PROGRESS & RESULTS IN THE DEVELOPMENTS OF
THE SENSITIVE, COOLED, RESOLVED ION BEAM
SPECTROMETER (SCRIBES)
University of Illinois @ Urbana/Champaign
Andrew Mills, Brian Siller, Michael Porambo, Manori Perera,
Holger Kreckel, Ben McCall
Ion Beam NICE-OHMS
•
•
•
•
•
•
•
Motivation
Ion beam setup
Line shape
N2+ signal
Sensitivity
Spectroscopy characteristics
Future plans
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 ions
Reactive, 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
High ion density
Ion-neutral discrimination
Low rotational temperature
Narrow linewidth
Cavity-enhanced spectroscopy
Mass Spectrum
Mass ID of Spectral Line
Hollow
Cathode
Supersonic
Expansion














Setup
CE Velocity Ion Beam Velocity
Modulation
Modulation














Sensitive
Cooled
Resolved
Ion
BEam
Spectroscopy
S _ R I Be S
Laser
retractable
Brewster
Faraday cup
window
Einzel lens 2
TOF beam modulation electrodes
electrostatic deflector 2
drift tube (overlap)
variable apertures
electrostatic deflector 1
steerers
ion source
wire beam profile monitors
Ion source – Currently uncooled
Ion optics
Current measurements
Co-linearity with laser
Mass spectrometer
Laser coupling
Concentration / velocity modulation
electron
multiplier
TOF detector
Einzel lens 1
Faraday
cup
Brewster window
EOM
Laser
Cavity Modes
Sideband spacing
Coe et al., JCP 90, 3893 (1989)
Mass ID
Mass ID of Spectroscopic Lines
Red
“Transition”
10
10865.05
10865.15
10865.20
1.0
5
0.8
0.8
0.8
0.6
0.6
0.6
0.4
0.4
0.4
-5
0.2
0.2
0.2
0.0
0.0
0.0
10865.25
6
4
2
0
0
-2
-4
-6
-10
9999.96
Ion Beam
Laser
9999.98
10000.00
10000.02
10000.04 9999.96
9999.98
10000.00
10000.02
10000.04 9999.96
9999.98
10000.00
10000.02
10000.04
10853.45 10853.50
10853.50 10853.55
10859.34410853.60
10865.20
10865.25
10853.45
10853.65
10853.70
-1
Frequency (cm )
Line Shape
'
2qV
 1
2

Mc
-6
x10
10865.10
1.0
Blue
“Transition”
x10
-6
1.0
Rest
“Transition”
ABSORPTION
fm 0
999.98
999.98
DISPERSION
CM Line shape
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
10000.00
10000.02
10000.04
2
9.99996
9.99998
10.00000
3
x10
10.00002
9999.98
10000.00
10000.0
1
0
-1
-2
10000.00
10000.02
1.5
10000.04
1.0
0.5
Overall line shape
0.0
-0.5
-1.0
-1.5
9999.96
ABSORPTION
DISPERSION
VM Line shape
1.5
1.0
0.5
0.0
Concentration Modulation Line shape
-0.5
-1.0
10 V
9999.98
9999.98
10000.00
vm
10000.00
10000.02
10000.02
-1.5
10000.04
1.5
10000.04
1.0
0.5
0.0
1
1
-0.5
-1.0
0
0
-1.5
9999.96
9999.98
10000.00
1000
9999.96
9999.98
10000.00
10000.02
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
9999.96
10000.00
10000.02
10000.04
-1
ABSORPTION
Velocity Modulation
-1
DISPERSION
-2
-2
-3
-3
9999.98
10000.00
10000.02
10
Example Spectra
DISPERSION
4.0
t =3.51 s
20
3.0
10
x10
-6
2.5
2.0
0
1.5
-10
1.0
0.5
-20
9999.98
10000.00
10000.02
10865.24
10000.04
10865.26
10000.04
-1
10000.00
Concentration
Modulation
10865.28
Frequency (cm )
4.0
VBeam ~ 3865 V
150
3.5
100
q
3.0
Q22(14.5) N2+
x10
-6
50
2.5
0
2.0
-50
1.5
-100
1.0
-150
0.5
Velocity Modulation
10865.24
10865.26
-1
Frequency (cm )
10865.28
t = 30 s
Sensitivity
Expected fractional signal strength:
Cavity
Path
Line
Ion
Heterodyne Loss
Finesse length Strength Density
Linewidth
Equivalent Fractional Absorption
Observed
Expected
Factor off
N2+
VM
2.71E-07
5.25E-07
1.94
N2+
CM
1.48E-07
4.62E-07
3.11
HN2+
VM
2.4E-06
Mid IR implications:
HN2+ without any rotational cooling
Spectroscopy of larger carbocations (like CH5+ and C3H3+) will require rotational cooling
S_RIBES Characteristics
Ion density
6x106 ion/cm3
Ion neutral discrimination







Sensitive
Cooled
Resolved
Ion
BEam
Spectroscopy
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
NICE-OHMS VMS DFG
Ti:Sapph
Frequency
comb
Wavemeter
YAG
Function
generator
PPLN
LP
Piezo
Preliminary Optimization
Increased finesse and refined laser locking
20 V
4.0
150
3.5
HF+
100
3.0
t = 30 s
50
x10
-6
2.5
0
2.0
-50
1.5
-100
-1
10865.24
-20
20
1.0
Frequency (cm )
-150
10865.26
4.0
10865.24
10865.26
10 V
Frequency (cm
-1
)
10865.28 0.5
3.5
1.0
3.0
-10
10
-6
1.5
2.5
0
0
2.0
2.0
6-
01x10
x
0.5
10865.28
2.5
1.5
10
-10
3.0
1.0
3.5
0.5
20
-20
4.0
10865.24
10865.26
-1
Frequency (cm )
10865.28
t = 10 s
Coe et al., JCP 90, 3893 (1989)
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.
Acknowledgments
Air Force Young
Investigator Award
Dreyfus New
Faculty, TeacherScholar Awards
NASA Laboratory
Astrophysics
NSF Chemistry,
Physics, Astronomy
Packard
Fellowship
Cottrell
Scholarship
Visit us at: http://bjm.scs.uiuc.edu
Sloan
Fellowship