Submillimeter-wave and far-infrared spectroscopy of high-J transitions of the ground and ν2=1 states of ammonia

Abstract

Complete and reliable knowledge of the ammonia spectrum is needed to enable the analysis and interpretation of astrophysical and planetary observations. Ammonia has been observed in the interstellar medium up to J=18 and more highly excited transitions are expected to appear in hot exoplanets and brown dwarfs. As a result, there is considerable interest in observing and assigning the high J (rovibrational) spectrum. In this work, numerous spectroscopic techniques were employed to study its high J transitions in the ground and ν2=1 states. Measurements were carried out using a frequency multiplied submillimeter spectrometer at Jet Propulsion Laboratory (JPL), a tunable far-infrared spectrometer at University of Toyama, and a high-resolution Bruker IFS 125 Fourier transform spectrometer (FTS) at Synchrotron SOLEIL. Highly excited ammonia was created with a radiofrequency discharge and a dc discharge, which allowed assignments of transitions with J up to 35. One hundred and seventy seven ground state and ν2=1 inversion transitions were observed with microwave accuracy in the 0.3–4.7 THz region. Of these, 125 were observed for the first time, including 26 ΔK=3 transitions. Over 2000 far-infrared transitions were assigned to the ground state and ν2=1 inversion bands as well as the ν2 fundamental band. Of these, 1912 were assigned using the FTS data for the first time, including 222 ΔK=3 transitions. The accuracy of these measurements has been estimated to be 0.0003–0.0006 cm−1. A reduced root mean square error of 0.9 was obtained for a global fit of the ground and ν2=1 states, which includes the lines assigned in this work and all previously available microwave, terahertz, far-infrared, and mid-infrared data. The new measurements and predictions reported here will support the analyses of astronomical observations by high-resolution spectroscopy telescopes such as Herschel, SOFIA, and ALMA. The comprehensive experimental rovibrational energy levels reported here will permit further refinement of the potential energy surface to improve ammonia ab initio calculations and facilitate assignment of new high-resolution spectra of hot ammonia.

HOT NH3SPECTRA FOR ASTROPHYSICAL APPLICATIONS

“…Many studies are of cold NH 3 and are useful when studying planetary atmospheres in our solar system (Brown & Margolis 1996). Some work has been done in determining empirical lower state energies (Brown & Margolis 1996) as well as on the rotation-inversion spectra (Sasada et al 1992;Yu et al 2010). Recently there has been considerable progress in the variational calculation of vibration-rotation energy levels and transitions from adjusted ab initio potential energy surfaces (Huang et al 2008(Huang et al , 2011aYurchenko et al 2009Yurchenko et al , 2011.…”

Circumstellar ammonia in oxygen-rich evolved stars

“…Yu et al 2010). These vibrational states are connected to the ground state (0 and 0.79 cm −1 ) by MIR photons of wavelengths between 6.1-6.3 µm.…”

“…(1) Kukolich (1967); (2) Kukolich & Wofsy (1970); (3) Poynter & Kakar (1975); (4) Belov et al (1998); (5) Cazzoli et al (2009); (6) Winnewisser et al (1996); (7) Yu et al (2010).…”

“…(a) The NH 3 experimental line frequencies and upper-level energies are retrieved from the calculations and compilations by Yu et al (2010). Code for original references are given in the last column.…”

High resolution far infrared laboratory spectroscopy of transient species: application to the SO radical (X3Σ)

“…Spectra of molecules and radicals excited at high rotational and vibrational temperatures (up to 2000 K) can be recorded using this technique allowing the observation of pure rotational and ro-vibrational transitions within the ground and vibrationally excited states. The experimental datasets on well-known interstellar molecules (also expected to be present in newly discovered exoplanets) H 2 O (Coudert et al 2004;Yu et al 2012), HCN, NH 3 (Yu et al 2010) and radicals OH and NH 2 have thus been completed to excited energy levels.…”