The Submillimeter/THz Spectrum of AlH (X 1 Σ + ), CrH (X 6 Σ + ), and SH + (X 3 Σ - ) DeWayne T. Halfen and Lucy M. Ziurys Department of Chemistry and Biochemistry Department of Astronomy Steward Observatory Arizona Radio Observatory University of Arizona June 25, 2015
Hydrides in Interstellar Chemistry Hydrogen most abundant element in the universe Hydrides major building blocks of interstellar chemistry Multiple diatomic hydrides present in diffuse and/or dense gas CH +, SH + tracers of energetic processes –Shocks, turbulence, shears or XDRs Diatomic Interstellar Hydrides CHOHSH + CH + HFHCl + HClOH + ArH + NHSH Metal-containing hydrides, CrH and FeH, observed in atmospheres of brown dwarfs Many metals have relatively high cosmic abundances Metal hydrides obvious possible interstellar species H: 1 C: 3 x O: 7 x Na: 2 x S: 2 x Al: 3 x Fe: 3 x Cr: 5 x 10 -7
Savage et al. (2004) measured pure rotational spectrum of SH + –N = 1 ← 0, J = 0 ← 1 near 345 GHz, J = 2 ← 1 near 526 GHz Past Pure Rotational Work Nagy et al. (2013) SH + detected in interstellar gas in 2011 by 526 GHz lines using Herschel (e.g. Menten et al. 2011, Nagy et al. 2013) Müller et al. (2014) observed SH + N = 1 ← 0, J = 0 ← 1 lines near 345 GHz in Orion Bar, shifted from predictions –Suggested that 345 GHz laboratory data needed to be examined Orion Bar Müller et al. (2014)
CrH/CrD and AlH/AlD rotational spectrum measured (Halfen et al. 2004a,b, 2010, 2014; Harrison, Halfen et al. 2006) N = 1 ← 0 for CrH; N = 2 ← 1 CrD J = 1 ← 0, 2 ← 1 for AlH; J = 2 ← 1 to 4 ← 3 for AlD Subsequent analysis of AlH by H. Müller (2014), based on previous data, suggested that J = 2 ← 1 was shifted
CrH/CrD and AlH/AlD rotational spectrum measured (Halfen et al. 2004a,b, 2010, 2014; Harrison, Halfen et al. 2006) N = 1 ← 0 for CrH; N = 2 ← 1 CrD J = 1 ← 0, 2 ← 1 for AlH; J = 2 ← 1 to 4 ← 3 for AlD Subsequent analysis of AlH by H. Müller (2014), based on previous data, suggested that J = 2 ← 1 off by a few MHz J = 1 ← 0
Millimeter-wave Velocity Modulation Spectrometer
Detector Radiation Source Gas Cell Reactant
Gas-Phase Synthesis For AlH, CrH, added 30 mtorr Ar, 25 mtorr H 2, and <1 mtorr either Al(CH 3 ) 3 or Cr(CO) 6 Cr(CO) 6 heated to ~40 o C in metal tube heater For SH +, added 50 mtorr Ar and 3 mtorr H 2 S AC discharge of W at 600 Ω Covered cell in μ-metal shield to protect against Earth’s magnetic field
Rotational Spectra Measured one rotational transition each of AlH, CrH, and SH + –Several spin components for CrH and SH +, along with hyperfine structure lines for SH + Re-measured J = 2 ← 1 of AlH near 755 GHz N = 2 ← 1, J = 3.5 ← 2.5 and 4.5 ← 3.5 of CrH near 730 and 735 GHz Re-examined N = 1 ← 0, J = 0 ← 1 and 2 ← 1 of SH + near 345, 526 GHz and measured J = 1 ← 1 near 683 GHz
F = 4.5 ← 3.5 F = 3.5 ← scans J = 2 ← 1
240 scans26 scans
200 scans
40 scans60 scans
250 scans
Species N NN J JJ F FF ν obs (MHz) ν obs - ν calc (MHz) AlH2 14.5 CrH2 13.5 2.54 3.55 SH + 1 00 10.5 11.5 10.5 Transition Frequencies
Spectroscopic Analysis Parameter a AlH (X 1 Σ + )CrH (X 6 Σ + )SH + (X 3 Σ - ) B (62) (17) (47) D10.944(10)10.685(28) b γ (10) (95) λ (71) (19) γsγs (25) bFbF -34.7(1.5)-56.68(15) c42.33(71)35.89(44) eQq-48.61(70) CICI 0.298(35) rms a In MHz. b Held fixed. Analyzed data with 1 Σ or Hund’s case (b) Hamiltonian (SPFIT) H eff = H rot + H sr + H ss + H sr (3) + H mhf + H eQq + H nsr D for CrH determined for first time, SH + fit globally for first time Constants agree very well with past work
New measurements could allow AlH, CrH to be detected in interstellar gas with Herschel data Accurate frequencies can be predicted to 2 THz with errors of <2 MHz SH + measurements confirm detection in Orion-Bar near 345 GHz (Müller et al. 2014) –Could allow for more ground-based detections of SH + Measure higher frequency lines (N = 2 ← 1) of SH + near THz –Determine D, make accurate predictions Conclusions and Future Directions
Prof. Lucy Ziurys Jie Min Julie Anderson John Keogh Deborah Schmidt Kyle Kilchenstein NSF and NASA Acknowledgements