Michael Porambo, Jessica Pearson, Craig Riccardo, Benjamin J. McCall

International Symposium on Molecular Spectroscopy, The Ohio State University

June 20, 2013

Expansion Discharge Source for Ion Beam Spectroscopy of

Cold Molecular Ions

Outline• Motivation• Review of Ion Beam Instrument• Ion Source Work• Summary

Molecular Ions

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

Atmospheric Reactions

Interstellar ChemistryCH5

+

C6H7+

H2CO+

Combustion Processes

Fundamental Chemical Physics

Oka Ion Factory, http://fermi.uchicago.edu/research/ch5plus/

http://www.navy.mil/view_single.asp?id=40674

Resolved, high precision spectroscopy to study these

fields

Challenges• Low number density• Small fraction of ions compared to

neutrals• High rotational temperature

Ion Production Methods

Low rotational temperature

No ion-neutral discrimination

Positive Column Velocity Modulation

Hollow Cathode Discharge Cell

No discrimination between ions and neutral molecules

Ion-neutral discrimination

High rotational temperature

Supersonic Expansion Discharge

Ion Production MethodsWay to bring low rotational temperature and

ion-neutral discrimination together?

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

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

- advances in technology open new opportunities- can obtain both ion-neutral discrimination and low rotational temperatures

Sensitive, Cooled, Resolved Ion BEam Spectroscopy - SCRIBES

TOF massspectrometer

Sourcechamber

Overlapregion

Electrostatic Bender2

Low ion density106 cm-3

Make up for this with cavity-enhanced spectroscopy

2Kreckel 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 DetectionEOM

NICE-OHMS Signal

Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy

Heterodyne/Frequency Modulation Detection for Lower Noise

Spectroscopic DetectionEOM

Lock-In Amplifier

NICE-OHMS Signal

Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy

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

Spectroscopic DetectionEOM

Lock-In Amplifier

NICE-OHMS Signal

Frequency calibrated with optical frequency comb

Line centers to ~1 MHz!

AccomplishmentsOperational mass spectrometer

Comb calibrated hot ion beam spectra in near-infrared (N2

+)

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

Mid-infrared optical system finished and optimized on CH4

First broadly tunable NICE-OHMS spectrometer in mid-IRFrom Porambo et al., Opt. Lett., 2012, 37, 4422.

Next: Supersonic Expansion Discharge Source

Produces ions with low rotational temperature

Designed and characterized with H3

+

cavity ring-down spectroscopy3

H3+ rotational

temperature of 80–110 K3

3Crabtree et al., Rev. Sci. Instrum., 2010, 81, 086103.

Supersonic expansion discharge source in ion beam source chamber producing a nitrogenic expansion

Instrument RearrangementIon source chamber moved to connect to Roots blower system

Leybold RUTA 13000 system

10” bellows connected to Roots blower

system

Unistrut brace to prevent

chamber from rising

Beam skimmer used to extract ions from expansion

Source Chamber

Beam SkimmersMachined in-house from EDM graphite4

Two designs:

4Jordan et al., Rev. Sci. Instrum., 1999, 70, 1640.5Keim, E. PhD Thesis, UC Berkeley, 1992.

1 2

Length

(in.)

Orifice Diameter

(mm)1 0.5 32 1.024 0.8

Difference Frequency Generation

Preliminary Studies: Characterization of HN2

+ Expansion

InSb

Double-pass setup to perform wavelength-modulation optical heterodyne spectroscopy

Rovibrational lines of HN2

+ will be used to estimate a rotational temperature

Instrument Rearrangement

Difference Frequency Generation

setup

Determine HN2+

rotational temperature by NICE-OHMS in ion beam

Summary• Molecular ion spectroscopy is challenging but

useful for studying many phenomena• Ion beam spectrometer development with ion-

neutral discrimination, NICE-OHMS, and cold ions

• Successful first signal and mid-infrared arrangement; now integrating source

• Source works well in setup, attempting to finish characterization of rotational temperature

• Preparing to extract ion beam from expansion

AcknowledgmentsMcCall Research Group Former Members

Brian SillerAndrew MillsManori PereraHolger KreckelKyle CrabtreeCarrie Kauffman

NASA Earth and Space Science Fellowship Program

Springborn Endowment