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

Overview Motivations Experimental Technique Indirect Rotational Spectroscopy Conclusions

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

HCO+ Background 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 (Gudeman et al.)c 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 c Gudeman, 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

Optical Heterodyne Velocity Modulation Spectroscopy (OHVMS) 35 kHz Plasma Modulation ni = np - ns OPO I P S Frequency Comb AOM Wave-meter EOM Lock-In Amplifier Lock-In Amplifier ν 80 MHz f = 35 kHz YDFL X & Y Channels X & Y Channels 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).

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

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

Comb Scanning Frequency 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

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).

Comb-Calibrated Rovibrational Transitions 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). e T. Amano, “The ν1 Fundamental Band of HCO+ by Difference Frequency Laser Spectroscopy” J. Chem. Phys. 1983, 79, 3595.

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).

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

Indirect Ground State Rotational Transitions J' J'' Present Work (MHz) Direct Meas. (MHz)f Present-Direct (MHz) 1 n/a 89188.5247 2 178374.6(17) 178375.0563 -0.5 3 267557.0(19) 4 356732.3(19) 356734.2230 -2.0 5 445903.9(21) 445902.8721 1.0 6 535061.0(23) 535061.5810 7 624207.4(26) 624208.3606 -1.0 8 713344.0(27) 713341.2278 2.8 9 802455.7(27) 802458.1995 -2.5 10 891558.4(27) 891557.2903 1.1 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

1ν1 Excited State Rotational Transitions J' J'' Present Work (MHz)d Uncertainty (MHz) 1 88486.7 1.9 2 176955.4 1.6 3 n/a f 4 353900.7 0.9 5 442366.0 1.1 6 530813.3 1.3 7 619257.7 8 707676.3 9 796093.7 10 884477.9 2.4 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). f Lattanzi, 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

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).

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

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