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 µm µm

4. 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 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

5. 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).

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 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

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
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.

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 SpectroscopyJ’ 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”

13. Indirect Ground State Rotational TransitionsJ'
J''
Present Work (MHz)
Direct Meas. (MHz)f
Present-Direct (MHz) 1
n/a 2
(17)
-0.5 3
(19) 4
(19)
-2.0 5
(21)
1.0 6
(23) 7
(26)
-1.0 8
(27)
2.8 9
(27)
-2.5 10
(27)
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

14. 1ν1 Excited State Rotational TransitionsJ'
J''
Present Work (MHz)d
Uncertainty (MHz) 1
1.9 2
1.6 3
n/a f 4
0.9 5
1.1 6
1.3 7 8 9 10
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

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