andrew mills, brian siller, michael porambo, manori perera, holger kreckel, ben mccall
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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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
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Ion Beam NICE-OHMS
• Motivation
• Ion beam setup
• Line shape
• N2+ signal
• Sensitivity
• Spectroscopy characteristics
• Future plans
Measured rotational temperature {maybe}
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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
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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
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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
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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
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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
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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
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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
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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
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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
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NICE-OHMS VMS DFG
Piezo
Functiongenerator
Ti:Sapph
PPLNBD
LP
Frequency comb
Wavemeter
YAG
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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+
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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.
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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