Laboratory Terahertz Spectroscopy of Gaseous Molecules and Ions for Herschel, SOFIA and ALMA Shanshan Yu, Brian Drouin and John Pearson Copyright 2009 California Institute of Technology. Government sponsorship acknowledged.

Outline  Introduction  Application of molecular spectroscopy in astrophysics  Laboratory spectroscopic work needed for Herschel, SOFIA and ALMA  Experimental setup  Data analysis and modeling  Experiment, data analysis and modeling of  Acetylene (C 2 H 2 and C 2 D 2 )  Protonated water (H 3 O + )  Methylamine (CH 3 NH 2 )  Laboratory measurements of interstellar weeds

Molecular spectroscopy  Studying the interaction of light and molecules  Interaction causes molecules to transit from one energy level to another  Molecules absorb light: transit from lower to higher energy level  Molecules emit light: transit from higher to lower energy level  The energy levels of a molecule are quantized and unique  Electronic energy levels (electronic transitions in visible and UV)  Vibrational energy levels (vibration transitions in infrared)  Rotational levels (pure rotation transitions in microwave)  Selection rules govern molecular transitions  Transition dipole moments govern transition intensities Vibration of the nucleirotation

Information contained in high-resolution spectra  Geometries, chemical bonding and electronic structure of molecules (line positions)  Temperatures of molecules (relative line intensities)  Concentrations of molecules (absolute line intensities) High resolution spectra of water (H 2 O) and carbon monoxide (CO) around 2000 cm -1 (Brown et al. Journal of Molecular spectroscopy (2005) 774, 111) CO CO stretching H O H H 2 O bending Frequency (cm -1 )

 Radio astronomy: cold objects, including interstellar gas and dust clouds (10-200K)  Infrared astronomy: objects colder than stars, such as planets  Optical astronomy: stars, galaxies and nebulae  Ultraviolet, X-ray and gamma ray astronomy: very energetic processes such as binary pulsars, black holes, magnetars Astrophysical observations The Planck function The Planck function at various temperatures (

Visible vs. infrared images of the constellation Orion Features that cannot be seen in visible light show up very brightly in the infrared. Visible Infrared

 Terahertz spectroscopy has historically been a technique challenge: limited radiation sources and detectors  Recent technical advances in terahertz sources and mixers have led to the development of powerful terahertz systems for astrophysics Terahertz spectroscopy: a new era for the cool Universe λ(μm) 1.00× × ×10 -3 ( )× × ×10 6 Cosmic Rays γ-Rays X-Rays UV Visible Infrared Microwave Radio  (Hz) 3.00× × ×10 16 ( )× × ×10 8  (cm -1 ) 1.00× × ×10 6 ( )× × ×10 -2 The electromagnetic spectrum Terahertz 1 terahertz = Hz = 300 μm = 33.3 cm -1

 Herschel-HIFI (Heterodyne Instrument for the Far-IR)  ESA and NASA joint mission  Launch: 2009 (3 years lifetime)  Telescope: 3.5 meter diameter, <100 K temperature  The only space facility dedicated to the terahertz part of the spectrum  Spectral coverage: 151–212 μm (1910–1410 GHz); 240–625 μm (1250–480 GHz)  Objectives: life cycle of gas and dust New terahertz telescopes (I)

 SOFIA (Stratospheric Observatory For Infrared Astronomy )  NASA and DLR, German Aerospace Center joint mission  Operation: 2010 (20 years lifetime)  The largest airborne observatory in the world  Telescope: 2.5 meter diameter, 240 K temperature  Spectral coverage: 1–700 μm (300000–430 GHz)  Objectives: identification of complex molecules in space, star birth and death etc. New terahertz telescopes (II)

New terahertz telescopes (III)  ALMA (Atacama Large Millimeter/Submillimeter Array)  Global collaboration mission: East Asia, Europe, and North America  Location: Chile, 5000 meters above sea level  Operation: 2012 (50 years lifetime)  Telescope: a system up to 66 high-precision dish antennas  Spectral coverage: μm (1000–31 GHz)  Objectives: the physics of the cold Universe

Laboratory work needed for these missions (I) Interstellar chemistry (Herbst & Klemperer 1973)  Many ions necessary for chemical network to function have not yet been indentified due to lack of laboratory data, e.g. CH 2 +, CH 3 +, NH +, NH 2 +, NH 3 +  Observed molecular ions in space (as of May 2009):  Seventeen positive ions: CH +, CO +, SO +, SH +, CF +, HCO +, HCS +, HOC +, N 2 H +, H 3 +, H 2 D +, HD 2 +, HCNH +, HCO 2 +, H 3 O +, H 2 COH +, HC 3 NH +  Four negative ions: C 4 H –, C 8 H –, C 3 N –, C 5 N – Woon, 2007

Laboratory work needed for these missions (II) Requiring lab frequency measurements better than 1 MHz for all known molecules to secure identifications of spectral features Ziurys, 2006

C2H2C2H2 Experimental setup RF Synthesizer FM PC Source Si bolometer at 2.4 K Lock-in Reference Gas cell Nine sources: THz

Static quartz cells for stable molecules Leaking rate: ~1 mTorr/week

H3O+H3O+ DC discharge cell for ions Pump Precusors, Ar/He Coolant out DC discharge Coolant In 2 kV, 100 mA Proton affinity:  H 2 : 422 KJ/mol  N 2 : 502 KJ/Mol  CH 4 : 546 KJ/Mol  CO: ~590 kJ/mol  H 2 O: 691 KJ/Mol  CH 3 OH: 754 KJ/Mol Forming protonated species, MH + : H 2 H e - H H 2 H H H M MH + + H 2 The proton affinity is the energy released in the M+H + MH + reaction

DC glow discharge A.V. Engel, Ionized Gases, 1965 Voltage Electric field Densities of positive charges Densities of negative charges Current densities of negative charges Current densities of positive charges Positive column: high voltage, high density of negative charges, high current Negative glow: high voltage, high density of positive charges, low current

Extended negative glow discharge cell (under development) 2.8 m 2.6 m Electrode 2.4 M 2.6 M Courtesy of Prof. T. Amano at U of Waterloo Keys:  Diameter of the cell  Shape and material of the electrodes  Magnetic field  Low cell temperature (liquid N 2 temperature)  Low pressure inside the cell

Radio frequency discharge cell (under development) Cell 1.4 MHz 5 kW 10 A 100 A 50  C LOAD C TUNE L EXT L ANT Plasma tank circuit

Data analysis and modeling Adjust molecular parameters Calculated positions Observed positions Comparison Molecular parameters Hamiltonian model Converge NO Yes Output parameters Line positions: Hamiltonian for 1  states (simplest case): G V, B V, D V Perturbation of two states using second order-perturbation theory Interaction Fitting software: SPFIT/SPCAT

Terahertz spectroscopy and global analysis of the bending vibrations of C 2 H 2 and C 2 D 2 Yu et al., Astrophys. J. 698 (2009) Yu et al., Astrophys. J. (in press) H CC H ++ ++ –– –– Zero net dipole moment: z x

Vibrational modes of C 2 H 2 and C 2 D 2 H CC H H CC H H CC H H CC H H CC H 3373 cm cm cm cm cm -1 C2H2C2H cm cm cm cm cm -1 C2D2C2D cm cm -1 (~3500 GHz) (~900 GHz)

Introduction to C 2 H 2  12 C 2 H 2 is highly abundant in the interstellar medium  Observed in the cold (<100K) gas with abundances from ~10 -9 to and in the warm ( K) gas with abundances up to ~10 -7 to (e.g., Evans et al. 1991; Carr et al. 1995; Lahuis and van Dishoeck 2000; Farrah et al. 2007; Sonnentrucker et al. 2007)  12 C 2 H 2 is present as traces in the upper atmosphere of  Titan (Coustenis et al. 2007)  Jupiter (Ridgway 1974)  Uranus (Encrenaz et al. 1998)  12 C 2 H 2 is also present as pollutant in  The terrestrial troposphere (Kanakidou et al. 1988)  the urban atmosphere (Goldman et al. 1981)

C 2 D 2 : a potential interstellar species  Observed multiply deuterated interstellar molecules  [D 2 CO]/[H 2 CO] = (Turner 1990; Ceccarelli et al. 1998, 2001, 2002; Loinard et al. 2000, 2001; Parise et al. 2006; Roberts and Millar 2007)  [NHD 2 ]/[NH 3 ] = (Roueff et al. 2000, 2005; Loinard et al. 2001; Gerin et al. 2006)  [ND 3 ]/[NH 3 ] = (Lis et al. 2002; van der Tak et al. 2002; Roueff et al. 2005)  [CHD 2 OH]/[CH 3 OH] = (Parise et al. 2002, 2004, 2006)  [CD 3 OH]/[CH 3 OH] = 0.01 (Parise et al. 2004)  [D 2 S]/[HDS] = 0.1 (Vastel et al. 2003)

Previous studies on C 2 H 2 and C 2 D 2  Spectroscopic information on all the five vibrational modes of C 2 H 2 and C 2 D 2 are available  But Spectroscopic study of 12 C 2 H 2 and 12 C 2 D 2 in the region of their 5 – 4 difference bands was very sparse  12 C 2 H 2 only ~300 lines measured by FTIR in 52–192 cm -1 with uncertainty of 9 MHz (Kabbadj et al. 1991)  12 C 2 D 2 only 10 lines were measured with microwave precisions (Lafferty et al. 1977; Deleon and Muenter, 1987 ) ~260 lines were measured in 30–100 cm -1 with uncertainty of 2.4 MHz (Huhanantti et al. 1979; Huet et al )

Experimental setup 150 mTorr C 2 H 2 /C 2 D 2 generated by passing H 2 O/D 2 O through CaC 2 powder Parabolic mirror FM RF Synthesizer Multiplier chain PC Si detector Lock-in Beamsplitter Sample cell Pump Rooftop reflector 2.8 meters Sample cell Sample ×6×6 ×2×2 ×3×3 …

Observed C 2 D 2 terahertz transitions C 2 D 2 lines observed 12 C 2 D 2 5 – C 2 D – ( )

Observed C 2 H 2 terahertz transitions C 2 H 2 lines observed

Multistate analysis of C 2 H 2 and C 2 D

Hamiltonian model for C 2 H 2 and C 2 D 2 (I)

2. Elements representing the vibrational l-type resonance and doubling 3. Elements accounting for the Darling-Dennison type interactions (C 2 D 2 only) 1. Elements responsible for the rotational l-type resonance and doubling Hamiltonian model for C 2 H 2 and C 2 D 2 (II)

Block form of the Hamiltonian model for C 2 D 2

A data set of 2092 transitions was constructed 72 parameters fitted to 1938 transitions (154 IR lines rejected) Reduced RMS = 1.3 Microwave RMS = MHz (261 MW data) IR RMS = cm -1 Fitting results for 12 C 2 D 2

A data set of 1406 lines was constructed 34 parameters fitted to 1390 transitions Reduced RMS = 0.5 Microwave RMS = MHz (20 MW data) IR RMS = cm -1 Fitting results for 12 C 2 H 2

Information obtained for the astronomy community  : the line position  E l : the lower state energy  E u : the upper state energy  The partition function  S': line strength or line intensity  g( - 0 ): line shape function  (Nl): column density The following parameters can be calculated based on our fitting results: Which can be used to calculate line strength: Line strength useful for simulating astrophysical spectra:

Herschel ALMA  (cm 2 /molecule) of the P ee and R ee branch lines of the band of C 2 H 2 SOFIA

Terahertz spectroscopy and global analysis of H 3 O + Yu et al., Astrophys. J. Suppl. Ser. 180 (2009)

Introduction to H 3 O + (I)  H 3 O + has a pyramidal structure and is iso-electronic to ammonia (NH 3 )  NH 3 : well-known radio frequency (~24 GHz) inversion splitting  H 3 O + : ground-state inversion splitting of ~1.6 THz  H 3 O + is a key molecular ion in interstellar oxygen chemistry  H 3 O + + e H 2 O + H  H 3 O + + e OH + 2H; OH + O O 2 + H  H 3 O + has been detected in the interstellar medium  Orion/KL, OMC-1 and Sgr B2 regions (Hollis et al. 1986; Wootten et al. 1986)  OMC-1 and Sgr B2 regions (Wootten et al. 1991)  W3 IRS 5, G and Sgr B2 (Phillips et al. 1992; Goicoechea & Cernicharo 2001; van der Tak et al. 2006; Polehampton et al. 2007)  Orion/KL, W3(OH), W51 M, and Orion BN-IRc2 (Phillips et al. 1992; Timmermann et al. 1996; Leratee et al. 2006)  Two prototypical active galaxies: M 82 and Arp 220 (van der Tak, 2008)

Introduction to H 3 O + (II)  H 3 O + was identified in the laboratory before its discovery in space  Infrared intensity ratios: 1:11:12:3 (Colvin et al 1983) H H H O 1 : O-H symmetric stretching 2 : inversion bending 3 : O-H asymmetric stretching 4 : perpendicular bending H H H O H H H O H H H O

Previous infrared studies on H 3 O +  3 : J ≤16 assigned  C and D K determined by observed  K-l)=3 forbidden transitions  Begemann et al. 1983, 1985; Stahn et al. 1987; Ho et al. 1991; Uy et al. 1997; Tang & Oka 1999  2 : J ≤ 16 assigned  0 – – 0 + inversion splitting determined to be (55) cm -1  Haese & Oka 1984; Lemoine & Destombes 1984; Davies et al. 1984, 1985; Liu & Oka 1985; Liu et al. 1986; Zheng et al  1 : J ≤ 10 assigned  Tang & Oka 1999  4 : J ≤ 7 assigned  Gruebele et al  ( ) – 2 and – 2 –  Davies et al. 1986; Ho et al. 1991

Previous submillimeter studies on H 3 O +  4 transitions around 350 GHz measured with an uncertainty of 100 kHz  Plummer et al. 1985; Bogey et al  24 transitions in THz measured with uncertainties of MHz by laser sideband spectroscopy  Verhoeve et al. 1988, 1989; Stephenson & Saykally 2005  J max =11

Perturbations in H 3 O + Tang and Oka, 1999  In previous studies:  Strong Coriolis interaction between 1 and 3 was not taken into account  About 200 assigned high-J lines could not be fitted  The largest observed-calculated frequency was cm -1

Experimental setup H 2 O: 30 mTorr, H 2 : 5 mTorr DC discharge: 80 mA, 2 kV Cell temperature: 230 K Magnetic field: 150 Gauss Coolant out Discharge H 2 O, H 2 Coolant in Sample cell Pump Beamsplitter Rooftop reflector FM Rf Synthesizer Multiplier chain PC Si detector Lock-in ×6×6 ×2×2 ×3×3 …

Observed H 3 O + terahertz transitions Observed for the first time 8 H 3 O + lines observed

Multistate analysis of H 3 O +

Block form of the Hamiltonian model for H 3 O +

Fitting results for H 3 O + A data set of 1114 transitions was constructed 113 parameters fitted to 1042 transitions (72 transitions rejected) Reduced RMS = 1.6; Microwave RMS = 1.22 MHz; IR RMS = cm -1 RMS of the eight lines measured in the present work: MHz

Terahertz spectroscopy of the ground state of methylamine (CH 3 NH 2 )

Introduction to CH 3 NH 2 Torsional motion of CH 3 Wagging motion of NH 2 Woon, 2007 Barrier heights 536 cm cm -1

Previous work on the ground state of CH 3 NH 2 ~700 lines in GHz with uncertainties of 20–500 kHz ~1200 lines in cm -1 with uncertainties of –0.001 cm -1  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)

Our interest in CH 3 NH 2  CH 3 NH 2 exists in the simulated atmosphere of Titan (Drouin, 2006) 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

Observed CH 3 NH 2 spectrum GHz GHz GHz GHz GHz

Energy levels of CH 3 NH 2 : G 12 group Ohashi and Hougen, 1987 B2 B1 A2 A1 Selections rules: A1 A2; B1 B2 E1 E1; E2 E2 (+1 +1; -1 -1; +1 -1)  J, K,  ′ Near-prolate asymmetric top J, K = K a SPFIT

Hamiltonian model for CH 3 NH 2 (I) Ohashi et al, 1987  Hamiltonian matrix elements are expanded in Fourier series

Hamiltonian model for CH 3 NH 2 (II) Ohashi et al, 1987

Hamiltonian model for CH 3 NH 2 (III)  K = ±2 term Ohashi et al, 1987

Hamiltonian model for CH 3 NH 2 (IV)  K = ±1 term Ohashi et al, 1987

SPFIT operators A1 A2 B1 B2 E1+1 E1-1 E2+1 E2-1 cos  cos  cos  –cos  –cos  cos  cos(  /3)+sin  sin(  /3) cos  cos( 2  /3)+sin  sin(2  /3) =  E E+037 h2v A E A E B E B E E E E E E E E2-1 1: ×cos  11: ×sin  Regular operators SPFIT operators

Fitting results for CH 3 NH 2 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  K = ±1 transitions with K′ or K″=1  Observed-calculated values range from 0.5 – 1 MHz

Observed vs. calculated CH 3 NH 2 spectrum J′=24, K′=5 J″=23, K″=5 A2 A1 B1 B2 E2-1 E2+1 E1-1 A1 A2 B2 B1 E Nuclear spin weights: A1:A2:B1:B2:E1+1:E1-1:E2+1:E2-1 4 : 4 :12 :12: 6 : 6 : 2 : 2

Experiments for interstellar weeds (I) Weeds THz GHz CH 3 OHXXXXXXXXX CH 3 OCHO X/X CH 3 OCH 3 XXXXX CH 3 CH 2 CNXXXXX SO 2 DONE isotopologues  What are interstellar weeds?  Abundant (10 -7 relative to H 2 )  Low-lying vibrational states  Dense rotational lines  Making observing less abundant species difficult

Experiments for interstellar weeds (II) Species THz GHz CH 3 OD x x CH 3 18 OH x x 13 CH 3 OH///x CH 2 DOHxxxxxxxx 13 CH 3 OCHO CH 3 O 13 CHO CH 3 18 OCHO CH 3 OCH 18 O CH 2 DOCHO CH 3 OCDO 13 CH 3 OCH 3 CH 2 DOCH 3 CH 3 18 OCH 3 13 CH 3 CH 2 CN CH 3 13 CH 2 CN CH 3 CH 2 13 CN CH 3 CH 2 C 15 N CH 2 DCH 2 CN CH 3 CHDCN

~20 lines/GHz 13 CH 3 OH spectrum around 0.9 THz

CH 3 CH 2 CN spectrum around 0.5 THz ~140 lines/GHz

Acknowledgements  Drs. Brian Drouin, John Pearson and Herb Pickett  Tim Crawford and William Chun  Alma Cardenas and Rowena Dineros  NPP/ORAU

H 2 O 2 2, 6 6,1 – 7 5,2 Work in progress  Global fitting of the X 3  g –, a 1  g, b 1  g + and B 3  u – states of the six isotopologues of O 2  Terahertz spectroscopy and global analysis of the ground state, 2, 2 2 and 4 states of NH 3  Terahertz spectroscopy and global analysis of the ground state, 2 2, 1, 3 states of H 2 O absorption emission

Future work  CH +, CH 2 +, CH 3 +, CH 2 D +, etc.  H 2 D +, HD 2 +  HD 2 O +, H 2 DO +  H 2 O +

Application of our results to astrophysics (I) Our model prediction for CO(CH 2 OH) 2 (DAH) transitions at 300K Submillimeter Array observations at 330 GHz towards G GHz

Application of our results to astrophysics (II) Our model prediction for CO(CH 2 OH) 2 (DAH) transitions at 300K Submillimeter Array observations at 340 GHz towards G19.6