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Terahertz spectroscopy of the ground state of methylamine (CH3NH2)
Shanshan Yu and Brian Drouin
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Introduction to CH3NH2 Torsional motion of CH3 Barrier heights
536 cm-1 Wagging motion of NH2 1366 cm-1 Woon, OSU 2007 (RH08)
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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
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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
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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)
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Observed CH3NH2 spectrum
GHz GHz GHz GHz GHz
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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
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Matrix elements for CH3NH2 (I)
Ohashi et al, 1987
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Matrix elements for CH3NH2 (II)
Ohashi et al, 1987
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Matrix elements for CH3NH2 (III)
DK = ±2 term Ohashi et al, 1987
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Matrix elements for CH3NH2 (IV)
DK = ±1 term Ohashi et al, 1989
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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 + ()
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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
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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
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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
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