Transcript Slide 1

INDIRECT TERAHERTZ SPECTROSCOPY OF
MOLECULAR IONS USING HIGHLY ACCURATE
AND PRECISE MID-IR SPECTROSCOPY
Andrew A. Mills, Kyle B. Ford, Holger Kreckel, Manori
Perera, Kyle N. Crabtree, Benjamin J. McCall
University of Illinois, Department of Chemistry
June 24, 2009
Indirect THz Spectroscopy Outline
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Motivations
How-to
Radiation source
Increasing accuracy and precision
Target ions
Sample production and detection
Future direction
THz or IR Spectroscopy
• THz
– The THz regime opens new possibilities for
interesting spectroscopy
– Growing yet limited availability of THz sources
– High sensitivity techniques infrequent
• IR
– Many available sources (OPO, DFG, QCL)
– Historical accuracy ~30 MHz
• Indirect THz via precise IR spectroscopy
5
Indirect THz - HNN+ & Other Linear Molecules
3280
J’
4
3260
Even Combination
differences
3
cm-1
IR Transitions
2
3240
80
Odd Combination
Differences
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
5
Reconstructed
Rotational
Transitions
cm-1
5
3300
40
4
20
3
0
2
1
0
0.0
-1.0
-0.5
0.0
0.5
1.0
J”
0.5
1.5
2.0
Test Molecule (HNN+) - Proof of Concept
Dn THz ~ DnIR + DnMicrowave
1.0
1.0
FIB #'s and the
Fit
Dv (Line Using
Accuracy)~30
MHz
Normalized Intensity (max @ 30K)
Normalized Intensity (max @ 30K)
J Lower Reconstruction
Terahertz
0.8 Fit Difference
0.8
Using FIB #'s and
the
J Lower Reconstruction
Terahertz
Difference
GHz
GHz
MHz
0
93.17
93.17
0
GHz
GHz
MHz
30 K
1
186.36
186.34
14
0
93.17
93.17
0
0.6
0.6
2
279.52
279.51
7
1
186.36
186.34
14
300 K
3
372.70
372.67
25
2
279.52
279.51
7
1000 K
4
465.84
465.82
12
3
372.70
372.67
25
0.4
0.4
5
559.00
558.97
31
4
465.84
465.82
12
6
652.12
652.10
20
5
559.00
558.97
31
7
745.24
745.21
34
6
652.12
652.10
20
8
838.33
838.31
18
7
745.24
745.21
34
0.2
0.2
9
931.42
931.39
35
8
838.33
838.31
18
10
1024.46
1024.44
14
9
931.42
931.39
35
11
1117.51
1117.48
27
10
1024.46
1024.44
14
0.0
0.0
12
1210.49
1210.49
8
11
1117.51
1117.48
27
3050
3100
3050 3150
3100 3200
3150
13
1303.48
1303.47
13
12
1210.49
1210.49
8
Frequency (cm-1)
14
1396.41
1396.42
9
13
1303.48
1303.47
13
15
1489.33
1489.34
5
14
1396.41
1396.42
9
16
1582.19
1582.23
32
15
1489.33
1489.34
5
1) Increase IR transition accuracy
17
1675.04
1675.08
33
16
1582.19
1582.23
32
17
1675.04
1675.08
33
3250
3200 3300
Frequency (cm-1)
Gudeman, C. S., Begemann, M. H. Pfaff, J.; Saykally, R. J. Velocity Modulated IR laser spectroscopy of Molecular Ions: The v1
band of HNN+ J. Chem. Phys 78(9) 1983 5837-8
P. Verhoeve, E. Zwart, M. Versluis, M. Drabbels, J.j. ter Meulen, W.Leo Meerts, A. Dymanus. Rev. Sci. Instrum. 61(6) 1990 16121625.
DFG
532 nm
pump
laser
Nd:YAG
1064 nm
Ti:Sapph
700 – 1000 nm
l/4
l/2
Reference
Cavity
AOM
InSb
PPLN
LP filter
l-meter
20 MHz Accuracy
Polarising
Prism
Achromat
l/2
Optical
Isolator
dichroic
Fabry-Perot Interferometer
Fiber-Optic to
Frequency Comb
Menlo Systems Optical Frequency Comb
• Stabilized mode-locked fs laser FT equally spaced lines in frequency
• Frequency reference OCXO disciplined to GPS
– 1x10-12 in 1s  200 Hz Accuracy

• Unknown laser
comb 
• Laser frequency ~ 1-100 kHz
Beat Frequency
l/2
l/2
PBS
Target ions – Small Linear Polyatomics
• IR transitions and J=10 Microwave
transitions recorded
• HNN+, HCS+, HOC+, HCO+, CO+, CH+, HCNH+
are known interstellar molecules.
• Probe of chemical intermediates and
conditions
• Make in plasma discharge
detector
Ion Beam Spectrometer
@ 925 nm
•NarrowingTi:Sapph
of line-widths
1st bender
lens
Source chamber
Optical Isolator
•Physically separate ions
l-meter
from neutrals
Etalon
FPI
dependent
Doppler
Finder
•Mass
splitting from cavity
AOM
•Characterize w/ N2+
apertures
lens beam modulator
2nd bender
drift
region
Mode
Matching
Optics
TOF
detector
cw-CRDS
cw-CRDS/Concentration Modulation CRDS
•99.99% RD Mirrors
33 ms time constant
amin =4x10-9 cm-1
•Concentration Modulation
Beam on/ Beam off
amin =5x10-10 cm-1
Reduces long term Drift
Representative Scans 1-0 A2Pu-X2Sg+
-9
Absorbance
x10 (cm-1)
x10-9
amin=4.5e-10
20
1.0
15
0.8
10
0.6
5
0.4
0
-5
0.2
-10
924.405
924.406
924.407
924.408
Wavelength (nm)
924.409
924.410
detector
Ion Beam Velocity Modulation
pzt
1st
•Use lower finesse cavity
lens
bender
Source chamber
•Use lock in detection to
further reduce noise and
increase signal
steerer
PLL
apertures
lens
2nd bender
•Modulate the velocity of the
ion beam by applying field to
drift region
beam modulator
drift
region
TOF
detector
Ion BeamReconstruction
+ Frequency
Comb
= SPRIBES
? Teraher
Terahertz Difference
DISCHARGE
Reconstruction
-1
cm-1
l (cm-1)
MHz
cmMHz
cm-1
3.108
3.108
9E-060
0.27
0.273.108
3.108
Low
Comb
6.216
6.216
0.0005
14.0
0.014
1
6.216 Accuracy
6.216
Accuracy
Reconstruction
Terahertz
Difference
DISCHARGE
SPRIBES
9.324
9.324
0.0002
6.89.324
0.007
2
9.324
-1
-1
cm 12.432
cm
l
(cm-1)
MHz
MHz
12.431
0.0008
25.2
0.025
3J”
12.432
12.431
3.108
3.108
9E-06
0.27
0.27
15.539
15.538
0.0004
11.8
0.012
4
15.539
15.538
6.216
6.216
0.0005
14.0
0.014
18.646
18.645
0.0010
31.1
0.031
5
18.646
18.645
9.324
9.324
0.0002
6.8
0.007
21.752
21.752
0.0007
19.9
0.020
6
21.752
21.752
12.432
12.431
0.0008
25.2
0.025
24.859
24.858
0.0011
33.7
0.034
7
24.859
24.858
15.539
15.538
0.0004
11.8
0.012
27.964
27.963
0.0006
18.2
0.018
8
27.964
27.963
18.646
18.645
0.0010
31.1
0.031
31.069
31.068
0.0012
34.8
0.035
9
31.069
31.068
21.752
21.752
0.0007
19.9
0.020
34.172
34.172
0.0005
14.4
0.014
10
34.172
34.172
24.859
24.858
0.0011
33.7
0.034
37.276
37.275
0.0009
27.4
0.027
11
37.276
37.275
27.964
27.963
0.0006
18.2
0.018
40.378
40.377
0.0003
7.59
0.008
12
40.378
40.377
31.069
31.068
0.0012
34.8
0.035
43.479
43.479
0.0004
13.4
0.013
13
43.479
43.479
34.172
34.172
0.0005
14.4
0.014
46.579
46.580
0.0003
9.5
0.009
14
46.579
46.580
37.276
37.275
0.0009
27.4
0.027
49.679
49.679
0.0002
5.4
0.005
15
49.679
49.679
40.378
40.377
0.0003
7.59
0.008
52.776
52.777
0.0011
31.8
0.032
16
52.776
52.777
43.479
43.479
0.0004
13.4
0.013
55.873
55.875
0.0011
33.1
0.033
17
55.873
55.875
46.579
46.580
0.0003
9.5
0.009
58.968
58.970
0.0021
63.6
0.064
18
58.968
58.970
49.679
49.679
0.0002
5.4
0.005
62.063
62.065
0.0025
73.8
0.074
19
62.063
62.065
52.776
52.777
0.0011
31.8
0.032
cm-1
0
1
2
3
40
51
62
73
84
95
6
10
7
11
8
12
9
13
10
14
11
15
12
16
13
17
14
18
15
19
16
• Ion beam 
Narrower line widths
• Narrower line widths 
accurate line frequencies
• Well calibrated lasers, 
highly accurate spectra
• Traditional accuracy 30 MHz,
expect 2-3 orders of magnitude
better than that
Conclusions
• The THz regime opens new possibilities for
interesting spectroscopy
• Indirect spectroscopy can be used to obtain the
transitions for many interesting molecular ions of
the interstellar medium
• Indirect spectroscopy requires
– Highly accurate and precise frequency references
such as a frequency comb
– Sub-Doppler line widths from fast ion beams
– Sensitive, cavity enhanced spectroscopy
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