1. Copyright 2014. All rights reserved.

2. Introduction to the hydronium ion (H 3 O + )  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 (Hollis et al. 1986; Wootten et al. 1986)  OMC-1 and Sgr B2(Wootten et al. 1991)  W3 IRS 5, G34.3+0.15 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)

3. Herschel/HIFI observation of H 3 O + (Lis et al. 2014)

4. H 3 O + = 0 + ortho energy levels and  K = ±3 forbidden transitions involving the J = K metastable levels

5. H 3 O + = 0 + para energy levels and  K = ±3 forbidden transitions involving the J = K metastable levels

6. Previous infrared studies on H 3 O +  Infrared intensity ratios: 3 : 2 : 4 : 1 : =12:11:3:1 (Colvin et al 1983)  Begemann et al. 1983, 1985; Stahn et al. 1987; Ho et al. 1991; Uy et al. 1997; Tang & Oka 1999  3 : J ≤16 assigned; C and D K determined by observed  (K-l)=3 forbidden transitions  Haese & Oka 1984; Lemoine & Destombes 1984; Davies et al. 1984, 1985; Liu & Oka 1985; Liu et al. 1986; Zheng et al. 2007  2 : J ≤ 16 assigned; GS inversion splitting determined [55.3462(55) cm -1 ]  Gruebele et al. 1987  4 : J ≤ 7 assigned  Tang & Oka 1999  1 : J ≤ 10 assigned  Davies et al. 1986; Ho et al. 1991  ( 2 + 3 ) – 2 and 2 2 + – 2 –

7. Previous submillimeter studies on H 3 O +  Jmax= 11, Kmax = 9  Plummer et al. 1985; Bogey et al. 1985  4 transitions around 350 GHz measured with ±100 kHz  Verhoeve et al. 1988, 1989; Stephenson & Saykally 2005  24 transitions in 0.9-3.1 THz measured with ±0.9 MHz to ±2 MHz by laser sideband spectroscopy  Yu et al. 2009  8 transitions in 0.9-1.6 THz measured with ± 0.3 MHz  Multistate analysis performed, including all submillimeter and IR data  Strong Coriolis interaction between 1 and 3 taken into account  Results incorporated to the JPL Spectral Line catalog and used for Herschel data analysis

8. Experimental setup H 2 O:H 2 :Ar = 1:2:12 mTorr DC discharge: 10 mA, 3 kV Cell temperature: 190 K Magnetic field: 150 Gauss Coolant out Discharge H 2 O, H 2, Ar Coolant in Sample cell (L=1.2m; ID=5cm) Pump FM Rf Synthesizer Multiplier chain PC Si detector Lock-in ×3×3 ×6×6 …

9. Summary of H 3 O + observations from this work 100 kHz uncertainty 25.6 MHz off from prediction based on previous work 6 s integration on each point 999 cm -1 in lower state energy J = K = 12 @1953 GHz  47 GS inversion transitions in 0.9-2.0 THz (22 new; 25 previously measured)  Experimental uncertainty from 100 to 300 kHz  Jmax = Kmax = 12 (J = K = 13 out of our source coverage)  Two factors for the improvement of this work compared to Yu et al. 2009  Extended negative glow enhanced H 3 O + production by a factor of 10  New multiplier chains provided more power and frequency coverage

10. Hamiltonian model  See Yu et al., Astrophys. J. Supp. Series, 180, 119 (2009) for details  The following  K = ±6 interaction was necessary to be added to fit the newly measured and improved positions to experimental accuracy:  See Yu et al. Astrophys. J. 786, 133 (2014) for the table of determined parameters

11. Sample H 3 O + frequency, uncertainty, difference from the current (a) and Yu et al. 2009 (b) analyses

12.  Professor Takayoshi Amano  This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.