IR EMISSION SPECTROSCOPY OF AMMONIA: LINELISTS AND ASSIGNMENTS.
R. Hargreaves, P. F. Bernath 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. TennysonDepartment of Physics and Astronomy, University College London, UK
OverviewOverview
Astronomical TemperaturesAstronomical Temperatures
0 1000 2000 3000 4000 5000 6000 7000 8000
The Sun (5800 K) (e.g., CN, OH, CH, NH)
Sunspots (3200 K) (e.g., H2O, TiO)
Royal Swedish Academy of Science
Earth (296 K)
Brown Dwarfs
Dwarf Stars
Stars
Exoplanets
H+
Diatomic MoleculesPolyatomic Molecules
Temperature / KHITRAN database at 296 K
NASA SOHO/EIT Consortium
H2O, NH3, CH4
Brown DwarfsBrown 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 (<79 MJ, mass of Jupiter). Nor are they planets because they are many times larger than the gas giants (>13 MJ).
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 CH4 and H2O absorption.
The proposed Y-class dwarfs are <700 K and are yet to be identified. It is predicted that NH3 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 NH3 absorption strengthen for ν2 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 NH3 bands.
NH3 ν2 umbrella mode
Experimental Setup (Emission)Experimental Setup (Emission)
FT-IR spectrometer under vacuum
KBr beam splitter
Mirror
Scanning mirror
MCT detector
CaF2 windows
Controllable tube furnace capable of maintaining temperatures of up to 1370 °C.
NH3 in
NH3 out to pump
Alumina (Al2O3) tube maintained at
1 Torr of pressure
Water cooling coils
CaF2 lens
Experiments: Experiments: R. J. Hargreaves, G. Li and P. F. Bernath, Hot NH3 for Astrophysical Applications, Astrophys. J. (in press) This arrangement is used to record high resolution (0.01
cm-1), hot emission spectra of NH3 for the temperatures 300, 400, 500, … 1200, 1300 and 1370°C.
The first range studied contained the ν4 bending mode between 1100 – 2200 cm-1 (9.0 - 4.5 μm).
The next range studied contained both ν2 and ν4 bending modes between 750 – 1500 cm-1 (13 - 6.7 μm).
Spectra also recorded for ν1 and ν3 stretching modes in 1800-4000 cm-1 region (5.5 – 2.5 μm), but not analyzed yet.
300 °CC1/
Edge of ν2 umbrella mode centred around
1000 cm-1
ν4 P-Branch
ν4 R-Branch
ν4 Q-Branch
Results forResults for νν44 band 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 RequirementsAstronomical Requirements An ammonia line list that can be used to simulate
astronomical spectra for T=500-2000 K
From Beer-Lambert law:
Need a lineshape function g(ν-ν10) (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, ν10, partition function, QT (easily calculated), line intensity, SJ′J″ (or S′), and the lower state energy, Elow.
Calibrate line positions and line intensities of observed spectra using HITRAN.
NlgSII 100 exp
kT
hν
kT
E
hcQε
SνπS low
T
JJ 10
0
102
exp1exp3
2
Empirical Lower State Empirical Lower State EnergiesEnergies From the line strength equation and taking the ratio for two
temperatures we get:
Rearranging to give:
in which T0 = 573 K is a reference temperature
Average error in Elow is147 cm-1 (3.5%) based on a comparisonwith HITRAN
Lower Lower State State EnergiesEnergies
Notice the weak ΔK=±3 and ±6 lines computed for ν2 in HITRAN
ν2 (a1)
ν4 (e)HITRAN
Empirical Lower State Energies
StrongStrongHITRANHITRANLinesLinesAddedAdded
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)K2 + ...] 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)•NH3-2010 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 (BYTe (BBarber-arber-Yurchenko-Tennyson) BYTe is designed for temperatures up to1500 K 1138 323 351 (1.1 billion) transitions from 0 to 12000 cm−1
Based on 1373 897 (1.4 million) energy levels below 18000 cm−1 with J≤36.
Assignment Overview740-2200 cm-1 region, 1300°C spectrum with 18370 lines
Assignment Summary
Focussed entirely on cold bands with origins in observed region: 2ν2 and ν4 (No hot bands yet).
Future Work
•Assign the next region, 2200-4000 cm-1, for cold bands and hot bands.
•Data in hand but not reduced for 4000-6000 cm-1 region
•Hot methane: spectra in hand for 800-4000 cm-1 region
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