Publications

Multiphoton excitation and dissociation of DN3 by short CO2 laser pulses is shown to be a collisionless process. The characteristic features of this multiphoton process are systematically studied. The average number of photons absorbed per DN3 molecule and the absolute dissociation yield show a strong dependence on the peak laser intensity. Resonantly enchanced coherent multiphoton excitation, rather than stepwise incoherent excitation, is suggested. The primary dissociation products of DN3 are ND[1delta] and N2. Formation of vibrationally excited ND[1delta] intermediates is suggested. The reactions of ND[1delta] with DN3 lead to chemiluminescent signals originating from the formation of electronically excited ND2[2A1] and ND[3II]. Formation of the ND[3II] intermediate is attributed to a reaction of ND[1delta] and vibrationally excited DN3 molecules: ND[1delta]+DN3-ND[3II]+ND[3-]+N2.

In the kinetic theory of non-equilibrium phenomena in dilute polyatomic gases there is a characteristic difference between the treatment of non- spherical and spherical particles. This difference stems from the fact that in a polyatomic gas, i.e. a gas of non-spherical particles, in the non-equilibrium state not only the distribution of molecular velocities becomes anisotropic, but that also the distribution of the orientations of the molecules is affected by macroscopic thermodynamic forces. These deviations from an isotropic distribution of angular momenta, or polarizations, are generally complicated in nature and may depend on both velocity and angular momentum. Their presence was first realized by Pidduck in 1922. In their book on non-uniform gases Chapman and Cowling were aware of the anisotropies in both the velocity and the rotational angular momentum, but in subsequent calculations they neglected the effects of the angular momentum dependent terms. Later in 1961, Kagan and Afanasiev showed that for a simplified classical model such terms give rise to sizable contributions to the transport properties. Experimental information on polarizations are obtained in various ways. First of all, field effects on transport phenomena, which as Kagan and Maksimov showed are a direct consequence of the existence of such polarizations, yield a wealth of data. Secondly, information is obtained by measuring the non-equilibrium birefringence caused by the anisotropy in the orientational distribution of the molecules. Additional information on certain aspects, e.g. relaxation times of polarizations, can be obtained from a study of phenomena which are determined by (equilibrium) fluctuations, such as the depolarized Rayleigh line broadening and nuclear magnetic resonance. Polarizations can be thought of as consisting of two parts: a tensorial factor (of rank one of higher) depending on the orientation of the molecule and the direction in which the molecule is moving, and a scalar factor depending on the magnitudes of both the molecular velocity and the rotational angular momentum. From studies of the dependence of field effects on the orientation of the field with respect to the gradient, the tensorial factors of polarizations produced by various macroscopic thermodynamic forces have been determined unambiguously. On the scalar structure, however, no information can be obtained from these experiments separately. An analogous situation exists with respect to the importance of higher order Sonine polynomials in the molecular velocity when one considers transport phenomena of noble gases. Measurement of one transport property by itself does not allow any conclusions to be drawn in this regard. One can, however, resort in this case to the verification of specific relations involving the Eucken factor. In a similar way internal consistency checks and a comparison of field effects with optical measurements might lead to more detailed information about the structure of polarization in polyatomic gases. So far such comparisons have not or only partially been carried out. In this thesis two experiments are described which enable some decisive consistency checks to be performed. In chapter II experiments on the influence of external electric and magnetic fields on the viscosity of some polar gases are described. Next, in chapter III, experiments on the influence of a magnetic field on diffusion in N2-noble gases are presented. These experiments–apart from yielding new numerical results–show very clearly that the structure of polarizations is much more complicated than was usually assumed so far, and that it is not possible to get detailed quantitative information on the scalar factor from experiments on field effects alone. For this reason a detailed analysis and comparison between the results of field effects and the results of optical measurements has been carried out. As we will see in chapter IV this comparison indeed clarifies the structure of polarizations. In order to present the results in a uniform and unambiguous way the kinetic theory of rotation molecules, which for this purpose has been modified and extended, is formulated at the beginning of this thesis in chapter I.

Collisional broadening of rotational Raman lines has been investigated for the gases nH2, nD2 and HD between 20 and 300 K. For H2 and D2 the dependence of the linewidth on ortho-para composition has been investigated. For a number of systems the pressure shift of the Raman lines has also been determined. The experimental results are compared with theory.

Experiments on the influence of a magnetic field on the thermal diffusion (Dt) have been performed. Both the transverse coefficient, (D-tr/T), as well as the difference between the longitudinal coefficients, (D-// T), were measured for binary mixtures of N2, nD2, HD and nH2 with the noble gases Ar, Ne and He and for the system pH2-Ar. For most of these systems, the results can be adequately described with the dominant angular momentum polarization of the form WJJ. For some mixtures, however, a significant contribution from a second polarization (viz. WJ) was found to be present. The results are expressed in terms of effective molecular cross sections. Using these results and those earlier obtained for the magnetic field effect on the thermal conductivity, an estimate is made about the magnitude of the Senftleben-Beenakker effect on the diffusion.

Experiments have been performed to study the influence of a magnetic field on the Dufour effect (or diffusion-thermo effect) in an equimolar N2-Ar mixture at room temperature. By comparing these experimental results with those obtained previously for the field effect on the thermal diffusion, an Onsager relation between these field effects is confirmed.

The influence of an external field on the transport phenomena in polyatomic gases leads to detailed information about the kinetic theory of such gases. In particular, a magnetic field gives rise to transverse transport, perpendicular to both the macroscopic gradient and the field. The present experimental results, along with other recent results from our laboratory, comprise a complete set of data on such transverse effects occurring in heat conducting and diffusing gas mixtures, viz., simple heat conduction and diffusion and also their cross effects, thermal diffusion and the diffusion-thermo effect. Special emphasis is given to the phenomenology of these effects and to experimental techniques.

Experimental data are reported on the influence of a magnetic field on the Dufour-effect, the reciprocal phenomenon of thermal diffusion, in an equimolar N2-Ar mixture at room temperature. An Onsager relation in the presence of a magnetic field is confirmed.