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A custom experimental apparatus is designed to perform through air drying under well-controlled drying conditions such as air temperature and mass-flowrate. Using a novel optical measurement technique, the spatial distribution of moisture content in paper during through air drying is quantified as a function of time. The technique is capable of measuring the moisture content distribution with high spatiotemporal resolution while air flows through a paper mat sitting on a permeable dryer fabric. Four commercially available fabrics with different structural design and properties are used in the investigations. The effect of the fabrics' structural properties, which are characterized using optical coherence tomography (OCT), is studied under various drying conditions. It is shown that the geometry of the contact spots of the fabrics has a significant impact on the drying time at high drying intensities. However, at low rates of drying (i.e., low air temperature and flowrate), no correlation between drying time and fabric properties is observed. After a cycle of through air drying, the permeability of paper increases irreversibly. This increased permeability is observed to be a function of the fabric structure. It is shown that the increase in permeability is larger for coarse fabric structures although no monotonic correlation with the fabric permeability can be observed. Comparing the spatial maps of moisture content with the paper grammage distribution reveals that there is a correlation between the local grammage and the spatial pattern of drying in a paper sheet. [ABSTRACT FROM AUTHOR]

Existing measurement techniques have prevented extensive investigations of the effect of dryer fabric structure on contact drying of paper. Using a novel optical measurement method, the moisture content (MC) of paper can be accurately quantified at high spatial and temporal resolution while it is sandwiched between the heater surface and the dryer fabric. To study the paper drying process, an experimental setup is designed to simulate realistic conditions of a typical paper dryer while providing optical access for the measurement system. Ten commercially available fabrics manufactured by weaving synthetic filaments are used in the investigations. The 3D structure of the fabrics is characterized using optical coherence tomography (OCT). The fabrics are used in the experiments to investigate the effects of the filament structure and paper/fabric contact on the drying process. It is shown the fabric structure affects the drying rate and the drying time. Fabrics that have a relatively large drying rate at high paper MC may have a relatively small drying rate at low levels of MC. The contact area and 3D arrangement of the filaments have the greatest impact on the drying process. Adjacent filaments result in larger blocked regions of the paper surface, which reduces the drying rate. The spatial distribution of moisture as a function of time reveals that frequent rewetted spots appear during the drying. These rewetting spots are caused by reabsorption of water condensed on the fabric filaments. [ABSTRACT FROM AUTHOR]

Collision cross sections for rotational and vibrational excitation of CO by fast H atoms are calculated for two potential energy surfaces, the older Bowman–Bitman–Harding potential and the recently constructed surface of Werner, Keller, and Schinke. Both quantum mechanical and classical calculations are performed. The results obtained with the new potential energy surface are very similar to those obtained with the older potential; in particular, they do not rectify the discrepancies between the experimental and theoretical cross sections for vibrationally elastic transitions into small rotational states of CO. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Rotational energy transfer in collisions of nitrogen molecules has been studied theoretically, using the N2–N2 rigid-rotor potential of van der Avoird et al. [J. Chem. Phys. 84, 1629 (1986)]. For benchmarking purposes, converged close coupling (CC) calculations have been carried out to a total energy of about 200 cm-1. Coupled states (CS) approximation calculations have been carried out to a total energy of 680 cm-1, and infinite order sudden (IOS) approximation calculations have also been carried out. The CC and CS cross sections have been obtained both with and without identical molecule exchange symmetry, whereas exchange was neglected in the IOS calculations. The CS results track the CC cross sections rather well: between 113–219 cm-1 the average deviation is 14%, with accuracy improving at higher energy. Comparison between the CS and IOS cross sections at the high energy end of the CS calculations, 500–680 cm-1, shows that IOS is sensitive to the amount of inelasticity and the results for large ΔJ transitions are subject to larger errors. State-to-state cross sections with even and odd exchange symmetry agree to better than 2% and are well represented as a sum of direct and exchange cross sections for distinguishable molecules, an indication of the applicability of a classical treatment for this system. This result, however, does not apply to partial cross sections for given total J, but arises from a near cancellation of the interference terms between even and odd exchange symmetries on summing over partial waves. In order to compare with experimental data for rotational excitation rates of N2 in the n=1 excited vibrational level colliding with ground vibrational level (n=0) bath N2 molecules, it is assumed that exchange scattering between molecules in different vibrational levels is negligible and direct scattering is independent of n so that distinguishable molecule rigid rotor rates may be used. With these assumptions good... [ABSTRACT FROM AUTHOR]

Using previously described close coupling (CC) and coupled states (CS) cross sections for N2–N2 collisions [J. Chem. Phys. 104, 7572 (1996)], we have calculated CARS linewidths at room temperature and below. Agreement with experimental values at room temperature is quite good but predictions become increasingly too large at lower temperatures, with errors reaching 10%–30% at 113 K. We believe these low temperature discrepancies reflect errors in the intermolecular potential used here. To obtain linewidths at higher temperatures we have used the energy corrected sudden (ECS) approximation, taking the fundamental cross sections, σ(0.0→J1,J2), from the CC–CS calculations extended to higher collision energies with additional CS and infinite order sudden (IOS) calculations: the ECS scaling distance, lc was chosen by fitting to the 300 K CC–CS results. In general, we find rather good agreement with experimental values to 1500 K, although it appears that smaller values of lc are more appropriate for higher temperatures and for higher rotational levels. This variability of lc is reasonable from physical arguments but somewhat diminishes the predictive utility of this approach. Agreement of these purely ab initio predictions with experimental data is nearly as good as that obtained from the best rate law model whose parameters were fitted to these data. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In fitting an ab initio potential for H2O–H2, Phillips et al. [J. Chem. Phys. 101, 5824 (1994)] excluded certain terms in the angular expansion they believed to vanish because of ‘‘the requirement that the potential is invariant to inversion of all coordinates through the origin.’’ However, there has been some question in the literature as to whether these terms must, in fact, vanish owing to spatial inversion symmetry. By providing counterexamples, it is demonstrated here that this is not required by fundamental spatial symmetry. However, these terms do appear to vanish for realistic molecular interactions and this symmetry may arise from the two-body nature of the electrostatic Hamiltonian. [ABSTRACT FROM AUTHOR]

The generalized Hess method (GHM) gives a line shape expression which is formally equivalent to the Rautian–Sobel’man hard collision model of Dicke narrowing, but differs radically in the definition of one of the relaxation terms. The relaxation term leading to pressure broadening is the same, but the term leading to Dicke narrowing and ultimately to Doppler line shapes at zero density differs in certain important respects: (1) in GHM it is a weighted sum of the pressure broadening coefficient and an optical diffusion coefficient and (2) there is no sharp distinction between ‘‘velocity changing’’ and ‘‘phase changing’’ collisions. The Dicke narrowing term should thus be understood as including both collision types irretrievably intermixed, with GHM providing a prescription for both relaxation terms. Applied to HF v=0→1, j→j±1 absorption spectra in a bath of Ar and using an accurate interaction potential obtained from spectra of the van der Waals complex and essentially exact close coupling scattering S matrices, GHM provides a rather good description of recently measured line shapes. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Collision cross sections were calculated for vibrational and rotational excitation of CO by H atoms at collisional kinetic energies from 0.7 to 1.9 eV. The BBH [J. M. Bowman, J. S. Bittman, and L. B. Harding, J. Chem. Phys. 85, 911 (1986)] potential energy surface was used and collision dynamics were treated within the quantum coupled states approximation, which is shown to be quite accurate for this system, and also using the infinite order sudden approximation for the rotational degree of freedom, which is shown to be less accurate than expected. Results are compared with experimental data and with quasiclassical trajectory values. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Le Roy–Hutson intermolecular potential for H2–Ar, which was derived from analysis of spectral data for the van der Waals complex and also shown to predict gas kinetic transport cross sections in good accord with experimental values, was subsequently found to provide rather poor predictions (small by nearly a factor of 2) for vibrational Q-branch Raman line shift cross sections. More recently, an ab initio intermolecular potential for this system has been calculated and scaled to agree with the earlier one in the region of the minimum where the latter is expected to be accurate [D. W. Schwenke et al., J. Chem. Phys. 98, 4738 (1993)]. We show here that this potential predicts line shifts in much better accord with experiment (about 10% large), but its predictions for linewidths are significantly less satisfactory. [ABSTRACT FROM AUTHOR]

An interaction potential for Ar–HF, which was recently determined from extensive spectroscopic data for the van der Waals dimer and which includes dependence on the HF vibrational state, has been used with accurate close-coupling molecular scattering calculations to predict line shape parameters for the pure rotational and the fundamental and first overtone vibrational bands of HF in a bath of Ar. Agreement with experiment is good; in fact, considering inconsistencies among the experimental values, the theoretical values may be the more reliable. This confirms the accuracy of the Ar–HF interaction potential, including the dependence on the HF vibrational level which is sensitively probed by the line shift cross sections. [ABSTRACT FROM AUTHOR]

Ab initio calculations were carried out for several Stokes–Raman Q branch line shapes of the v=0→1 transition of D2 in helium in the low pressure regime where Dicke narrowing is observed. Five different approximations were studied: a generalization of Hess’s method, which is based on the equivalent of a Bhatnagar–Gross–Krook model of the Waldmann–Snider kinetic equation; two high pressure asymptotes of Hess’s method; a moment method of the Chapman–Cowling type due to Corey and McCourt; and a collision kernel method that has found extensive use lately in rarefied gas dynamics and other highly nonequilibrium states. Using generalized cross sections incorporating S-matrices calculated in the close coupled approximation with an ab initio potential energy surface, most of the methods agreed moderately well with the most recent line width determinations at most densities. Of the five methods, the generalized Hess and collision kernel methods agreed best with experiment over the whole range. At the lowest densities where the collision kernel method might be expected to have convergence problems none were observed, and at high densities the ab initio generalized Hess method may not be flexible enough. [ABSTRACT FROM AUTHOR]

Collision induced widths and shifts for Raman Q-branch transitions of D2 in He were calculated from S matrices obtained with converged close coupling scattering calculations on an accurate theoretical interaction potential. Results agree well with experimental values. Discrepancies between experimental line shifts and theoretical values from an earlier study are traced to a computational error in that work. The effects of vibrational inelasticity and of centrifugal distortion on energy levels and on potential matrix elements, all of which were ignored in the earlier study, are explicitly considered here and found to be small. [ABSTRACT FROM AUTHOR]

The pressure broadening and line coupling cross sections in the Fano–Ben Reuven theory of line shapes are calculated for bending bands of CO2 in a bath of He atoms. Molecular collision dynamics are simplified by invoking the infinite order sudden (IOS) approximation for molecular rotational and vibrational angular momentum in a manner similar to but not identical with the method developed by Clary and shown to be accurate for CO2–He. Numerical values are obtained using a pairwise additive interaction potential developed by Clary. Predictions are in good accord with data for various infrared bands and pure rotational Raman spectra. It is found that all the pressure broadening and state-to-state cross sections depend on only a few dynamical factors (generalized IOS cross sections) and are therefore closely interrelated. Results are used to assess models developed previously to analyze line shapes in this and similar systems. [ABSTRACT FROM AUTHOR]

Theoretical potentials for rotational excitation of H2O by He were constructed via several methods, all of which start with a large basis set SCF interaction. The semiempirical Hartree–Fock with damped dispersion (HFD) model adds a damped long-range attraction with parameters adjusted to fit experimental total differential cross sections. Purely ab initio potentials add correlation energies obtained via perturbtion theory (MP2 and MP4) or a variational method (ICF1). Scattering calculations were performed on all surfaces to compare with available beam scattering and pressure broadening data and to assess sensitivity of state-to-state rates to uncertainties in the potential. From comparison with the limited experimental data, the ICF1 surface appears to be marginally better than the MP4 surface. Thermal rates calculated from this surface should be accurate to better than 50%, at least for the larger, more important rates. [ABSTRACT FROM AUTHOR]

The spectral pressure broadening formalism of Ben-Reuven [Phys. Rev. 145, 7 (1966)] is applied to rotational transitions of a closed-shell linear molecule with nuclear quadrupole hyperfine structure (hfs) due to a nucleus of spin I. If, as expected, nuclear spin does not affect molecular collision dynamics, generalized pressure broadening cross sections can be expressed in terms of the spin–free collisional S matrices. For the three hfs components of the lowest j=0–1 rotational transition, the line shape is a simple sum of three noninterfering Lorentzians each of which has the same width and shift as would be expected in the absence of nuclear spin. For higher rotational transitions, however, the line shape is no longer so simple; in general, each hfs component is described by a different width and shift, and collisions transfer intensity among them. Numerical results for HCN broadened by He atoms are presented using both the accurate close coupling (CC) collision formalism and also the infinite order sudden (IOS) approximation. For the case that broadening is very large compared with the hfs splittings it is shown (numerically, within the IOS approximation) that the line shape is nearly (but not exactly, except for j=0–1 as noted above) a Lorentzian with the same width as would be expected in the absence of nuclear spin. [ABSTRACT FROM AUTHOR]

The high pressure dependence of the linewidths of the D2 polarized Q branch of the Stokes Raman ν=0→1 transition is calculated numerically and compared with the experimental linewidths. These linewidths are computed using the Shafer–Gordon formula for the line broadening cross section. The requisite S matrices are determined from an ab initio D2–He interparticle potential averaged over D2 vibrational wave functions. These wave functions were also calculated with an ab initio D–D potential. Thus, there were no adjustable parameters for the linewidth calculations. In the high pressure limit, the calculated linewidths matched the experimental linewidths to within about 5%. [ABSTRACT FROM AUTHOR]

We examine the question, given a set of state-to-state rate constants k( j=0 → j’|T) for collision-induced rotational transitions in a diatomic molecule, where j and j’ are initial and final rotational quantum numbers and T is the translational temperature, can one use scaling analyses to predict a full set of j → j’ rate constants? To answer this we consider a rigid rotator model of CO in a bath of Ar at 500 K, and we calculate accurate quasiclassical k( j → j’|T) for j=0, 10, and 20 and j’=0–29. These are used to test the energy sudden (ES) and energy-corrected sudden (ECS) scaling procedures. Both procedures are used to predict the j=10 and j=20 rate constants from the j=0 values. The ES procedure, which has no adjustable parameters, overestimates the rates out of excited states by a factor of about 1.5 with a rms error of about 60%. The ECS procedure, in contrast, when the one parameter bc (a critical impact parameter) is about 1.75–2.0 Å, yields excellent excited-state rates on the average and has a rms error of less than 20%. The value of bc can be estimated by a weighted average impact parameter leading to inelastic collisions from a j=0 initial state. [ABSTRACT FROM AUTHOR]