Alignment of molecules in gaseous transport: Alkali dimers in supersonic nozzle beams

From measurements of the degree of polarization of the molecular fluorescence, it is shown that one can determine the coefficients of the first three even‐ordered Legendre polynomials in the expansion of the spatial distribution of J vectors (M_J population), n(θ)=1 + a₂P₂(cosθ) + a₄P₄(cosθ), where θ is the angle between the angular momentum vector J of the molecule and the beam direction. This method is applied to determine the alignment of Na₂ molecules in a supersonic nozzle beam generated by expansion of sodium vapor through a converging nozzle (throat diameter = 0.375 mm and channel length = 0.750 mm) with stagnation temperature (oven temperature) up to ∼1015°K, corresponding to stagnation pressures (oven pressures) up to ∼155 torr. The 4880 Å line of a cw argon‐ion laser is used to excite the transition (ν″=3, J″=43 → ν′=6, J′=43) in the ${\mathrm{B\hspace{0.17em}}}^1{\Pi }_u{\mathrm{-X\hspace{0.17em}}}^1{\Sigma }_g^{+}$ band system of Na₂ molecules and the degree of polarization of a single line of the resulting fluorescence series (Q series) is measured as a function of (1) stagnation pressure p₀ and (2) the angle θ₀ between the electric vector of the exciting laser light and the molecular beam direction. A decrease in the degree of polarization from its isotropic value of 0.50 for θ₀=0 to a value of 0.44 at a stagnation pressure of 155 torr is found, showing increasing alignment of dimer molecules with their angular momenta preferentially pointing perpendicular to the beam direction. At the stagnation pressure of 155 torr the coefficients of the Legendre polynomials are found to be a₂=−0.203±0.006 and a₄=−0.14±0.03. For this distribution the ratio of molecules with angular momentum J parallel and perpendicular to the beam axis is ∼2:3. Classical models involving the nonreactive collisions between hard spheres (atoms) and ellipsoids of revolution (molecules) and the chemical exchange between atoms and dimers predict the production of alignment in qualitative agreement with experiment.