Transcript The physics, technology, and applications of the submillimeter

THE PHYSICS, TECHNOLOGY, AND
APPLICATIONS OF THE SUBMILLIMETER
SPECTRAL REGION.
Frank C. De Lucia
Ohio State University
Columbus, 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
The Energetics
Atoms and Molecules
Temperature
kT (300 K) = 200 cm-1
kT (1.5 K) = 1
cm-1
E (electronic) ~ 50000 cm-1
3
kT (0.001 K) = 0.0007 cm-1
E (vibrational) ~ 1000 cm-1
1
E (rotational) ~ 10 cm-1
[low lying vibration, libration, . . .]
Fields
qE (electron) >> 100000 cm-1
mE (1 D) ~ 1 cm-1
mB (electronic) ~ 1 cm-1
mB (nuclear) ~ 0.001 cm-1
E (fine structure) ~ 0.01 cm-1
Radiation
UV/Vis > 3000 cm-1
2 IR 300 - 3000 cm-1
FIR 30 - 300 cm-1
The THz has defined itself broadly
and spans kT
SMM has left itself less wiggle
room
Jumping the ‘gap in the
electromagnetic spectrum is not
THz 3 - 300 cm-1
SMM 10 – 100 cm-1
MMW 1 - 10 cm-1
RF/MW < 1 cm-1
The Central Theme: hn/kT
Physics
Rich rotational spectrum: hn < kT
Interactions are very strong – peak ~ 1THz
Vibration/rotation Spectroscopy
Collisional Spectroscopy
Play god with kT vs hn vs 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
Spectra as a Function of Molecular Size
Population of levels
A  B  C  25 GHz
Jmax  18
A  B  C  10 GHz
Jmax  30
A  B  C  3 GHz
Jmax  55
A  B  C  1 GHz
Jmax  96
A  B  C  0.1 GHz
Jmax  305
Absorption Coefficients
Number
Density
 mn
NFm

c
Boltzmann
Factor
Einstein Photon
Coefficient Size
h n mn / kT
1

e

 Bmn hn mn
2 1
8 3 
 m mg n  n
2
3h g x, y, z

hn mn / kT
n3 Effect: Degeneracy/rotational partition function
Emission vs. Absorption
Photon Size
(in long wavelength limit)
Frequency and Temperature Factors
mn
2

8 2 N Fm
2

n mn   m mg n 
3ck n T
g x, y, z


1
mn 
3
T5 /2
n mn
T
5 /2
(Partition function and degeneracy)
(Pressure broadening = Doppler broadening)
10 GHz - 1000 GHz: 106
300 K - 3 K:
105
1000 K - 1 K:
3 x 107
Collisional Spectroscopy
Classical at Ambient Temperature
Quantum at Low Temperature
Ambient Collisional Spectrosocopy
Collisions provide a ‘low resolution’ source of radiation
Collisions provide a source of radiation of high multipole moment
Near room temperature, multiplicity of open channels
for a source with these characteristics leads
to near classical results
Quantum Collisional Spectroscopy
at Low Temperature
hnrot ~ kT ~ Ewell
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 Principle
The 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.
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 Channels
hard theory
classical results
Preclusion of many cooling techniques
Technology: Photon >> kT
Molecule Envy?
Collisional/Buffer Gas Cooling
MH07 L. Sarkozy et al.
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
Technology
The Terahertz Gap – Solid-State Sources
[From Tom Crowe UVA/VDI]
In
1 MHz
Solid-State THz Source s (CW)
1019 K
10000
1018 K
1000
1017 K
1015 K
1014 K
Power (mW)
1016 K
100
10
1
1013 K
0.1
1012 K
0.01
1011 K
0.001
10
100
1,000
Frequency (GHz)
10,000
100,000
The THz is VERY Quiet even for CW
Systems in Harsh Environments –
it is NOT ‘Plagued by Noise’
Experiment: SiO vapor
at ~1700 K
109
All noise from 1.6 K
detector system
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
MH09 D. Petkie et al.
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
The Energetics of HNO3
Iv 
1400
H
O
1200
N
1000
O
2n6
3n9
n7+2n9
n4
n3
2n7
n6+n9
n7+n8 n5+n9
n6+n7
n8+n9
n7+n9
2n9
O
800
ma = 1.98 D
mb = 0.88 D
n5+n7
n6+n8
600
a-type Ka = 0, 2
Kc = 1, 3
400
b-type Ka = 1, 3
Kc = 1, 3
200
n5
n8
n6
n7
n9
kT
0
1
I gs
1000
FC01 Z. Kisiel et al.
Perturbations in 2n9 in ClONO2
Perturbation of > 1 GHz are fit to <0.1 MHz
85 Years of Submillimeter Spectroscopy
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
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