is shown that the above parameters can be determined with maximum uncertainties of hundredths or thousandths of a wave number.

Optimally chirped multimodal CARS microscopy based on a single Ti:sapphire oscillator

We demonstrate high performance coherent anti-Stokes Raman scattering (CARS) microscopy of live cells and tissues with user-variable spectral resolution and broad Raman tunability (2500 - 4100 cm(-1)), using a femtosecond Ti:Sapphire pump and photonic crystal fiber output for the broadband synchronized Stokes pulse. Spectral chirp of the fs laser pulses was a user-variable parameter for optimization in a spectral focusing implementation of multimodal CARS microscopy. High signal-to-noise, high contrast multimodal imaging of live cells and tissues was achieved with pixel dwell times of 2-8 micros and low laser powers (< 30 mW total).

Crystalline Quartz as an Internal Pressure Calibrant for High-Pressure Infrared Spectroscopy

Raman spectra of aqueous phospholipid bilayers and aqueous micellar solution of surfactants at high pressure have been studied in this laboratory. However, some normal modes in these systems are Raman inactive or weakly active, and most of the normal modes of the methylene chains in these systems are highly dispersed. In order to further understand the pressure effects on the vibrational dynamics in these systems, we have extended our pressure studies to the Fourier transform infrared spectra of these aqueous systems, using a diamond anvil cell.

A Generalized Approach to Derivative Spectroscopy

A general approach to obtaining smoothed even-order derivative spectra using Fourier transforms is developed. The method is easier to use and yields higher signal-to-noise ratios than do techniques which involve the use of convolution functions.

Fourier transform methods for spectral resolution enhancement

Abstract Two mathematical procedures of band narrowing using Fourier transforms are discussed. The methods of Fourier self-deconvolution and Fourier derivation are easier to use and to control than procedures which rely on the use of convolution functions.

A New Line-Narrowing Procedure Based on Fourier Self-Deconvolution, Maximum Entropy, and Linear Prediction:

We propose a new methodology to reduce the widths of spectral lines. This procedure, which is performed entirely in the time domain, combines elements of the methods of Fourier Self-Deconvolution (FSD), Maximum Entropy (MEM), and Linear Prediction (LP) in such a way as to take advantage of the efficacies of each of these procedures. The procedure is illustrated with examples.

The frustrated motion of benzene on the surface of Si(111)

Benzene adsorption on Si(111)‐7×7 is studied with scanning tunneling microscopy. Benzene diffusion is found to be inhibited. Ordinarily surface diffusion is controlled by a substantially lower energy of activation than is desorption. In this case diffusion is frustrated by a barrier to diffusion that is comparable to that for desorption. Both desorption and diffusion are monitored. On average, for every two adsorbate disappearances, one readsorption is observed and one molecule desorbs. Site‐specific activation barriers of 0.94±0.01 eV and 0.95±0.01 eV for center faulted and corner faulted adatom sites, respectively, are extracted. Residence times increase as coverage decreases, implying adsorbate crowding causes the strength of the surface–adsorbate bond to weaken. Diffusion is generally found to involve jumps to sites beyond nearest neighbors. It emerges that the adsorbate largely breaks its existing bond to a surface site before forming a substantial bonding interaction with a new site. We surmise that...

Smoothing of spectral data in the Fourier domain.

Smoothing of spectral data using Fourier transforms is described and demonstrated with Lorentzian, sinc(2), and sine smoothing functions. Four parameters are defined and used to study the smoothing operation. It is also concluded that the best smoothing function is a sinc function if we require that the distortions due to the smoothing operation are smaller than the residual noise. Sine smoothing using Fourier transforms is also compared to least square smoothing in the frequency domain, and the advantages of sine smoothing are illustrated.