FT-IR measurements of NH3 line intensities in the 60 – 550 CM−1 using Soleil/Ailes beamline

Abstract

Ammonia (NH$_3$) has been found ubiquitous, $it{e.g.}$, in the interstellar medium, low-mass stars, Jovian planets of our solar system, and possibly in the low temperature exoplanets. Their spectroscopic line parameters are essential in the accurate interpretation of the planetary and astrophysical spectra observed with Herschel, SOFIA, ALMA, and JWST. In our previous paperfootnote{S. Yu, et al. J. Chem. Phys. (2010) 174317/1-174317/14.}, the NH$_3$ line positions in the far-IR region were studied for the ground state and $nu_2$ in an unprecedented accuracy, which revealed significant deficiencies in the NH$_3$ intensities, for instance, some weak $Delta$K = 3 lines were predicted to be ~100 times stronger. Measurement of line intensity for these lines in a consistent manner is demanded because the $Delta$K = 3 forbidden lines are only way other than collisions and $l$-doubled states to excite NH$_3$ to $K > 0$ levels. Recalling that NH$_3$ transition lines in the high $J$ and $K$ up to 18 were detected toward the galactic center in the star forming region of Sgr B$_2$, their accurate intensity measurements are critical in explaining the observed high $K$ excitation, which will provide insights into radiative-transfer $it{vs.}$ collision excitation mechanics of interstellar NH$_3$. For this, we obtained a series of spectra of $^{14}$NH$_3$ in the 50 � 550 cm$^{-1}$ using a Fourier-transform spectrometer, Bruker 125HR, and AILES beam line at Synchrotron SOLEIL, France. Line positions, intensities, and pressure-broadened half-widths have been measured using non-linear least squares spectrum fitting algorithm. In this presentation we report and discuss preliminary results of line position and intensity measurements for the inversion transitions in the ground state, $nu_2$, 2$nu_2$, $nu_4$ and for the vibration-rotation transitions of $nu_2$, 2$nu_2$, $nu_4$, 2$nu_2-nu_2$, $nu_4-nu_2$ and $nu_4-2nu_2$ in this region. Comparison of the new measurements with the current databases and {it ab initio} calculations will be discussed