Transcript Methanol_OSU.ppt

The ground state rotational
spectrum of methanol
Rogier Braakman
Chemistry & Chemical Engineering
California Institute of Technology
John C. Pearson
Brian J. Drouin
Geoffrey A. Blake
Jet Propulsion Laboratory
California Institute of Technology
Geological & Planetary Science
California Institute of Technology
Outline
• Motivation 1: Astronomy – Herschel
• Motivation 2: Complete theory picture
• History of methanol spectrum
• Data set:
– New assignments
– Analysis of global line list
Astronomy - Herschel
• Methanol one of most abundant ‘hot core’ molecules
• Coupled with an intrinsically strong and dense spectrum
gives strong lines in almost any ‘hot core’ spectrum
• With Herschel unprecedented access to vast regions of
interstellar THz radiation
• Critical to characterize target molecules & known
molecules!
• Methanol obvious place to start
Simulated spectrum 0 – 2 THz
HIFI
So many lines..!
HIFI
AM spectrum at 900 GHz
Optically thick!!
Completing theory picture
Previous:
•
Coles (1948): First high resolution MW spectra
•
1948 – 1967: Further studies of MW spectra
•
Lees & Baker (1968): First mm-wave study
•
De Lucia, Herbst, Anderson (1989,1992): Fit spectrum to microwave accuracy,
eventually extended analysis to J=27
•
Baskakov & Pashaev (1992): Assignments and analysis to J=41
•
Moruzzi et al. (1995, Methanol Atlas): FTIR survey 0-1258 cm-1, basis for most
high K, J assignments
•
Tsunekawa et al. (1995): Copendium of complete spectra 7-200 GHz
Present goal:
•
Compile global line list including new assignments and previous data
•
Extend analysis and global fit to higher K & J
K – progressions A state
3000
2500
GHz
2000
1500
1000
500
0
0
2
4
6
8
K
10
12
14
16
Data Set
Frequency coverage:
• Almost complete up to 1200 GHz
• Pieces from 1575 – 2530 GHz
• Measured on direct multiplier, flow/static cell
spectrometer at JPL (B. Drouin, WI08)  very high S/N!
Assignments:
• Over 2400 new assignments, 3800 total lines in GS
• Identified many additional b-type transitions up to K=14
• Extended branches to high J: aR to J = 39, P = 38, Q = 46
Power series fittings
Method:
• Fit A state as symmetric top molecule, E state as linear
molecule
• Treat K-stacks as vibrational states, separated by
Energy term
Result:
• A state works well until K=9, then diverges
• E state diverges at lowest K
 Perturbations!!
Level crossing K=9 – Vt1 K=5
A levels relative to K=0, vt=1
-2
0
5
10
15
20
-2
-2
Energy (cm^-1)
-3
-3
-3
-3
-3
-4
-4
-4
J
25
30
35
Energy level interactions
Level crossing:
• Both states: low-K stacks cross at large
• A state: K=9 crosses K=5 in Vt=1
• Perturbation inversely proportional to K
• Mapping transitions near crossing give
interaction constant unique for K
State mixing:
• States close in E (even if no crossing) have
some mixing resulting in extra lines
• Several such transitions identified
Loops
J+4
J+5
Asymmetry splitting (A state)
J+3
J+4
J+2
J+3
J+1
J+2
J+1
J+1
J+1
J
J
K+1
K
(K+1)+ K+
K-
(K+1)-
Loop results
~80% of lines in loop in A & E state
General: low K better than high K
b-type transitions in loops  K stacks connected
Level crossings in loops  accurate interaction
terms
Ready for global fit!
K-progressions in A state
3000
2500
GHz
2000
1500
1000
500
0
0
2
4
6
8
K
10
12
14
16
Summary
• Extensive line list for methanol GS
compiled, 3800 total
• J extended to J ~ 40 in aR, P and Q
branches
• Many high b-type branches and lines
identified up to K=14
• Several level crossings identified
Next: Global fit!
Acknowledgements
Blake group
NASA & NSF