Terahertz spectroscopy of the ground state of methylamine

Terahertz spectroscopy of the ground state of methylamine (CH 3 NH 2) Shanshan Yu and Brian Drouin

Introduction to CH 3 NH 2 Torsional motion of CH 3 Barrier heights 536 cm-1 Wagging motion of NH 2 1366 cm-1 Woon, OSU 2007 (RH 08)

Previous work on the ground state of CH 3 NH 2 q Hershberger and Turkevich (1947): first microwave spectrum q The Shimoda, Nishikawa and Itoh group (1954, 1956, 1957) q The Lide group (1952, 1953, 1954, 1957) q Ohashi et al (1987): far-infrared transitions q Kreglewski and Wlodarczak (1992) q Kreglewski et al (1992) q Ilyushin et al (2005) q Ilyushin and Lovas (2007) ü~700 lines in 3 -470 GHz with uncertainties of 20– 500 k. Hz ü~1200 lines in 6 -140 cm-1 with uncertainties of 0. 0007– 0. 001 cm-1

Our interest in CH 3 NH 2 q CH 3 NH 2 exists in the simulated atmosphere of Titan (Drouin, 2006) § q CH 3 NH 2 observed in the interstellar medium (Fourikis et al. 1974; Kaifu et al. 1974) CH 3 NH 2: G 12 symmetry § Isoelectronic to CH 3 OH 2+ § Comprehensive experimental data available

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

Observed CH 3 NH 2 spectrum 390 -464 GHz 770 -859 GHz 876 -901 GHz 1061 -1198 GHz 1575 -1625 GHz

Energy levels of CH 3 NH 2: G 12 group Ohashi and Hougen, 1987 J, K, ′ SPFIT J, K = Ka Near-prolate asymmetric top Selections rules: A 1 A 2; B 1 B 2 E 1; E 2 (+1 B 2 B 1 A 2 +1; -1 -1; +1 -1) A 1

Matrix elements for CH 3 NH 2 (I) Ohashi et al, 1987

Matrix elements for CH 3 NH 2 (II) Ohashi et al, 1987

Matrix elements for CH 3 NH 2 (III) DK = ± 2 term Ohashi et al, 1987

Matrix elements for CH 3 NH 2 (IV) DK = ± 1 term Ohashi et al, 1989

SPFIT operators Regular operators 2 1: ×cosa 3 11: ×sina 4 5 6 7 8 9 2 100000 -3. 0984147127856 E+003 1. 000000 E+037 h 2 v A 1 = a +-3. 0984147127856 E+003 ( ) 2 -1000000011 1. 000000 A 2 2 -1000000022 3. 0984147127856 E+003 -1. 000000 B 1 2 -1000000033 3. 0984147127856 E+003 -1. 000000 B 2 2 -1000000044 -1. 5492073563928 E+003 0. 500000 E 1+1 2 -11000000055 -2. 6833058527323 E+003 0. 866025 E 1 -1 2 -1000000066 1. 5492073563928 E+003 -0. 500000 E 2+1 2 -11000000077 -2. 6833058527323 E+003 0. 866025 E 2 -1 A 2 B 1 B 2 E 1+1 E 1 -1 cost cosa –cosacos(p/3)+sinasin(p/3) E 2+1 E 2 -1 cosacos(2 p/3)+sinasin(2 p/3)

Fitting results for CH 3 NH 2 ü A data set of 2800 transitions was constructed (~900 lines from the present study)) ü 60 parameters fitted to 2500 transitions (300 lines rejected) ü Reduced RMS = 1. 5 ü Microwave RMS = 0. 231 MHz ü IR RMS = 0. 00082 cm-1 q Rejected lines are all from DK = ± 1 transitions with K′ or K″=1 q Observed-calculated values range from 0. 5 – 1 MHz

Observed vs. calculated CH 3 NH 2 spectrum B 2 E 1+1 B 1 E 1+1 E 1 -1 E 2 -1 E 2+1 J′=24, K′=5 J″=23, K″=5 B 1 B 2 A 1 A 2 Nuclear spin weights: A 1: A 2: B 1: B 2: E 1+1: E 1 -1: E 2+1: E 2 -1 4 : 12: 6 1062370 1062450 : 6 : 2

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