The ground state rotational spectrum of methanol Rogier Braakman Chemistry & Chemical Engineering California Institute of Technology John C. Pearson Brian J. Drouin Jet Propulsion Laboratory California Institute of Technology Geoffrey A. Blake 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..!

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 cm -1, basis for most high K, J assignments Tsunekawa et al. (1995): Copendium of complete spectra 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, 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

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 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 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

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 groupNASA & NSF