Rotational spectra and van der Waals potentials of Ne-Ar

Abstract

The high sensitivity and resolution of Fourier-transform microwave spectroscopy using a pulsed jet coaxial to a Fabry-Perot resonator have been exploited to measure pure rotational transitions of several isotopomers of the weakly polar Ne-Ar van der Waals dimer in natural abundance. Transitions of the most abundant isotopomer, ²⁰Ne-⁴⁰Ar, could be observed with an excellent signal-to-noise ratio with a single polarization pulse. The ground-state rotational constants for this species yield a zero-point separation of R₀≃360.7 pm. Simple model van der Waals potentials have been fit to the microwave transitions for the various isotopomers, providing estimates of the equilibrium spacing at the well minimum of R_e=348.0(2) pm. More elaborate potentials based on ab initio calculations or on molecular-beam scattering cross sections and thermodynamic and transport properties have also been tested. The induced electric dipole moment is estimated to be μ₀=7.3(1.6)×10⁻³³ C m [0.0022(5) D] by comparison of π/2 polarization pulses with a reference molecule (Ar-CO₂) whose dipole moment is known from Stark effect splitting measurements. Uncertainties in parentheses are one standard deviation.