1. Terahertz spectroscopy of the ground state of methylamine (CH3NH2)
Shanshan Yu and Brian Drouin

2. Introduction to CH3NH2 Torsional motion of CH3 Barrier heights
536 cm-1
Wagging motion of NH2
1366 cm-1
Woon, OSU 2007 (RH08)

3. Previous work on the ground state of CH3NH2
Hershberger and Turkevich (1947): first microwave spectrum
The Shimoda, Nishikawa and Itoh group (1954, 1956, 1957)
The Lide group (1952, 1953, 1954, 1957)
Ohashi et al (1987): far-infrared transitions
Kreglewski and Wlodarczak (1992)
Kreglewski et al (1992)
Ilyushin et al (2005)
Ilyushin and Lovas (2007)
~700 lines in GHz with uncertainties of 20–500 kHz
~1200 lines in cm-1with uncertainties of –0.001 cm-1

4. Our interest in CH3NH2
CH3NH2 exists in the simulated atmosphere of Titan (Drouin, 2006)
CH3NH2 observed in the interstellar medium (Fourikis et al. 1974; Kaifu et al. 1974)
CH3NH2: G12 symmetry
Isoelectronic to CH3OH2+
Comprehensive experimental data available

5. Frequency Multiplier Submillimeter Spectrometer (FMSS)
Experimental setup
30 mTorr CH3NH2
Pump
Sample cell
Si detector
Multiplier chain … ×6 ×2 ×3 FM PC
Rf Synthesizer
Lock-in
Reference
Frequency Multiplier Submillimeter Spectrometer (FMSS)

6. Observed CH3NH2 spectrum
GHz
GHz
GHz
GHz
GHz

7. Energy levels of CH3NH2: G12 group
Ohashi and Hougen, 1987 
J, K, ′
Near-prolate asymmetric top
J, K = Ka
SPFIT B2 B1
Selections rules:
A1 A2; B1 B2
E1 E1; E2 E2 ( ; ; ) A2 A1

8. Matrix elements for CH3NH2 (I)
Ohashi et al, 1987

9. Matrix elements for CH3NH2 (II)
Ohashi et al, 1987

10. Matrix elements for CH3NH2 (III)
DK = ±2 term
Ohashi et al, 1987

11. Matrix elements for CH3NH2 (IV)
DK = ±1 term
Ohashi et al, 1989

12. SPFIT operators SPFIT operators = a +  () Regular operators
1: ×cosa
11: ×sina
Regular operators
E E+037 h2v A1
E A2
E B1
E B2
E E1+1
E E1-1
E E2+1
E E2-1
A1 A B1 B E E E E2-1
cost cosa cosa –cosa –cosa cosacos(p/3)+sinasin(p/3) cosacos(2p/3)+sinasin(2p/3)
= a +  ()

13. Fitting results for CH3NH2
A data set of 2800 transitions was constructed (~900 lines from the present study))
60 parameters fitted to 2500 transitions (300 lines rejected)
Reduced RMS = 1.5
Microwave RMS = MHz
IR RMS = cm-1
Rejected lines are all from DK = ±1 transitions with K′ or K″=1
Observed-calculated values range from 0.5 – 1 MHz

14. Observed vs. calculated CH3NH2 spectrum
J′=24, K′= J″=23, K″=5
Nuclear spin weights:
A1:A2:B1:B2:E1+1:E1-1:E2+1:E2-1
4 : 4 :12 :12: : 6 : : 2

15. Acknowledgements Herbert Pickett NASA Postdoctoral Program (ORAU)
NASA Astrophysics Research and Analysis Program (APRA)
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California
National Aeronautics and
Space Administration