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