New THz Spectroscopic Tools
for Laboratory Astrochemistry
Susanna L. Widicus Weaver
Department of Chemistry
Emory University
Motivation: Understanding COMs in the ISM
Grain surface formation
hν
• Simple molecules form in ice via
single-atom addition reactions
• Organic radicals form in ice via
photolysis of simple molecules
• Radicals react during warm-up
to form larger organics
H2O, CO, CH3OH,
NH3 , H2CO
Ice mantle
H
O
Gas phase formation
• Molecules are released from ices
H N
H
C
H
CH3OH2+
H
or H3+
H N
H
H
H
H
O
O
C
HCOOH
-H2O
C
protonated
aminomethanol
aminomethanol
• Gas-phase molecules are ionized
• Ion-molecule reactions drive
gas-phase organic chemistry
H
H
+ O
H N
H
C
H
H
glycine
Transient molecules are the driving forces for both grain-surface and gas-phase chemistry.
2 atoms
3 atoms
4 atoms
5 atoms
6 atoms
7 atoms
8 atoms
9 atoms
10 atoms
11 atoms
12 atoms
13 atoms
H2
C3
c-C3H
C5
C5H
C6H
CH3C3N
CH3C4H
CH3C5N
HC9N
C6H6
HC11N
AlF
C2H
l-C3H
C4H
l-H2C4
CH2CHCN
HC(O)OCH3
CH3CH2CN
(CH3)2CO
CH3C6H
C2H5OCH3
AlCl
C2O
C3N
C4Si
C2H4
CH3C2H
CH3COOH
(CH3)2O
(CH2OH)2
C2
C2S
C3O
l-C3H2
CH3CN
HC5N
C7H
CH3CH2OH
CH3CH2CHO
CH
CH2
C3S
c-C3H2
CH3NC
CH3CHO
H2C6
HC7N
CH+
HCN
C2H2
H2CCN
CH3OH
CH3NH2
CH2OHCHO
C8H
CN
HCO
NH3
CH4
CH3SH
c-C2H4O
l-HC6H
CH3C(O)NH2
CO
HCO+
HCCN
HC3N
HC3NH+
H2CCHOH
CH2CHCHO
C8H–
CO+
HCS+
HCNH+
HC2NC
HC2CHO
C6H–
CH2CCHCN
C3H6
CP
HOC+
HNCO
HCOOH
NH2CHO
SiC
H2O
HNCS
H2CNH
C5N
HCl
H2S
HOCO+
H2C2O
l-HC4H
KCl
HNC
H2CO
H2NCN
l-HC4N
NH
HNO
H2CN
HNC3
c-H2C3O
NO
MgCN
H2CS
SiH4
H2CCNH
NS
MgNC
H3O+
H2COH+
NaCl
N2H+
c-SiC3
C4H–
OH
N2O
CH3
PN
NaCN
SO
OCS
SO+
SO2
SiN
c-SiC2
SiO
CO2
SiS
NH2
CS
H3+
HF
H2D+, HD2+
SH
SiCN
HD
AlNC
FeO
SiNC
O2
HCP
CF+
SiH
PO
Detected Interstellar Molecules
~40% are Radicals and Ions
Production Methods
•
•
•
•
Small quantities (low efficiency)
High temperatures = weak signals
Interference from stable molecules
Reactivity/instability of products
Discharges
hν
Photolysis
Matrix
Isolation
Supersonic Expansions
High-Sensitivity Cavity-Enhanced Spectroscopy
~2 – 50 GHz
> 1000 cm-1
The ‘THz Gap’
3 GHz
0.1 cm-1
30 GHz
1 cm-1
300 GHz
10 cm-1
FTMW
10 cm
3 THz
100 cm-1
30 THz
1000 cm-1
THz
1 cm
1 mm
300 THz
10,000 cm-1
3000 THz
100,000 cm-1
CRDS
100 µm
ALMA Spectral Coverage
10 µm
1 µm
100 nm
Laboratory Spectral Cataloging
First light April 1, 2009!
1 – 50 GHz
Frequency
Synthesizer
To Computer
VDI Multiplier chain
50 GHz – 1.2 THz
Detector
Gas Flow Cell
Sample Input
Methanol
To Vacuum Pump
Ethyl Cyanide
CRDS High Finesse Cavity
R = 99.99%
Radiation
Source
Supersonic
Source
Detector
Mode Matching
Optics
cavity ringdown
recorded
IR mirrors → dielectric coated
Losses due to transmission
THz mirrors → metal coated
Losses due to skin depth
FTMW High Finesse Cavity
R ≈ 98%
Radiation
Source
Aperture: r << l
Supersonic
Source
Switch
X
Detector
free-induction
decay recorded
microwave mirrors → aperture
Large λ, small losses
THz mirrors → ?
Small λ, large losses with any aperture!
Proposed THz-CRDS Spectrometer
To Computer
Supersonic
Source
HEB Detector
Gold - Coated
Mirror
Off-Axis
Parabolic Mirror
Wire Grid
Polarizers
Transmission =
≤700 GHz
R = 99.99%
10-4
Mode Matching
Optics
VDI multiplier chain
50 GHz – 1.2 THz
1 – 50 GHz
Frequency
Synthesizer
Progress Toward THz-CRDS
Beam profiling completed,
mode-matching calculations performed
Polarizer reflectivity
tested up to 300 GHz
R = 99.9 – 99.99%
Progress Toward THz-CRDS
Benchtop optics setup completed
Next Steps:
• Finish bench-top cavity tests:
requires QMC HEB detector
(expected to ship next week)
• Place cavity in vacuum chamber
• Test fully-integrated system
• Begin molecular spectroscopy
• Extend system to higher frequencies
• Extend concept to broadband
spectral acquisition
Future Work: FT-THz Spectroscopy
Similar cavity setup to THz-CRDS
Sample is polarized with high-power
radiation pulse, FID recorded
Lower mirror R required: Optics are less
demanding, modes are broader than CRDS
Hardware requirements are beyond current
(commercially) available technology:
•Nb HEB heterodyne detector
(can be custom-built by Prober group)
•Higher power amplifiers at higher
frequencies (none commercially
available; spare Herschel amps??)
In collaboration with Geoff Blake at
Caltech and Dan Prober at Yale
Future Work: TD-THz-CRDS
Spherical
Mirror
Ti:Sapph
Laser
ZnTe
Polarizer
THz radiation
ZnTe
Polarizer
• Broadband THz radiation is generated via
optical rectification of laser output
• THz radiation is coupled into cavity; cavity
output signal is upconverted to visible light
Beamsplitter
Detector
Detector
Mirror
Mirror
• Ringdown signal is monitored directly
• Second beam is coupled into interferometer;
time-dependence of ringdown is monitored
• Broadband frequency spectrum is deconvolved
In collaboration with Tim Lian
and Brian Dyer at Emory
Using THz Spectroscopy to
Trace Prebiotic Chemistry in Space
What do we plan to measure?
• Photolysis branching ratios for complex organics
• Spectra of small, reactive organics produced via O(1D) insertion
• Spectra of molecular ions with complex internal motion
• THz spectral catalogs of “interstellar weeds”
Acknowledgements
The Widicus Weaver Group:
Mary Radhuber
Jay Kroll
Brandon Carroll
Jake Laas
Thomas Anderson
Brett McGuire (not pictured)
Emory University &
Emory Dept. of Chemistry
Geoffrey Blake, Caltech
Dan Prober, Yale
Tim Lian and Brian Dyer, Emory
Virginia Diodes, Inc.
QMC Instruments, Ltd.