High Precision, Sensitive, Near-IR Spectroscopy in a Fast Ion Beam

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

MWAM 2011University of Illinois at Urbana-Champaign

22 October 2011

Outline

• Introduction

• Description of Instrument

• Results

• Summary

• Future Work

Molecular Ions in Astrochemistry

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

HCN

CH3NH2

CH3CN

C2H5CN

N, e

NH3, e

HCN, eCH3

CN, e

e

CO, e

H2O, e

CH3OH, e

CH

CH2CO

CH3OH

CH3OCH3

CH3+

C2H5+e

C2H4

e

C3H2

e

C3H

e

C2H

Experimental laboratory spectra needed to aid in theoretical, observational work.

Ion Production Techniques

Oka, Saykally, McCall Maier, Nesbitt

Ion-neutral discrimination

Low rotational temperature

Narrow linewidth

Compatible with cavity-enhanced spectroscopy

Positive Column

Supersonic

ExpansionHollow

Cathode

Hirota, Amano

High ion column density

We want…

Sensitive, Cooled, Resolved Ion BEam Spectrometer – SCRIBES

Ion Production Techniques

Oka, Saykally, McCall Maier, Nesbitt

Ion-neutral discrimination

Low rotational temperature

Narrow linewidth

Compatible with cavity-enhanced spectroscopy

Mass spectrometry of laser-probed ions

Spectral identification of ion mass

Velocity

Modulation

Supersonic

ExpansionSCRIBES

Hollow

Cathode

Hirota, Amano

High ion column density

McCall

So we want…

First Generation Ion Beam Instrument

Coe, J. V. et al. J. Chem. Phys. 1989, 90, 3893–3902.

Direct Laser Absorption Spectroscopy in Fast Ion Beams –DLASFIB.

Pioneered by Saykally group in late 1980s– early 1990s.1,2,3

Studied HF+, HN2+,

HCO+, H3O+, NH4+ in the

mid-infrared, with no supersonic expansion.

Lacked sensitivity to see larger or more complex ions, especially at high temperature.

1Coe et al., J. Chem. Phys. 1989, 90, 3893–3902.2Owrutsky et al., J. Phys. Chem. 1989, 93, 5960–5963.3Keim et al., J. Chem. Phys. 1990, 93, 3111–3119.

Ion Beam Setup

Ion Beam Setup

Ion Beam Setup

Source Chamber

Cold Cathode Ion SourceNote: No rotational

cooling

Beam Deflector7

4Kreckel, H. et al. Rev. Sci. Instrum. 2010, 81, 063304.

Time-of-Flight Region

Mass Spec Detector

Cavity Mirror

Detector

Spectroscopy: Heterodyne Detection

Single Carrier Frequency

Carrier + Other Frequencies

Electro-optic modulator

EOMAnalyte

113 MHz 113 MHz

Relative Frequency (MHz)

Relative Frequency (MHz)

Heterodyne DetectionAbsorption

Dispersion

+-

Cavity EnhancementNoiseImmuneCavityEnhancedOpticalHeterodyneMolecularSpectroscopy (NICE-OHMS)5,6

Analyte

Optical cavity increases pathlength by factor of ~100

EOM

5Ye, Ph.D. Dissertation, University of Colorado Department of Physics, 1997.6Foltynowicz et al., Appl. Phys. B 2008, 92, 313–326.

Relative Frequency (MHz)

Cavity Modes

Laser Frequencies

Spectroscopy LayoutPZT

EOM

~113 MHz

Lock-In Amplifier

Lock-In Amplifier

Lock-In Amplifier

Lock-In Amplifier

Vel. Mod.~ 4 kHz

Dispersion

AbsorptionDetector

DispersionAbsorption

Ion Beam

Doppler Splitting

0red blue

Amount of shifts depend on the mass of the ion

Relative Frequency

First Spectroscopic Target• Obtain rovibronic spectral transitions of

A 2u – X 2g+ 1–0 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

A 2u – X 2g+, qQ22(14.5) line

Frequency (cm−1)

Fra

ctio

nal A

bsor

ptio

n (×

10−

7 )

No absorption observed!

Absorption Lock-In Amplifier Output

Dispersion Lock-In Amplifier Output

NICE-OHMS absorption signal strongly affected by saturation; saturation of the ions decrease the absorption to below the

noise

Spectral Signals

• FWHM ≈ 120 MHz (at 4 kV)• Noise equivalent absorption ~ 4 × 10−11 cm−1 Hz−1/2 (50× lower than last ion beam instrument)• Within ~1.5 times the shot noise limit

A 2u – X 2g+, qQ22(14.5) line,

red- and blue-shifted

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 SpectraA 2u – X 2g

+, qQ22(14.5) line,

red- and blue-shifted

Only ~8 MHz from linecenter obtained in N2+ positive column work.6

Confident in improvements in the mid-IR.6Siller, B. M. et al. Opt. Express 2011, Accepted.

Average the line centers

Average the line centers

Summary and Conclusions• Fast ion beam spectroscopy can be very

effective for general molecular ion spectroscopy.

• Integrated NICE-OHMS and velocity modulation spectroscopy for performing sensitive measurements of ion beam.

• Operational spectroscopy on rovibronic transitions of the Meinel band of N2

+ – first direct spectroscopy of electronic transition in fast ion beam.

• Performed precisely calibrated measurements with optical frequency comb to get line centers to an accuracy of ~8 MHz.

Present and Future WorkVibrational spectroscopy in the mid-IR

• Finished construction of mid-IR DFG laser at 3.0 µm.

• Produced HN2+ in

the ion beamSupersonic Expansion Discharge Source7

• Enable rotational cooling

H3+

HN2+

Increasing N2

Time-of-flight mass spectra of hydrogenic ion beam with increasing amounts of N2

• 750 K with cold cathode;<100 K with supersonic source?

• Vibrational spectroscopy of rotationally cooled molecular ions (CH5

+, C3H3+, etc.)

Supersonic expansion discharge source7Crabtree, K. N. et al. Rev. Sci. Instrum. 2010, 81, 086103.

AcknowledgmentsMcCall Research Group

Machine Shop

Electronics Shop

Jim Coe

Rich Saykally

Sources of Funding– Air Force – NASA– Dreyfus– Packard– NSF

– U of Illinois– Springborn Endowment