Indirect Rotational Spectroscopy of HCO+Adam J. Perry, James N. Hodges, Brian M. Siller, and Benjamin J.

McCall68th International Symposium on Molecular Spectroscopy

The Ohio State University19 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+

60-670 µm0.3-1600 µm

3-400 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

c Gudeman, C. S.; Begemann, M. H.; Pfaff, J.; Saykally, R. J. “Velocity-Modulated InfraredLaser Spectroscopy of Molecular Ions: The ν1 Band of HCO+” Phys. Rev. Lett. 1983, 50, 727–731

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

Optical Heterodyne Velocity Modulation Spectroscopy (OHVMS)

YDFL

EOMLock-In

Amplifier

X & Y Channels

Lock-In Amplifier

X & Y Channels

Wave-meter

80 MHz

90o Phase Shift

f = 35 kHz

ni = np - ns

AOM

Frequency Comb

ν

35 kHzPlasma

Modulation

OPOIPS

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+ ProductionPlasma 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 calculatedνidler= νpump- νsignal

Comb Scanning

Frequency

Comb Modes

Pump offset locked (~20 MHz) to nearest comb mode

Rep. rate tuned so that signal beat lies within bandpass filter on frequency counterFrequency correction applied by

AOM keeps signal beat within the bandpass

Bandpass regions (on frequency counter)

AOM

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

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

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).

ed

Improved precision by nearly two orders of magnitude

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

B. M. Siller, J. N. Hodges, A. J. Perry, and B. J. McCall, “Indirect Rotational Spectroscopy of HCO+” J. Phys. Chem. A (in press).

Total RMS error ~1.7 MHz

0

80

60

40

20

0

2.01.51.00.50.0-0.5-1.0

80

60

40

20

0

2.01.51.00.50.0-0.5-1.0

3320

3300

3280

3260

3240

2.01.51.00.50.0-0.5-1.0

3320

3300

3280

3260

3240

2.01.51.00.50.0-0.5-1.0

12

3

4

5

6

01

2

3

4J’

cm-1

cm-1

J”

Indirect Rotational Spectroscopy

v = 1

v = 0

Indirect Ground State Rotational Transitions

J' J'' Present Work (MHz) Direct Meas. (MHz)f Present-Direct (MHz)

0 1 n/a 89188.5247 n/a

1 2 178374.6(17) 178375.0563 -0.5

2 3 267557.0(19) n/a n/a

3 4 356732.3(19) 356734.2230 -2.0

4 5 445903.9(21) 445902.8721 1.0

5 6 535061.0(23) 535061.5810 -0.5

6 7 624207.4(26) 624208.3606 -1.0

7 8 713344.0(27) 713341.2278 2.8

8 9 802455.7(27) 802458.1995 -2.5

9 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)

0 1 88486.7 1.9

1 2 176955.4 1.6

2 3 n/a f n/a f

3 4 353900.7 0.9

4 5 442366.0 1.1

5 6 530813.3 1.3

6 7 619257.7 1.6

7 8 707676.3 1.9

8 9 796093.7 1.9

9 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

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).

P(5) line of ν1 fundamental band of HCO+

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 SillerJames

Hodges

• Funding Agencies