Nuclear Spin Symmetry Conservation Studied by Cavity Ring-Down Spectroscopy of Ammonia in a Seeded Supersonic Jet from a Pulsed Slit Nozzle

We report high-resolution cw laser spectra of the symmetric stretching fundamental in ammonia14NH3measured with a cavity ring-down setup for mid-IR transitions (around3300cm-1). In a supersonic jet expansion (0.0001 <= x(NH3)<= 0.04seeded in Ar), we have obtained rotational temperatures below7K. For the gas expansion, a pulsed slit nozzle with40mmlength and 70 mu m width has been used. The measured relative line intensities demonstrate that the A and E nuclear spin symmetry isomers do not interconvert in the supersonic jet expansion under these conditions (nuclear spin symmetry conservation). The lateral velocity of the gas expansion has been probed by a Doppler profile measurement, showing a preference for an expansion with an angular distribution following Lambert's cosine law perpendicular to the slit direction and an approximate molecular velocity of about470m/s. The experimental setup is described in detail, also in view of high-resolution spectroscopy of chiral molecules as candidates to measure the parity violating energy difference between enantiomers with a barrier to interconversion around2500cm-1.