IR EMISSION SPECTROSCOPY OF AMMONIA: LINELISTS AND ASSIGNMENTS. R. Hargreaves, P. F. Bernath Department of Chemistry, University of York, UK N. F. Zobov, S. V. Shirin, R. I. Ovsyannikov, O. L. Polyansky Russian Academy of Sciences, Nizhny Novogorod, Russia S. N. Yurchenko, R. J. Barber, J. Tennyson Department of Physics and Astronomy, University College London, UK

Overview

Astronomical Temperatures The Sun (5800 K) (e.g., CN, OH, CH, NH) Sunspots (3200 K) (e.g., H 2 O, TiO) Royal Swedish Academy of Science Earth (296 K) Brown Dwarfs Dwarf Stars Stars Exoplanets H+H+ Diatomic Molecules Polyatomic Molecules Temperature / K HITRAN database at 296 K NASASOHO/EIT Consortium H 2 O, NH 3, CH 4

Brown Dwarfs  Brown dwarfs fall into the L and T-classes and mainly emit radiation in the infrared.  They are not classed as stars because hydrogen fusion does not take place ( 13 M J ).  Their atmospheres are much cooler than stars (typically 700 – 2,500 K) and rich in molecules.  The L-class is mainly determined by FeH and CrH absorption, while the T-class has strong CH 4 and H 2 O absorption.  The proposed Y-class dwarfs are <700 K and are yet to be identified. It is predicted that NH 3 will become a major absorption feature.

Hot Ammonia: proposed ‘Y’ dwarfs G, K, M, L, T & now Y-dwarfs? Cushing et al., ApJ 648, 614 (2006) see NH 3 absorption strengthen for ν 2 mode at 10.6 microns using Spitzer Space Telescope. New Y class of sub-brown dwarfs with T<700 K is predicted to be characterized by strong NH 3 bands. NH 3 ν 2 umbrella mode

Experimental Setup (Emission) FT-IR spectrometer under vacuum KBr beam splitter Mirror Scanning mirror MCT detector CaF 2 windows Controllable tube furnace capable of maintaining temperatures of up to 1370 °C. NH 3 in NH 3 out to pump Alumina (Al 2 O 3 ) tube maintained at 1 Torr of pressure Water cooling coils CaF 2 lens

Experiments: Experiments: R. J. Hargreaves, G. Li and P. F. Bernath, Hot NH 3 for Astrophysical Applications, Astrophys. J. (in press)  This arrangement is used to record high resolution (0.01 cm -1 ), hot emission spectra of NH 3 for the temperatures 300, 400, 500, … 1200, 1300 and 1370°C.  The first range studied contained the ν 4 bending mode between 1100 – 2200 cm -1 ( μm).  The next range studied contained both ν 2 and ν 4 bending modes between 750 – 1500 cm -1 ( μm).  Spectra also recorded for ν 1 and ν 3 stretching modes in cm -1 region (5.5 – 2.5 μm), but not analyzed yet.

300 °CC1/ Edge of ν 2 umbrella mode centred around 1000 cm -1 ν 4 P-Branch ν 4 R-Branch ν 4 Q-Branch Results for ν 4 band 400 °CC1/500 °CC1/600 °CC1/700 °CC1/800 °CC1/900 °CC1/1000 °CC1/1100 °CC1/1200 °CC1/1300 °CC1/ 1370 °CC1/ Line width ≈ 0.02 cm -1

Astronomical Requirements  An ammonia line list that can be used to simulate astronomical spectra for T= K  From Beer-Lambert law: Need a lineshape function g( ν-ν 10 ) (assumed to be Voigt) and a line strength S ’ given by (SI units, from Bernath, Spectra of Atoms and Molecules):  Therefore need a line position, ν 10, partition function, Q T (easily calculated), line intensity, S J′J″ (or S′), and the lower state energy, E low.  Calibrate line positions and line intensities of observed spectra using HITRAN.

Empirical Lower State Energies  From the line strength equation and taking the ratio for two temperatures we get:  Rearranging to give: in which T 0 = 573 K is a reference temperature Average error in E low is 147 cm -1 (3.5%) based on a comparison with HITRAN

Lower State Energies Notice the weak ΔK=±3 and ±6 lines computed for ν 2 in HITRAN ν 2 (a 1 ) ν 4 (e) HITRAN Empirical Lower State Energies

Strong HITRAN Lines Added

Assignments: Assignments: N.F. Zobov, S.V.Shirin, R.I. Ovsyannikov, O.L. Polyansky, S.N. Yurchenko, R.J. Barber, J. Tennyson, R. Hargreaves and P.F. Bernath, Analysis of high temperature ammonia spectra from 780 to 2100 cm -1, J. Mol. Spectrosc. (in press) Problems with ammonia spectra: 1. Perturbations as vibrational density of states increases. 2. Inversion motion causes complications. 3. Light molecule so rotational energy level expression based on perturbation theory [BJ(J+1)+(C-B)K ] converges slowly. Solution: Variational calculation of energy levels and wavefunctions of the vibration-rotation-inversion Hamiltonian with a high quality potential energy surface. Transitions and intensities are calculated with the help of an ab initio dipole moment surface. (Accounts for perturbations automatically.)

Ammonia Calculations TROVE program: S.N. Yurchenko, W. Thiel, P. Jensen, J. Mol. Spectrosc. 245, 126 (2007) NH spectroscopically-determined potential energy surface: S. N. Yurchenko, R. J. Barber, J. Tennyson, W. Thiel, and P. Jensen, J. Mol. Spectrosc. (in press) Ab initio dipole moment surface: S.N. Yurchenko, R.J. Barber, A. Yachmenev, W. Thiel, P. Jensen, and J. Tennyson, J. Phys. Chem. A 113,11845 (2009) BYTe 1500 K hot ammonia line list: S.N. Yurchenko, R.J. Barber and J. Tennyson, Mon. Not. R. Astron. Soc. 413, 1828 (2011)

BYTe (Barber- BYTe (Barber-Yurchenko-Tennyson) BYTe is designed for temperatures up to1500 K (1.1 billion) transitions from 0 to cm −1 Based on (1.4 million) energy levels below cm −1 with J≤36.

Assignment Overview cm -1 region, 1300°C spectrum with lines

Assignment Summary Focussed entirely on cold bands with origins in observed region: 2 ν 2 and ν 4 (No hot bands yet).

Future Work Assign the next region, cm -1, for cold bands and hot bands. Data in hand but not reduced for cm -1 region Hot methane: spectra in hand for cm -1 region