Initial Development of High Precision, High Resolution Ion

Beam Spectrometer in the Near-Infrared

Michael Porambo, Brian Siller, Andrew Mills, Manori Perera, Holger Kreckel, Benjamin J. McCall

International Symposium on Molecular SpectroscopyThe Ohio State University

18 June 2012

Outline

• Introduction: Why a Fast Ion Beam?• Ion Beam Description• NIR Spectra• Summary and Future Work

Molecular IonsImportant in many areas of nature and science

H2+

H3+

CH+

CH2+

CH3+

CH5+

CH4

C2H3+

C2H2

C3H+

C3H3+

H2

H2

H2

H2

H2

C

e

C+

e

C+

OH+

H2O+

H3O+

H2O

OHe

O

H2

H2

HCO+

CO

HCNCH3NH2

CH3CN

C2H5CN

N, e

NH3, e

HCN, eCH3CN, e

eCO, e

H2O, e

CH3OH, e

CHCH2CO

CH3OH

CH3OCH3

CH3+

C2H5+e

C2H4

e

C3H2

e

C3H

e

C2H

AstrochemistryAtmospheric science

Fundamental physics and chemistry

CH5+

From White et al. Science, 1999, 284, 135–137.From B. J. McCall, Ph.D. Thesis, Univ. of Chicago, 2001.

NASA Picture of the Day, Expedition 13 Crew, International Space Station, NASA

Challenge: How to produce ions in the laboratory effectively to study them?

Ion Production MethodsHollow Cathode

Supersonic Expansion

Positive Column

Way to bring low rotational temperature and ion-neutral discrimination together?

No ion-neutral discrimination

Low rotational temperature

No ion-neutral discrimination

Ion-neutral discrimination with velocity modulation

No low rotational temperature

Ion Beam Spectroscopy-last attempted in 1980s–1990s1

-advances in technology open newopportunities

1Coe et al. J. Chem. Phys. 1989, 90, 3893.

Sensitive, Cooled, Resolved Ion BEam Spectroscopy – SCRIBES

TOF massspectrometer

Sourcechamber

Overlapregion

Laser incavity

Electrostatic Bender2

Rigorous ion-neutral discrimination

Can perform low temperature spectroscopy with a supersonic discharge source

Low ion densityMake up for this with cavity-enhanced spectroscopy2Kreckel et al. Rev. Sci. Instrum. 2010, 81, 063304.

Sensitive, Cooled, Resolved Ion BEam Spectroscopy – SCRIBES

Spectroscopic Detection

Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy

Cavity enhancement for longer pathlength (× Finesse/π)

Spectroscopic Detection

Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy

Heterodyne/Frequency Modulation Detection for Lower Noise

EOM

NICE-OHMS Signal

Spectroscopic DetectionEOM

Lock-In Amplifier

NICE-OHMS Signal

Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy

Also velocity modulate the ion beam and demodulate at this signal.

Ion Beam

Doppler Splitting

nred nblue

Mass information encoded in the optical

spectrum!

First Spectroscopic Target• Obtain rovibronic spectral transitions of Meinel band

of N2+

• Near-infrared transitions probed with commercial tunable titanium–sapphire laser (700–980 nm)

• N2+ formed in cold cathode ion source; no rotational

cooling

Experimental N2+ Signal

Frequency (cm−1)

Frac

tiona

l Abs

orpt

ion

(× 1

0−7)

No absorption observed!

Absorption

Dispersion

• Absorption signal strongly attenuated by saturation.3 Not observable!• Saturation parameters: 30,000 carrier, 6300 sidebands.• Dispersion signal attenuated by a factor of 2 due to saturation.3Ma et al. J. Opt. Soc. Am. B 2008, 25, 1144–1155.

Spectral Signals

• Obtain line centers, linewidths, and amplitudes from fits• FWHM ≈ 120 MHz (at 4 kV)

From Mills et al. J. Chem. Phys. 2011, 135, 224201.

TOF MS

Mass spectrum of nitrogenic ion beam. Energy spread in inset corresponds to an expected linewidth of 120 MHz.

From Mills et al. J. Chem. Phys. 2011, 135, 224201.

Spectral Signals

• Obtain line centers, linewidths, and amplitudes from fits• FWHM ≈ 120 MHz (at 4 kV)• Noise equivalent absorption ~ 2 × 10−11 cm−1 Hz−1/2 (50× lower than last ion

beam instrument)1

• Within ~1.5 times the shot noise limit!

From Mills et al. J. Chem. Phys. 2011, 135, 224201.

1Coe et al. J. Chem. Phys. 1989, 90, 3893.

Ultra-High Resolution Spectroscopy

• Rough calibration with Bristol wavelength meter (~70 MHz precision)

• Precisely calibrate with MenloSystems optical frequency comb (<1 MHz accuracy)

Frequency Comb Calibrated Spectra

Only ~8 MHz from line center obtained in N2+ positive column work.4

Confident in improvements in the mid-IR.4Siller, B. M. et al. Opt. Express 2011, 19, 24822.

Average the line centers

Average the line centers

Summary and Conclusions• Ion Beam Spectroscopy – effective in studying

molecular ions.• High sensitivity spectroscopy used to study ion

beam – high S/N, Doppler splitting.

• Spectroscopy on rovibronic transitions of N2+ –

first direct spectroscopy of electronic transition in fast ion beam.

• Accurate frequency calibration with optical frequency comb.

Present and Future Work• Ro-vibrational spectroscopy in the mid-

IR• Integration of supersonic cooling

Stay tuned to MG05 for more information!

AcknowledgmentsMcCall Research Group Machine ShopElectronics ShopJim CoeRich SaykallySources of Funding

– Air Force – NASA– Dreyfus– Packard– NSF

– Sloan–Research Corp.– Springborn Endowment