L. J. F. Hermans

Slow evaporation and condensation

In a previous paper an explicit condition for the jump coefficients in a one-component fluid was found within the context of non-equilibrium thermodynamics for the occurence of, on the one hand, an inverted temperature profile in the vapor phase between an evaporating and a condensing liquid surface and of, on the other hand, the supersaturation of the vapor close to the evaporating surface. By making comparisons with the kinetic theory description in the vapor phase we obtained explicit expressions for these jump coefficients. Using these expressions, the conditions found using non-equilibrium thermodynamics reduced to those given in the context of kinetic theory for these phenomena. For a fluid that obeys Troutons rule, as most fluids do, the conditions were found to be satisfied. In the present paper we analyse how a low concentration of dissolved material modifies this situation. For this purpose, the jump conditions for the temperature, the chemical potentials and the pressure are formulated for a multicomponent system in the context of non-equilibrium thermodynamics. Explicit conditions for the occurence of supersaturation and an inverted temperature profile are then given in terms of the jump coefficients. It is found that the inverted temperature profile and supersaturation no longer occur under the same conditions. A comparison is made with the results from kinetic theory, which makes it possible to give explicit expressions for these jump coefficients. We then find that the inverted temperature profile will already disappear for concentrations of dissolved material of the order of the mean-free path divided by the typical size of the system while supersaturation occurs under essentially the same conditions as in the one-component system. The disappearance of the inverted temperature profile is related to the build-up of the concentration of dissolved material near the condensing surface and the corresponding temperature increase.

Slow evaporation and condensation. II: A dilute mixture

Abstract In a previous paper an explicit condition for the jump coefficients in a one-component fluid was found within the context of non-equilibrium thermodynamics for the occurence of, on the one hand, an inverted temperature profile in the vapor phase between an evaporating and a condensing liquid surface and of, on the other hand, the supersaturation of the vapor close to the evaporating surface. By making comparisons with the kinetic theory description in the vapor phase we obtained explicit expressions for these jump coefficients. Using these expressions, the conditions found using non-equilibrium thermodynamics reduced to those given in the context of kinetic theory for these phenomena. For a fluid that obeys Troutons rule, as most fluids do, the conditions were found to be satisfied. In the present paper we analyse how a low concentration of dissolved material modifies this situation. For this purpose, the jump conditions for the temperature, the chemical potentials and the pressure are formulated for a multicomponent system in the context of non-equilibrium thermodynamics. Explicit conditions for the occurence of supersaturation and an inverted temperature profile are then given in terms of the jump coefficients. It is found that the inverted temperature profile and supersaturation no longer occur under the same conditions. A comparison is made with the results from kinetic theory, which makes it possible to give explicit expressions for these jump coefficients. We then find that the inverted temperature profile will already disappear for concentrations of dissolved material of the order of the mean-free path divided by the typical size of the system while supersaturation occurs under essentially the same conditions as in the one-component system. The disappearance of the inverted temperature profile is related to the build-up of the concentration of dissolved material near the condensing surface and the corresponding temperature increase.

Experimental investigation of the rotational‐ and vibrational‐state dependence of the HF–Rg interactions

A systematic investigation of the rotational‐ and vibrational‐state dependence of the HF‐Rg (Rg = He, Ar, Kr, Xe) intermolecular interactions has been performed using the technique of light‐induced drift. Data are presented of the relative change in collision rate upon excitation Δν/ν≡(νe−νg)/νe of HF with respect to Rg. We studied the P‐ and R‐branch of the fundamental vibrational band (v=0→1) with the rotational quantum number J ranging from J=0 through 6 using a continuously tunable F‐Center Laser (λ≊2.5 μm). The results indicate that v and J have independent additive influences upon the collision rate ν. This allows one to determine the v‐ and J‐dependence of ν separately. It is found that, upon vibrational excitation v = 0 → 1, ν increases by ≊0.15% for HF‐He; ≊1.0% for HF‐Ar, Kr and ≊1.5% for HF‐Xe. A remarkable J‐dependence of ν is observed: for HF‐Ar, Kr and Xe, the collision rate ν first decreases by ≊5% for J=0→1, subsequently reaches a minimum for J=2 and then increases again for higher J. By c...

Description of light-induced drift in terms of transport mean paths

Light-induced drift is often described in terms of a change Delta nu in the collision rate (or of the transport collision rate) caused by optical excitation of atoms or molecules immersed in a buffer gas. One assumes (at least tacitly) that the collision rates are independent of velocity, though this is true only for heavy particles in a light buffer gas. The recently observed anomalous light-induced drift of molecular gases suggests using a speed-dependent Delta nu which, however, is difficult to justify. The formulation in terms of speed-dependent transport mean paths, proposed here, is free of such ambiguities. For molecules a Wang-Chang-Uhlenbeck-de Boer multilevel formalism leads to extended transport mean paths, whereupon anomalous light-induced drift can be interpreted in a plausible way.

Angular momentum polarization produced by molecule–surface collisions in a Knudsen flow

In a flowing Knudsen gas, molecular angular momentum polarization can be produced by tangential forces resulting from nonspherical molecule–surface interaction. This is investigated by studying the change in the flow due to molecular precession around an external magnetic field. Various types of polarization are observed in CH4, N2, and HD flowing through a Au‐plated channel. The results suggest a strong correlation between type of polarization produced and molecular structure. These data call for model calculations which include surface corrugation.

Intermolecular versus intramolecular interactions in nuclear spin conversion: Experiments on 13CH3F–O2

Ortho–para conversion for gaseous 13CH3F is measured in mixtures with O2. As collision partner, O2 is found to be 4 times less efficient for conversion that CH3F itself. This demonstrates that intramolecular rather than intermolecular magnetic interactions provide the main pathway leading to nuclear spin conversion for such molecules.

Molecular Angular momentum polarization produced in a Knudsen flow

In a Knudsen flow of polyatomic molecules, polarization of the angular momenta can occur due to non-spherical molecule-surface interactions. The polarization has been studied by observing a change in the particle flux caused by the precession of the molecules in an external magnetic field. It is show how the experimental results as a function of field orientation and field strength allow a determination of the type and magnitude of the polarization. The experimental technique is described and results are given for N2 flowing through a Au-coated channel at 300 K. It is found that the dominant types of polarization are Jy and JxJz, where x is the flow direction and z is the normal to the surface.

Experiments on surface light-induced drift for various systems

Surface light-induced drift of a low-pressure gas results from velocity-selective optical excitation combined with state-dependent molecule-surface interaction. This effect has been studied for CH3F and OCS, rovibrationally excited by a CO2 laser, interacting with various surfaces. For the R(4, 3) transition of 13CH3F, an increase in tangential momentum accommodation coefficient α upon excitation is found ranging from δα = 1.0 × 10-3 for stainless steel to δα = 14 × 10-3 for LiF(100) at 295 K. For this transition, the effect was also studied as a function of temperature for a quartz surface, resulting in a 40% decrease of δα as the temperature is raised from 300 to 700 K. Experiments for the P(5) transition of OCS yield δα = -0.53 × 10-3 on a quartz surface at 295 K.

Use of electric fields to study nuclear spin conversion in molecules. Application to CH3F

Abstract The ortho-para nuclear spin conversion rate of gaseous 13 CH 3 F molecules subjected to an external electric field has been calculated in the framework of intramolecular state mixing. Results are given for the mixing induced by the magnetic dipole-dipole interaction between the molecular nuclei. It is found that the Stark shift of the rotational levels can speed up the conversion rate by a few orders of magnitude at gas pressures appropriate to the experiment. Measurements of this effect will provide information about the role of state mixing in the conversion and the type of intramolecular interaction involved.

Angular momentum polarization in a Knudsen flow; Results for various gases on a Au-surface

In a Knudsen flow of polyatomic molecules, non-spherical molecule-surface interaction can give rise to polarization of the angular momenta. This phenomenon is studied for various gases flowing through a Au-coated channel, by observing the magnetic field effect on the flow. Different types of polarization are found to be produced in different types of molecules. This is qualitatively explained with simple models for a molecule-wall collision. The effects in strongly polar gases are found to be very small. This is attributed to non-instantaneous scattering.