The ground state rotational spectrum of methanol Rogier

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

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, WI 08) 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: a. R 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 – Vt 1 K=5

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 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

Summary • Extensive line list for methanol GS compiled, 3800 total • J extended to J ~ 40 in a. R, 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