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