Transcript NH3-Columbus2011.ppt

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
Sunspots (3200 K)
(e.g., H2O, TiO)
Earth (296 K)
The Sun (5800 K)
(e.g., CN, OH, CH, NH)
Royal Swedish Academy of Science
NASA
SOHO/EIT Consortium
Exoplanets
H2O, NH3,
CH4
Dwarf
Stars
Brown
Dwarfs
0
1000
2000
Stars
3000
4000
5000
6000
7000
8000
Temperature / K
HITRAN database
at 296 K
Polyatomic Molecules
H+
Diatomic Molecules
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 (<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)
FT-IR
spectrometer
under vacuum
KBr beam
splitter
Mirror
Scanning
mirror
MCT
detector
Alumina (Al2O3)
tube maintained at
1 Torr of pressure
CaF2 lens
NH3 out
to pump
NH3
in
CaF2
windows
Water
cooling
coils
Controllable tube
furnace capable of
maintaining
temperatures of up
to 1370 °C.
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.
Results for ν4 band
1370 °CC1/
1000
1100
1300
1200
400
600
300 °CC1/
500
700
800
900
Edge of ν2
umbrella mode
centred around
1000 cm-1
Line width ≈ 0.02 cm-1
ν4 Q-Branch
ν4 P-Branch
ν4 R-Branch
Astronomical Requirements

An ammonia line list that can be used to simulate
astronomical spectra for T=500-2000 K

From Beer-Lambert law:
I  I 0 exp  S g   10 Nl 
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):
2π 2ν10 S J J 
 Elow 
 hν10 
S 
exp  
1  exp  

3ε0 hcQT
 kT 
 kT 


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.
Empirical Lower State Energies

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 is
147 cm-1 (3.5%)
based on a comparison
with HITRAN
HITRAN
ν2 (a1)
Notice the
weak ΔK=±3
and ±6 lines
computed for
ν2 in HITRAN
ν4 (e)
Lower
State
Energies
Empirical Lower
State Energies
Strong
HITRAN
Lines
Added
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 (Barber-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 Overview
740-2200 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