Biomedical subjects

AD May

Publications and source records attributed to AD May.

15 recordsLinked to original sources

PubMed1999

Lineshifts in the Fundamental Band of CO: Confirmation of Experimental Results for N(2) and Comparison with Theory.

We have used a three-channel version of a tunable difference frequency laser spectrometer to measure the collisionally induced lineshifts at room temperature for 26 lines of the fundamental band of CO perturbed by nitrogen. Each lineshift was obtained directly by comparing the line center positions of two simultaneous recordings, one for a pressure-shifted line, and the other for the same line in pure CO line at very low pressure. The experimental results are found to be in complete agreement with earlier measurements and confirm that shifts as small as 3 MHz may be measured in such a system. Our results are compared with theoretical calculations. The part of the shifting coefficient antisymmetric with respect to a change in sign of the line number m, is in disagreement with the calculations. Copyright 1999 Academic Press.

Journal Article↗

PubMed1999

Spectral Profiles for Atmospheric Absorption by Isolated Lines: A Comparison of Model Spectra with P- and R-Branch Lines of CO in N(2) and Ar.

There is a need in atmospheric research to accurately model absorption profiles over a wide range of pressures and temperatures. Here we explore the suitability of four profiles to describe absorption by an isolated line. Three of the profiles, a Lorentzian, a Voigt profile, and a hard-collision model, are speed independent. The fourth profile is an empirical speed-dependent hard-collision model. We test the quality of the profile and the pressure dependence of the broadening by fitting to precise experimental absorption curves for CO broadened by N(2) and Ar. Of the four profiles, we find that only the empirical model is satisfactory. This profile also meets the need of atmospheric modeling in that it is easily evaluated. Copyright 1999 Academic Press.

Journal Article↗

PubMed1999

Line Broadening and the Temperature Exponent of the Fundamental Band in CO-N(2) Mixtures.

We have measured the width, Gamma, of many P and R lines of the fundamental vibration-rotation band of CO perturbed by N(2) at 348 K and pressures of about 50 kPa. We have also extended the measurements made earlier at room temperature. The broadening coefficients, gamma = Gamma/pressure, were obtained with an accuracy of 0.3% by fitting with a Lorentzian, a Voigt, and an empirical lineshape model that blends together a hard-collision model and a speed-dependent Lorentzian profile. In all cases the results are represented by an empirical exponential power law polynomial in the line number, m. Combining the data at the two temperatures yields the exponent n in the scaling law gamma(T(1))/gamma(T(2)) = (T(1)/T(2))(-n), as a function of line number. The broadening coefficients and the variation of the temperature exponent line number are compared with theory. In addition, the line-mixing parameters are also reported at 348 and 301 K. Copyright 1999 Academic Press.

Journal Article↗

PubMed1999

An Algorithm for Calculating a Speed-Dependent Lorentzian Profile.

We present an algorithm for calculating speed-dependent Lorentzian profiles. Specifically, we show that the sum of two simple Lorentzians, subject to certain constraints, accurately reproduces a Lorentzian profile with a speed-dependent broadening. Further, the algorithm may be used in more complicated lineshapes in which the speed-dependent Lorentzian is embedded. The benefits resulting from this construction are simplicity of form, the ability to use the form in further symbolic manipulations, and the use of preexisting fast computer codes for the evaluation of the function. Such practical considerations make it possible to use speed-dependent lineshapes in applications such as atmospheric modeling, which demand accurate but computationally efficient absorption curves. In addition, several biases or problems, that occur when speed-dependent experimental profiles are fitted with a simple Lorentzian, are discussed. Copyright 1999 Academic Press.

Journal Article↗

PubMed1998

Line Broadening in the Fundamental Band of CO in CO-He and CO-Ar Mixtures.

We present accurate broadening coefficients for many lines of the fundamental band of CO highly diluted in He and Ar at 301.5 K. It is shown that a soft collision model gives a reasonable fit to the data at pressures up to 1 atm. A distinction is drawn between the accuracy with which fitting parameters may be determined and the accuracy with which a spectral profile fits experimental data. Differences between a model profile and the experimental profile are quantified, and the underlying physics omitted from the spectral model is briefly discussed. Copyright 1998 Academic Press.

Journal Article↗

PubMed1998

Line Mixing and Broadening in the Raman Q Branch of HD at 304.6 K.

The Q-branch lines of pure HD were measured at densities ranging from 1 to 7 Amagat at 304.6 K. Each profile was fitted to the well-known Rosenkranz expression to extract the size of the asymmetry due to line mixing as well as to the linewidth. Line mixing and broadening coefficients were obtained by fitting the asymmetries and widths to a straight line as a function of gas density. Apart from a single existing measurement for the Q(0) line, our mixing coefficients are the first direct measurements of the asymmetry due to line mixing in HD. Our broadening coefficients are consistent with the best earlier measurements but are an order of magnitude more precise. Agreement is found with some existing semiclassical calculations of broadening. We have fitted our HD broadening coefficients to a variety of empirical energy gap laws. Our conclusions are that none of the exponential gap law (EGL), the modified exponential gap (MEG) law, and the statistical power gap (SPG) law successfully models our broadening coefficients. We present a modified version of the EGL and the MEG laws, which are successful in reproducing the experimental results. Using the fitted parameters of the new gap law, we have calculated the relaxation matrix of HD at room temperature. With this relaxation matrix, we have simulated the Q-branch spectrum at a number of densities between 49.1 and 490 Amagat and compared the results with previous high-density measurements. At all densities and frequencies, the simulated spectral intensity was found to agree with the measured strength within about 5% of the peak of the spectrum. In addition, the comparison provides evidence of a nonlinear vibrational dephasing shift in HD. Copyright 1998 Academic Press.

Journal Article↗

PubMed1997

Direct Measurements of Line-Mixing Coefficients in the nu1 + nu2 Q Branch of CO2

High-resolution measurements of the (nu1 + nu2) Q branch of pure CO2 were made using a difference frequency spectrometer with resolution of 5 x 10(-5) cm-1 and a signal-to-noise ratio of 2000:1. Lines Q(2) through Q(32) were measured with up to 14 lines in a single spectrum. The analysis of the branch has been performed on data taken at 301 K and pressures less than 11 kPa. The spectra were analyzed on a line-by-line basis within the Rosenkranz approximation of weak overlap [P. W. Rosenkranz, IEEE Trans. Antennas Propagation AP-23, 498 (1975)]. The lineshape profile included Doppler broadening and Dicke narrowing [R. H. Dicke, Phys. Rev. 89, 472 (1953)] using a modified hard collision model [S. G. Rautian and I. I. Sobel'man, Sov. Phys. Uspekh. 9, 701 (1967)] with line mixing. For each line the broadening, Dicke narrowing, and line-mixing coefficients were determined. The broadening coefficients are in good agreement with measurements of lines belonging to different CO2 bands. Our measured line-mixing parameters are compared to those predicted by a relaxation matrix which was calculated from an exponential power gap (EPG) law [L. L. Strow, D. C. Tobin, and S. E. Hannon, J. Quant. Spectrosc. Radiat. Transfer 52, 281 (1994)]. The vibrational band intensity and the linear pressure shift of the branch were also measured. Copyright 1997Academic Press

Journal Article↗

PubMed1997

Testing Lineshape Models: Measurements for v = 1-0 CO Broadened by He and Ar

We present measurements of the spectral profiles of many P and R branch lines in CO-He and CO-Ar mixtures, all at 301.5 K. The lineshapes are compared with several semi-classical models and in particular with models that include the speed dependence of both the translational motion and the relaxation of the optical coherence. Using a correlated speed-dependent Galatry model proposed by Ciurylo and Szudy we find an apparent reduction of the Dicke narrowing, amounting to almost 100% in some cases with a strong dependence on the rotational quantum number. However, while the parameters of this and several other models may be adjusted to fit the experimental data within the noise, no single model is free of criticism. Several shortcomings of existing models are discussed. Plots of broadening coefficients versus line number are also given. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗

PubMed1996