Transcript Slide 1

Brian Siller, Michael Porambo & Benjamin McCall
Chemistry Department
University of Illinois at Urbana-Champaign

Applications
◦ Astrochemistry
◦ Fundamental physics

Goals
◦ Completely general (direct absorption)
◦ High resolution



Molecular ions are important
to interstellar chemistry
Ions important as reaction
intermediates
>150 Molecules observed in
ISM
CH
Only ~20 are ions
CH OCH
Need laboratory data to
CH OH
provide astronomers with CH CO
HO
spectral targets
HO
2


C6H7+
H2
3
C2H2
C4H3+
H
C4H2+
C3H2
C3H3
OH
3
e
H2
C+
C2H2
C2H
e
C2H3
C2H5
+
+
e
C+
CH3+
CH4
e
CH5+
3
C2H5CN
H2
CH3CN
H2
CH3NH2
CH3+
CH2+
CH
HCN
H2
+
H2
+
C3H+
2
e
C3H
C
e
3
2
C6H6
C6H5+
4
e
e
CH+
H2O+
H2
C
OH+
O
H3
+
H2
H2+
HCO+
CO


Combination differences to compute THz
transitions by observing rovibrational transitions
in the mid-IR
Support for Herschel, SOFIA, and ALMA THz
60-670 µm
0.3-1600 µm
3-400 µm
observatories
5
3280
J’
4
Even
Combination
differences
3260
2
3240
Odd
Combination
Differences
80
1
0
-1.0
-0.5
0.0
0.5
1.0
0. 0
1.5
2.0
0. 5
6
60
1-0 Rotational
Transition
Reconstructed
Rotational
Transitions
3
cm-1
IR Transitions
5
cm-1
5
3300
40
4
20
3
0
2
1
0
0-1.0
. 0 -0.5
0.0
0.5
1.0
J”
1.5 . 5
2.0
0

CH5+ is a prototypical carbocation
◦ SN1 reaction intermediates
◦ Highly fluctional structure
◦ Spectrum completely unassigned
E.T. White, J. Tang, and T. Oka, “CH5+: The Infrared Spectrum Observed”,
Science, 284, 135-137 (1999).
Animation from Joel Bowman, Emory University

Positive Column
◦ High ion density
◦ Simple setup

Ion Beam
◦ Rigorous ion-neutral
discrimination
◦ Mass-dependent Doppler
shift

Positive column discharge cell
◦ High ion density, rich chemistry
◦ Cations move toward the cathode
+1kV
-1kV
Plasma Discharge Cell

Positive column discharge cell
◦ High ion density, rich chemistry
◦ Cations move toward the cathode
◦ Ions absorption profile is Doppler-shifted
+1kV
-1kV
Laser
Plasma Discharge Cell
Detector

Positive column discharge cell
◦ High ion density, rich chemistry
◦ Cations move toward the cathode
◦ Ions absorption profile is Doppler-shifted
-1kV
+1kV
Laser
Plasma Discharge Cell
Detector

Positive column discharge cell
◦ High ion density, rich chemistry
◦ Cations move toward the cathode
◦ Ions absorption profile is Doppler-shifted

Drive with AC voltage
◦ Ion Doppler profile alternates red/blue shift
◦ Laser at fixed wavelength
◦ Demodulate detector signal at modulation frequency
Laser
Plasma Discharge Cell
Detector
0
1


Want strongest absorption possible
Signal enhanced by modified White cell
◦ Laser passes through cell unidirectionally
◦ Can get up to ~8 passes through cell
Laser
Plasma Discharge Cell

Detector
Also want lowest noise possible, so
combine with heterodyne spectroscopy

Single-pass
direct
absorption
0
1

Single-pass
Heterodyne @
1GHz
2

Doppler-broadened lines
◦ Blended lines
◦ Limited determination of line centers

Sensitivity
◦ Limited path length through plasma

Improve by combining
with cavity enhanced
absorption spectroscopy
Ti:Sapph
Laser
PZT
Polarizing
Beamsplitter
Detector
EOM
Detector
AOM
30MHz
Cavity
Transmission
Quarter
Wave Plate
0.1-60kHz
Lock
Box
<100Hz
Error Signal
Audio Amplifier
Transformer
Laser
Cavity Mirror Mounts
40 kHz
Lock-In
Amplifier


Absorption Strength (Arb. Units)

Doppler profile shifts back and forth
Red-shift with respect to one direction of the
laser corresponds to blue shift with respect to
the other direction
Net absorption is the sum of the absorption
in each direction
Relative Frequency (GHz)


Demodulate detected signal at twice the
modulation frequency (2f)
Can observe and distinguish ions and
neutrals
◦ Ions are velocity modulated
◦ Excited neutrals are concentration modulated
◦ Ground state neutrals are not modulated at all

Ions and excited neutrals are observed to be
~75° out of phase with one another






Cavity Finesse 150
30mW laser power
N2+ Meinel Band
N2* first positive band
Second time a Lamb dip of
a molecular ion has been
observed (first was DBr+ in
laser magnetic resonance
technique)1
Used 2 lock-in amplifiers
for N2+/N2*
B. M. Siller, A. A. Mills and B. J. McCall, Opt. Lett., 35, 1266-1268. (2010)
1M.
Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991)
0
1
2


Line centers determined
to within 1 MHz with
optical frequency comb
Sensitivity limited by
plasma noise
A. A. Mills, B. M. Siller, and B. J. McCall, Chem. Phys. Lett., 501, 1-5. (2010)

Noise Immune Cavity Enhanced Optical
Heterodyne Molecular Spectroscopy
Cavity Modes
Laser Spectrum
J. Ye, L. S. Ma, and J. L. Hall, JOSA B, 15, 6-15 (1998)
Ti:Sapph
Laser
PZT
Polarizing
Beamsplitter
EOM
Detector
AOM
30MHz
Quarter
Wave Plate
Lock
Box
Detector
Ti:Sapph
Laser
PZT
EOM
Detector
Ti:Sapph
Laser
Detector
PZT
EOM
EOM
90°
Phase
Shift
113 MHz
Cavity FSR
Lock-In
Amplifier
X
Y
Lock-In
Amplifier
X
Y
Absorption Dispersion
Signal
Signal
40 kHz
Plasma
Frequency
113 MHz
Sidebands
1 Cavity FSR
Dispersion
Lock-In X
Lock-In Y
Absorption
Dispersion
Absorption
Lock-In X
Lock-In Y
No center
Lamb dip in
absorption
Spectra calibrated with optical frequency comb
Frequency precision to <1 MHz!
Ultra-High Resolution Spectroscopy
Dispersion
Absorption
113MHz
Sub-Doppler fit based on pseudo-Voigt
absorption and dispersion profiles
(6 absorption, 7 dispersion)
Line center from fit: 326,187,572.2 ± 0.1 MHz
After accounting for systematic problems, line center measured to
within uncertainty of ~300 kHz!
VMS
CEVMS
OHVMS
NICE-OHVMS
NICE-OHVMS

Better sensitivity than traditional VMS
◦ Increased path length through plasma
◦ Decreased noise from heterodyne modulation


Retained ion-neutral discrimination
Sub-Doppler resolution
◦ Better precision & absolute accuracy with comb
◦ Resolve blended lines

Can use same optical setup for ion beam
spectroscopy
Ion Beam
Instrument
Ti:Sapph
Laser
Detector
PZT
EOM
EOM
Lock-In
Amplifier
X
Y
Lock-In
Amplifier
X
Y
Absorption Dispersion
Signal
Signal
40 kHz
Plasma
Frequency
Laser
S _ R I Be S
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
Einzel lens 1
Faraday
cup
Brewster window
wire beam profile monitors electron
multiplier
TOF detector
Ion source
Ion optics
Current measurements
Co-linearity with laser
Mass spectrometer
Laser coupling
Velocity modulation
±5V ~ ±100MHz
Ground
4kV
2kV



Ion density ~5×106 cm-3
Cavity finesse ~450
Lock-in τ=10s



4kV float voltage
±5V modulation
~120MHz linewidth
'
2qV
 1
2

Mc
Ion mass
Float voltage

Positive Column
◦ High ion density
◦ Simpler setup
◦ Direct measurement of
transition rest frequency

Ion Beam
◦ Rigorous ion-neutral discrimination
◦ Simultaneous mass spectroscopy
◦ Mass identification of each spectral
line
◦ No Doppler-broadened component
of lineshape

Positive Column
◦ Mid-IR OPO system
 ~1W mid-IR idler power
 Pump and signal lasers referenced to optical frequency comb
◦ Liquid-N2 cooled discharge cell

Ion Beam
◦ Mid-IR DFG laser
 Ti:Sapph referenced to comb
 Nd:YAG locked to I2 hyperfine transition
◦ Supersonic expansion discharge source

McCall Group
◦ Ben McCall
◦ Michael Porambo

Funding