The ground state rotational spectrum of methanol

Rogier BraakmanChemistry & Chemical EngineeringCalifornia Institute of Technology

John C. PearsonBrian J. Drouin

Jet Propulsion LaboratoryCalifornia Institute of Technology

Geoffrey A. Blake

Geological & Planetary ScienceCalifornia 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 HIFI

So many lines..!

AM spectrum at 900 GHz

Optically thick!!

Completing theory picturePrevious:

• 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

0

500

1000

1500

2000

2500

3000

0 2 4 6 8 10 12 14 16

K

GH

z

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 fittingsMethod:• 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=5A levels relative to K=0, vt=1

-4

-4

-4

-3

-3

-3

-3

-3

-2

-2

-2

0 5 10 15 20 25 30 35

J

En

erg

y (

cm

^-1

)

Energy level interactionsLevel 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

KK+1

J

J+5

J+1

J+2

J+3

J+4

J+1

J+2

J+3

J+4

K-K+ (K+1)-(K+1)+

J

J+1 J+1

Asymmetry splitting (A state)

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

0

500

1000

1500

2000

2500

3000

0 2 4 6 8 10 12 14 16

K

GH

z

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