Indirect Rotational Spectroscopy of HCO +

1. Indirect Rotational Spectroscopy of HCO+ Adam J. Perry, James N. Hodges, Brian M. Siller,and Benjamin J. McCall 68th International Symposium on Molecular Spectroscopy The Ohio State University 19 June 2013

2. Overview Motivations Experimental Technique Indirect Rotational Spectroscopy Conclusions

3. Motivations • General technique for acquiring rotational spectra of molecular ions • Technology more developed in mid-IR • Support observations by new telescopes/arrays • ALMA • SOFIA • Herschel • Testing out this technique on HCO+ 3-400 µm 0.3-1600 µm 60-670 µm

4. HCO+ Background a Buhl, D.; Snyder, L. E. “Unidentified Interstellar Microwave Line” Nat. 1970, 228, 267–269 b Klemperer, W. Carrier of the Interstellar 89.190 GHz Line. Nat. 1970, 227, 1230–1230 cGudeman, C. S.; Begemann, M. H.; Pfaff, J.; Saykally, R. J. “Velocity-Modulated Infrared Laser Spectroscopy of Molecular Ions: The ν1 Band of HCO+” Phys. Rev. Lett. 1983, 50,727–731 • First observed via telescope in 1970 by Buhl and Snydera,b • Known as “X-ogen” until future confirmation of identity • First ion studied by Velocity Modulation Spectroscopy (Gudemanet al.)c

5. Optical Heterodyne Velocity Modulation Spectroscopy (OHVMS) 35 kHz Plasma Modulation ni = np - ns I P S OPO Frequency Comb AOM Wave-meter EOM Lock-In Amplifier Lock-In Amplifier f = 35 kHz 80 MHz YDFL X & Y Channels X & Y Channels ν 90o Phase Shift B. M. Siller, J. N. Hodges, A. J. Perry, and B. J. McCall, “Indirect Rotational Spectroscopy of HCO+” J. Phys. Chem. A (in press).

6. HCO+ Production • Plasma Conditions: • 30 mTorr CO • 500 mTorr H2 • 35 kHz , 140 mA discharge Trot ~ 166 K

7. Frequency Calibration • MenloSystems FC1500 • 100 MHz repetition rate • Used to measure pump and signal beam frequencies • Idler frequency is then calculated νidler= νpump- νsignal

8. Comb Scanning AOM Rep. rate tuned so that signal beat lies within bandpass filter on frequency counter Comb Modes Frequency correction applied by AOM keeps signal beat within the bandpass Pump offset locked (~20 MHz) to nearest comb mode Bandpass regions (on frequency counter) Frequency

9. Comb-Calibrated OHVMS Scan P(5) line of ν1 fundamental band of HCO+ • S/N ~300 (~100 for weakest lines) • Lines fit to 2nd derivative of Gaussian function • 4-7 scans for each line • Average linecenter statistical uncertainty ~600 kHz B. M. Siller, J. N. Hodges, A. J. Perry, and B. J. McCall, “Indirect Rotational Spectroscopy of HCO+” J. Phys. Chem. A (in press).

10. Comb-Calibrated Rovibrational Transitions d e Improved precision by nearly two orders of magnitude d B. M. Siller, J. N. Hodges, A. J. Perry, and B. J. McCall, “Indirect Rotational Spectroscopy of HCO+” J. Phys. Chem. A (in press). eT. Amano, “The ν1 Fundamental Band of HCO+ by Difference Frequency Laser Spectroscopy” J. Chem. Phys. 1983, 79, 3595.

11. Fitting the Spectroscopic Data • Rovibrational transitions fit to simple linear molecule Hamiltonian: • Included terms up to sextic distortion • Upper and lower state sextic constants constrained to be equal Total RMS error ~1.7 MHz B. M. Siller, J. N. Hodges, A. J. Perry, and B. J. McCall, “Indirect Rotational Spectroscopy of HCO+” J. Phys. Chem. A (in press).

12. Indirect Rotational Spectroscopy J’ 4 IR Transitions Even Combination differences Odd Combination Differences Known Rotational Transition Reconstructed Rotational Transitions 3 v = 1 cm-1 2 1 0 6 5 cm-1 v = 0 4 3 2 1 0 J”

13. Indirect Ground State Rotational Transitions f G. Cazzoli, L. Cludi, G. Buffa, and C. Puzzarini, “Precise THz Measurements of HCO+, N2H+, and CF+ for Astronomical Observations” Astrophys. J. Sup. 2012, 203, 11

14. 1ν1 Excited State Rotational Transitions • Deduced 9 new excited rotational transitions • Never directly observed • Uncertainty < 3MHz • Should be able to facilitate astronomical observations in “hot” environments • Hot cores • Circumstellar envelopes d B. M. Siller, J. N. Hodges, A. J. Perry, and B. J. McCall, “Indirect Rotational Spectroscopy of HCO+” J. Phys. Chem. A (in press). fLattanzi, V.; Walters, A.; Drouin, B. J.; Pearson, J. C. Rotational Spectrum of the Formyl Cation, HCO+, to 1.2 THz. Astrophys. J. 2007, 662, 771–778

15. Future Improvements to Linecenter Determination • Sub-Doppler Spectroscopy • Achieved with cavity enhancement • NICE-OHVMS • Narrower sub-Doppler features should provide more accurate & precise linecenter determination P(5) line of ν1 fundamental band of HCO+ Feature width ~50 MHz B. M. Siller, J. N. Hodges, A. J. Perry, and B. J. McCall, “Indirect Rotational Spectroscopy of HCO+” J. Phys. Chem. A (in press).

16. Conclusions • Performed infrared spectroscopy on the ν1 fundamental band of HCO+ and calibrated 20 rovibrational transitions with an optical comb • Lines fit with average precision of ~600 kHz • Demonstrated a general technique for obtaining rotational spectra of molecular ions using infrared transitions • Current/future directions: • Employ cavity enhancement • New targets • CH5+ • HO2+ • Others

17. Acknowledgments • Advisor: Ben McCall • Group Members: • Brian Siller • James Hodges • Funding Agencies