.Mid-Infrared NICE-OHMS Spectrometer for the Study of

Cold Molecular IonsMichael Porambo, Jessica Pearson, Courtney Talicska, Benjamin J. McCallISMSJune 18, 2014WG12

Outline

• Background and Motivation• Spectrometer/Instrumentation• Present and Future Work

Why molecular ions?Why spectroscopy?

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

Janne Karaste, http://commons.wikimedia.org/wiki/File:Midsummer_bonfire_closeup.jpg

Combustion

Efficiency of engines†

†Thomas, Mass Spec. Rev., 2008, 27, 485–530.

Atmospheric Chemistry

Interstellar Reactions

•

http://earthobservatory.nasa.gov/IOTD/view.php?id=7373

Resolved, high precision spectroscopy to study

these fields All molecular models drawn in Avogado (http://avogadro.cc/wiki/Main_Page)

Laboratory Astrochemistry

Complex, congested spectra from interstellar medium

Simpler, standard spectra from lab of hypothesized molecules

Better understanding of the ISM (e.g., detection of more molecules)

ESO/C. Malin, http://www.eso.org/public/images/ann13016a/

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

neutrals• High rotational temperature

Overcoming Low Number Density

• Cavity enhancement to increase signal

• Heterodyne/frequency modulation spectroscopy to decrease noise

EOM

Heterodyne/Frequency Modulation

Heterodyne/Frequency Modulation

EOM

113 MHz

Relative Frequency (MHz)

EOM

Relative Frequency (MHz)

113 MHz

FMS Signal

Cavity Enhancement

Heterodyne/Frequency Modulation

Noise Immune Cavity-Enhanced

Optical Heterodyne Molecular

Spectroscopy (NICE-OHMS)†

†Ye et al., JOSA B, 1998, 15, 6–15.

EOM

NICE-OHMS

Sideband SpacingCavity Longitudinal Modes

Laser Spectrum

Spectroscopy sidebands must couple into longitudinal modes

NICE-OHMS/NICE-OHVMS Projects

Plasma discharge

Supersonic expansion

Overcoming high rotational temperature

Other talks:MK06FA01

Continuous Supersonic Expansion Discharge Source

Source running with nitrogen (N2+ discharge)

H3+ temperature: 80–110 K†

High pressure in line (≈2 atm)

100 mTorr chamber pressure

Tiny pinhole (0.5 mm diameter)

Low temperature ions

Spectroscopy of this “zone of

silenceӠCrabtree et al., Rev. Sci. Instrum., 2010, 81, 086103.

Difference Frequency Generation

Ti:Sapph (700–1100 nm)

Nd:YAG (1064 nm)

PPLN

EOM

2.8–4.8 µm

Mid-IR NICE-OHMS Spectrometer

Difference Frequency Generation

setup

To EOM

RF Generator (113 MHz)

One of the first broadly tunable mid-IR NICE-OHMS spectrometers†

†Porambo et al., Opt. Lett., 2012, 37, 4422–4424.

NICE-OHMS Signal

Mid-IR NICE-OHMS Spectrometer

Difference Frequency Generation

setup

To EOM

Lock-In Amplifier

RF Generator (113 MHz)

IGBT Switching

Circuit

Function Generator (1–10 kHz)

Adapted from diagram by Thomas Houlahan, Jr.

-1 kV

Source

15 V Power Supply

1-10 kHz

Opto-Isolator

DC/DC Isolator

IGBT Driver

IGBT

Insulated-Gate Bipolar Transistor (IGBT) Switching Circuit

V+

V1GND1In

V2GND2Out

+Vin

-Vin

+Vout

-Vout

VInGND

Out

V-

13000 m3/hr Roots blower system

Temperature Determination with Methane

Estimated methane rotational temperature of 300±28 K

Lower State Energy (cm-1)

ln (I/S)

Present and Future Work• H3

+ spectroscopy to test temperature of ion

• HN2+ spectroscopy to test efficient cooling

• High precision spectroscopy of ions of astrochemical/fundamental interest

High Precision Spectroscopy

Difference Frequency Generation

setup(2.8–4.8 µm)

Optical Frequency Comb

H2CO+C3H3+

Summary• Molecular ions key in combustion,

atmospheric, interstellar reactions• Challenging to study, but overcome

with NICE-OHMS on supersonic expansion discharge source

• Preliminary temperature determinations promising; ultimate goal is precision spectroscopy of cooled ions

Acknowledgments

Springborn Endowment, UIUCNASA Earth and Space Science Fellowship

Thomas Houlahan, Jr.