Terahertz measurements of the hot hydronium ion

with an extended negative glow discharge

Shanshan Yu and John C. Pearson Jet Propulsion Laboratory, California Institute of Technology, USA

Copyright 2014. All rights reserved.

Published in Astrophys. J. 786, 133, 2014 May 10

Introduction to the hydronium ion (H3O+)

H3O+ has a pyramidal structure and is iso-electronic to ammonia (NH3)

NH3: well-known radio frequency (~24 GHz) inversion splitting H3O+: ground-state inversion splitting of ~1.6 THz

H3O+ is a key molecular ion in interstellar oxygen chemistry

H3O+ + e H2O + H H3O+ + e OH + 2H; OH + O O2 + H

H3O+ 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)

Herschel/HIFI observation of H3O+ (Lis et al. 2014)

H3O+ n = 0+ ortho energy levels and DK = ±3 forbidden transitions involving the J = K metastable levels

H3O+ n = 0+ para energy levels and DK = ±3 forbidden transitions involving the J = K metastable levels

Previous infrared studies on H3O+

Infrared intensity ratios: n3: n2: n4: n1: =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 n3: J ≤16 assigned; C and DK determined by observed D(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 n2: J ≤ 16 assigned; GS inversion splitting determined [55.3462(55) cm-1]

Gruebele et al. 1987 n4: J ≤ 7 assigned

Tang & Oka 1999 n1: J ≤ 10 assigned

Davies et al. 1986; Ho et al. 1991 (n2+n3) – n2 and 2n2

+ – n2–

Previous submillimeter studies on H3O+

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 n1 and n3 taken into account Results incorporated to the JPL Spectral Line catalog and used for

Herschel data analysis

Experimental setup

H2O:H2:Ar = 1:2:12 mTorr

DC discharge: 10 mA, 3 kV

Cell temperature: 190 K

Magnetic field: 150 Gauss

Coolant out

Discharge

H2O, H2, Ar

Coolant in Sample cell(L=1.2m; ID=5cm)

Pump

FM

Rf Synthesizer

Multiplier chain

PC

Si detector

Lock-in

×3

×6

…

Summary of H3O+ 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 H3O+ production by a factor of 10 New multiplier chains provided more power and frequency coverage

Hamiltonian model

See Yu et al., Astrophys. J. Supp. Series, 180, 119 (2009) for details The following DK = ±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

Sample H3O+ frequency, uncertainty, difference from the current (a) and Yu et al. 2009 (b) analyses

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.

Acknowledgements