Nuclear spin symmetry conservation studied by cavity ring-down spectroscopy of ammonia in a seeded supersonic jet from a pulsed slit nozzle

Abstract

We report high-resolution cw laser spectra of the symmetric stretching fundamental in ammonia NH measured with a cavity ring-down setup for mid-IR transitions (around ). In a supersonic jet expansion ( seeded in Ar), we have obtained rotational temperatures below . For the gas expansion, a pulsed slit nozzle with length and 70 µm 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 about . 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 around .