Theoretical studies of rotational relaxation in para‐hydrogen are presented. By using a set of theoretically deduced state‐to‐state rotational rate constants the master kinetic equations and the relevant fluid mechanical equations are solved numerically. Calculations are performed over the range 100⩽T⩽1100 °K to simulate conditions for free jet expansion, shock tube, and sound absorption experiments. The over‐all rotational energy relaxation rates extracted from these calculations are compared with available experimental data and are shown to be in good qualitative agreement. The magnitudes and the temperature dependence of these rates depend critically on the degree and direction of the initial departure from equilibrium as well as the multilevel nature of the relaxation process. The apparent discrepancies in the measured rates from the different experiments are shown to be qualitatively self‐consistent in light of the present calculations.

Rotational energy relaxation in molecular hydrogen

Values for the depolarization ratio ( ϱ T V ) of Rayleigh scattered light are reported for a large number of gas molecules. When possible, the results are combined with refractive index data to yield the anisotropy in the optical polarizability.

Measurements of depolarization ratios and polarizability anisotropies of gaseous molecules

Experimental capabilities of molecular spectroscopy and its applications nowadays require a sub-percent or even sub-per mille accuracy of the representation of the shapes of molecular transitions. This implies the necessity of using more advanced line-shape models which are characterized by many more parameters than a simple Voigt profile. It is a great challenge for modern molecular spectral databases to store and maintain the extended set of line-shape parameters as well as their temperature dependences. It is even more challenging to reliably retrieve these parameters from experimental spectra over a large range of pressures and temperatures. In this paper we address this problem starting from the case of the H2 molecule for which the non-Voigt line-shape effects are exceptionally pronounced. For this purpose we reanalyzed the experimental data reported in the literature. In particular, we performed detailed line-shape analysis of high-quality spectra obtained with cavity-enhanced techniques. We also report the first high-quality cavity-enhanced measurement of the H2 fundamental vibrational mode. We develop a correction to the Hartmann–Tran profile (HTP) which adjusts the HTP to the particular model of the velocity-changing collisions. This allows the measured spectra to be better represented over a wide range of pressures. The problem of storing the HTP parameters in the HITRAN database together with their temperature dependences is also discussed.

The implementation of non-Voigt line profiles in the HITRAN database: H2 case study

We have measured state‐resolved rotational energy transfer rates of H2 (v=1) in collisions with H2 (v=0) at 295 K, using optical pump/probe techniques. A single rotational level was populated using stimulated Raman pumping, and the degree of collision‐induced population transfer was monitored using either coherent anti‐Stokes Raman spectroscopy (CARS) or resonance‐enhanced multiphoton ionization spectroscopy (REMPI). The CARS and REMPI results are in good agreement; averages of the two sets of rates are 180±27×106 s−1 amagat−1 for j=0→2 and 50±8×106 s−1 amagat−1 for j=1→3. We also observed transfer of vibrational excitation from paraspin v=1 states to ortho v=0 states due to v–v exchange, measuring a rate of 1.9±0.8×106 s−1 amagat−1 for this process.

Raman pump/probe measurements of state‐to‐state energy transfer rates in the v=1 state of H2

A unified classification of binary collision cross sections is presented. This is accomplished with the aid of a collision superoperator, in terms of which both differential and kinetic theory cross sections are defined. Connections and comparisons are then made with standard expressions for chemical reaction cross sections and with those ’’spectral’’ cross sections describing collisional widths, shifts and overlapping of spectral lines. A detailed treatment of rotational invariance is included. Finally, it is shown how this general framework encompasses the more complicated cross sections that arise in the study of the field dependence of gas transport coefficients.

Definitions and properties of generalized collision cross sections

The pressure broadening of the depolarized Rayleigh line in pure gases of linear molecules

The pressure broadening of the rotational Raman lines of hydrogen isotopes

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.

H.F.P. Knaap

Papers

The pressure broadening of the depolarized Rayleigh line in pure gases of linear molecules

The pressure broadening of the rotational Raman lines of hydrogen isotopes

The broadening and shift of the ratational Raman lines for hydrogen isotopes at low temperatures

The pressure broadening of the depolarized rayleigh line in N2-noble-gas mixtures

Lineshape of the depolarized Rayleigh line in gases of linear molecules