PROGRESS & RESULTS IN THE DEVELOPMENTS OF THE SENSITIVE, COOLED, RESOLVED ION BEAM

SPECTROMETER (SCRIBES)

Andrew Mills, Brian Siller, Michael Porambo, Manori Perera, Holger Kreckel, Ben McCall

University of Illinois @ Urbana/Champaign

Ion Beam NICE-OHMS

• Motivation

• Ion beam setup

• Line shape

• N2+ signal

• Sensitivity

• Spectroscopy characteristics

• Future plans

Measured rotational temperature {maybe}

Motivations for studying molecular ions

Fundamental:

Structure of molecular ions

Astrochemical Systems:

Drive chemistry in interstellar medium (ISM)

Need spectra to locate in ISM

Challenges to studying ionsReactive, transient species:

Production under harsh conditions (discharge)

Discharges often rotationally and vibrationally excited ions

Weak signal from dilute analyte

Large background of neutrals and other excited species

Direct Absorption Ion Spec. Techniques

Ion-neutral discrimination

Low rotational temperature

Narrow linewidth

Cavity-enhanced spectroscopy

CE VelocityModulation

Supersonic

Expansion

Hollow Cathode

High ion density

Ion Beam Velocity Modulation

Mass Spectrum

Mass ID of Spectral Line

Setup

drift tube (overlap) variable apertures

electrostatic deflector 1

steerers

Einzel lens 1

Einzel lens 2

electrostatic deflector 2

TOF beam modulation electrodes

wire beam profile monitors

retractableFaraday cup

electronmultiplierTOF detector

ion source

Brewsterwindow

Brewster windowFaradaycup

S _ R I Be S

Ion source – Currently uncooledIon opticsCurrent measurementsCo-linearity with laserMass spectrometerLaser coupling

Coe et al., JCP 90, 3893 (1989)

Concentration / velocity modulation

SensitiveCooledResolvedIonBEamSpectroscopy

EOM Laser

Cavity ModesSideband spacing

Laser

Mass ID

Mass ID of Spectroscopic Lines

Ion Beam

Laser

2

21

'

Mc

qV

10

5

0

-5

-10

x10-6

10853.7010853.6510853.6010853.5510853.5010853.45

Frequency (cm-1

)

-6

-4

-2

0

2

4

6

x10-6

10865.2510865.2010865.1510865.1010865.05

10859.344 10865.20 10865.2510853.45 10853.50

1.0

0.8

0.6

0.4

0.2

0.0

10000.0410000.0210000.009999.989999.96

1.0

0.8

0.6

0.4

0.2

0.0

10000.0410000.0210000.009999.989999.96

1.0

0.8

0.6

0.4

0.2

0.0

10000.0410000.0210000.009999.989999.96

Rest “Transition”

Blue “Transition”

Red “Transition”

Line Shape

CM Line shapeABSORPTION DISPERSION

1.0

0.8

0.6

0.4

0.2

0.0

10000.0410000.0210000.009999.989999.961.0

0.5

0.0

-0.5

-1.0

10000.0410000.0210000.009999.989999.961.0

0.5

0.0

-0.5

-1.0

10000.0410000.0210000.009999.989999.96

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

10.0000410.0000210.000009.999989.99996x10

3

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

10000.0410000.0210000.009999.989999.96

2

1

0

-1

-2

10000.0410000.0210000.009999.989999.96

Overall line shape

0fm

VM Line shapeABSORPTION DISPERSION

1.0

0.5

0.0

-0.5

-1.0

10000.0410000.0210000.009999.989999.96

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

10000.0410000.0210000.009999.989999.96

-3

-2

-1

0

1

10000.0410000.009999.96

ABSORPTIONDISPERSION

Concentration Modulation Line shape

Velocity Modulation

-3

-2

-1

0

1

10000.0410000.009999.96

-3

-2

-1

0

1

10000.0410000.009999.96

vm

10 V

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

10000.0410000.0210000.009999.989999.96

1.0

0.5

0.0

-0.5

-1.0

10000.0410000.0210000.009999.989999.96

Example SpectraDISPERSION

Velocity Modulation

Concentration Modulation

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

10000.0410000.0210000.009999.989999.96

VBeam ~ 3865 V

qQ22(14.5) N2+

= 30 s

= 1 s

-150

-100

-50

0

50

100

150

x10-6

10865.2810865.2610865.24

Frequency (cm-1

)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

20

10

0

-10

-20

x10-6

10865.2810865.2610865.24

Frequency (cm-1

)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

SensitivityExpected fractional signal strength:

CavityFinesse

Pathlength

LineStrength

IonDensity

LinewidthHeterodyne Loss

Mid IR implications:

HN2+ without any rotational cooling

Spectroscopy of larger carbocations (like CH5+ and C3H3

+) will require rotational cooling

Observed Expected Factor off

2.71E-07

1.48E-07

5.25E-07

4.62E-07

1.94

3.11

VM

CM

VM

N2+

N2+

HN2+ 2.4E-06

Equivalent Fractional Absorption

S_RIBES CharacteristicsSensitiveCooledResolvedIonBEamSpectroscopy

Ion density 6x106 ion/cm3

Ion neutral discrimination

Complete spatial, and modulation discrimination from excited neutrals.

Rotational temperature

~ 600 K. Surprisingly low temperature.

Supersonic cooling available.

Linewidth

~ 120 MHz in the NIR. ~33 MHz in midIR.

Mass spectrometry of ions

Confirms species probed… Optimize plasma conditions.

Mass ID of spectral lines

Compare with OKA Saykally VM of positive column.Hirota, Amano, Hollow CathodeSupersonic Expansion Maier, Nesbit

NICE-OHMS VMS DFG

Piezo

Functiongenerator

Ti:Sapph

PPLNBD

LP

Frequency comb

Wavemeter

YAG

Preliminary OptimizationIncreased finesse and refined laser locking

-150

-100

-50

0

50

100

150

x10-6

10865.2810865.2610865.24

Frequency (cm-1

)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

20 V

20

10

0

-10

-20

x10-6

10865.2810865.2610865.24

Frequency (cm-1

)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

10 V

= 30 s

= 10 s Coe et al., JCP 90, 3893 (1989)

HF+

Conclusions

• Using NICE-OHMS, an N2+ equivalent absorption

signal has been obtained from our ion beam. • The NICE-OHMS-S_RIBES technique:

– Yields narrow linewidth spectral lines– Yields mass ID for every spectral line– Yields complete ion/neutral discrimination– Is compatible with supersonic cooling– Is sensitive enough to compensate for low ion density

• The signal strength matches up with expected values.• Construction of a mid-IR DFG NICE-OHMS setup will

soon begin.

AcknowledgmentsAir Force Young

Investigator Award

Visit us at: http://bjm.scs.uiuc.edu

NASA LaboratoryAstrophysics

NSF Chemistry, Physics, Astronomy

Dreyfus New Faculty, Teacher-Scholar Awards

Packard Fellowship

CottrellScholarship

SloanFellowship