the physics, technology, and applications of the submillimeter spectral region. frank c. de lucia
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THE PHYSICS, TECHNOLOGY, AND APPLICATIONS OF THE SUBMILLIMETER
SPECTRAL REGION.Frank C. De Lucia
Ohio State UniversityColumbus, OH 43210
The scope of interest in the submillimeter (a.k.a. terahertz, far infrared, millimeter, near millimeter, etc.) region of the electromagnetic spectrum is growing at an ever-expanding rate. High-resolution molecular spectroscopy continues not only to be at the core of this interest, but also is expanding its impact on emerging fields and their technology. This talk will focus on the relation of the underlying physics and technology of the submillimeter to past, present, and future applications. Emphasis will be on the high-resolution applications most closely associated with this meeting: spectroscopy, chemical physics, astronomy, atmospheric remote sensing, and diagnostics.
Physics
TemperaturekT (300 K) = 200 cm-1
kT (1.5 K) = 1 cm-1
kT (0.001 K) = 0.0007 cm-1
FieldsqE (electron) >> 100000 cm-1
E (1 D) ~ 1 cm-1
B (electronic) ~ 1 cm-1
B (nuclear) ~ 0.001 cm-1
The THz has defined itself broadly and spans kTSMM has left itself less wiggle roomJumping the ‘gap in the electromagnetic spectrum is not the same as closing it
The EnergeticsAtoms and Molecules
E (electronic) ~ 50000 cm-1
E (vibrational) ~ 1000 cm-1
E (rotational) ~ 10 cm-1
[low lying vibration, libration, . . .]
E (fine structure) ~ 0.01 cm-1
Radiation
UV/Vis > 3000 cm-1
IR 300 - 3000 cm-1
FIR 30 - 300 cm-1
THz 3 - 300 cm-1
SMM 10 – 100 cm-1
MMW 1 - 10 cm-1
RF/MW < 1 cm-1
1
3
2
The Central Theme: h/kT Physics Rich rotational spectrum: h < kT Interactions are very strong – peak ~ 1THz Vibration/rotation Spectroscopy Collisional Spectroscopy Play god with kT vs hvs IMP
Technical Detectors/background: The THz is very quiet: 1 mW in 100 Hz ~1018 K Sources: lasers vs classical sources – size scales
Applications – why the SMM? Astrophysical Atmospheric Spectroscopy Sensors: Remote and Local
GHzCBA 25 Jmax 18
GHzCBA 10 Jmax 30
GHzCBA 3 Jmax 55
GHzCBA 1 Jmax 96
GHzCBA 1.0 Jmax 305
Spectra as a Function of Molecular SizePopulation of levels
mn NFmc
1 e hmn / kT Bmn hmn
Absorption CoefficientsNumber Boltzmann Einstein PhotonDensity Factor Coefficient Size
8 3
3h2 m g n2
gx, y, z
1
hmn / kT (in long wavelength limit)
Effect: Degeneracy/rotational partition function Emission vs. Absorption Photon Size
Frequency and Temperature Factors
mn 8 2
3ckN
FmT
mn2 m g n
2
gx, y, z
mnT 5 / 2 (Partition function and degeneracy)
1 (Pressure broadening = Doppler broadening)
mn 3
T 5 / 210 GHz - 1000 GHz: 106
300 K - 3 K: 105
1000 K - 1 K: 3 x 107
Collisional Spectroscopy
Classical at Ambient TemperatureQuantum at Low Temperature
Collisions provide a ‘low resolution’ source of radiation
Collisions provide a source of radiation of high multipole moment
Near room temperature, multiplicity of open channelsfor a source with these characteristics leadsto near classical results
Ambient Collisional Spectrosocopy
Quantum Collisional Spectroscopy at Low Temperature
Only a few Rotational States Energetically Available
Low Energy/Temperature lead to Quasi-bound States
Collisions have Small Angular Momentum Quantum Numbers
Collisional Spectroscopy can be ‘High Resolution’
Correspondence PrincipleThe predictions of the quantum theory for the behavior of any physical system must correspond to the prediction of classical physics in the limit in which the quantum numbers specifying the state of the system become very large.
hrot ~ kT ~ Ewell
Atom Envy, Molecule Envy: [the Grass is Greener on the Other Side of the Fence]
Atom Envy:
Science: Rotational and Vibrational Partition Function
Dilution of Oscillator Strength
Complexity of ‘Open’ Collisional Channelshard theoryclassical results
Preclusion of many cooling techniques
Technology: Photon >> kT
Molecule Envy?Collisional/Buffer Gas Cooling
Why Else are We Interested?To explore new experimental regime
A regime in which ‘exact’ calculations are possible
Collisions in the astrophysical regime
We can
MH07 L. Sarkozy et al.
Technology
The Terahertz Gap – Solid-State Sources[From Tom Crowe UVA/VDI]
Solid-State THz Sources (CW)
0.001
0.01
0.1
1
10
100
1000
10000
10 100 1,000 10,000 100,000
Frequency (GHz)
Pow
er (m
W)
1019 K
1018 K
1017 K
1016 K
1015 K
1014 K
1013 K
1012 K
1011 K
In 1 MHz
The THz is VERY Quiet even for CW Systems in Harsh Environments –
it is NOT ‘Plagued by Noise’Experiment: SiO vapor at ~1700 K
All noise from 1.6 K detector system
109
Design Space: The FASSST Spectrometer as an example
FASSST Spectrum
MH08 S. Fortman et al.TH05 I. Medvedev et al.TH06 C. Casto et al.
Applications
Quantitative end-to-end
designs based on known signatures
Ro-Vibrational Spectroscopy
With the growth in resolution of infrared instruments in both the laboratory and the field and the increase in spectral coverage of microwave techniques what were two separate field studying two separate problems (rotational and vibrational spectroscopy) have truly become one.
This merger however is very complex because of the amount of data
Data bases have provided an invaluable basis for transferring information to our customers
Impact on careers of young scientists – citations
Data bases have been very good about showing the sources of information
We need to help them
MH09 D. Petkie et al.
1400
1200
1000
800
600
400
200
0
7
9
6
8
5
29
7+9
27
39
6+9
8+9
6+7
2634
7+8 5+9
6+8
5+77+29
The Energetics of HNO3
kT
gsv II1000
1
a = 1.98 Db = 0.88 D
a-type Ka = 0, 2
Kc = 1, 3
b-type Ka = 1, 3
Kc = 1, 3
N
O O
O
H
Perturbations in 29 in ClONO2
FC01 Z. Kisiel et al.
Perturbation of > 1 GHz are fit to <0.1 MHz
85 Years of Submillimeter Spectroscopy
Wireless communications industry will have made sources and detectors ~ free But clever system design will still be at a premium
Down Looking smart arrays from orbit to look down (MLS) and up (Herschel)
Penetrability will be widely exploited (but is a steep inverse function of frequency)
ALMA will have become as famous as Hubble – a great success
We will continue to develop techniques to detect ever smaller signals and quantities of material – taking advantage of spectral brightness and electronic frequency control
There will still be many unassigned lines in relatively common molecules
This will still be an exciting community to work in and the people in it will still be a joy to work with
MONDAY, JUNE 11, 2038 – 7:30 A. M.Auditorium, Independence Hall
Chairman: Jay Gupta, Chair, Department of Physics, Ohio State University, Columbus, OH 43210
MA2. EIGHT-FIVE YEARS OF SUBMILLIMETER WAVES . . . . . . . . . . 10 min.
Frank C. DeLucia, Department of Physics, Ohio State University, Columbus, OH, 43210
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