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 ${}^{14}$NH${}_3$ measured with a cavity ring-down setup for mid-IR transitions (around $3300{\mathrm{c}\mathrm{m}}^{-1}$). In a supersonic jet expansion ($0.0001\le x\left({\mathrm{N}\mathrm{H}}_3\right)\le 0.04$ seeded in Ar), we have obtained rotational temperatures below $7\mathrm{K}$. For the gas expansion, a pulsed slit nozzle with $40\mathrm{m}\mathrm{m}$ 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 $470\mathrm{m}/\mathrm{s}$. 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 around $2500{\mathrm{c}\mathrm{m}}^{-1}$.

GRAPHICAL ABSTRACT

Keywords:

• Nuclear spin symmetry
• supersonic jet expansion
• cavity ring-down absorption spectroscopy
• NH${}_3$
• mid-IR high-resolution laser spectroscopy

Acknowledgments

Our work is supported financially by an ERC Advanced Grant and by an ETH Zurich grant. We also enjoyed discussions with and help and support from Ziqiu Chen, Csaba Fábri, Karen Keppler, Carine Manca Tanner, Roberto Marquardt, Andreas Schneider, Jürgen Stohner, and Daniel Zindel. Special thanks go to Frédéric Merkt for continuous support and to Andres Laso for his important contribution to the construction of the slit nozzle and other aspects of the experimental setup. We dedicate this publication to Richard N. Zare on the occasion of his 80th birthday.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The initial part of this work was supported by funding from the European Research Council under the European Union's Seventh Frame Work Programme (FP7) 2007-5302013 Grant 20925.