THE PHYSICS, TECHNOLOGY, AND APPLICATIONS OF THE SUBMILLIMETER SPECTRAL REGION. Frank C. De Lucia Ohio State University Columbus, OH 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

Temperature kT (300 K) = 200 cm -1 kT (1.5 K) = 1 cm -1 kT (0.001 K) = cm -1 Fields qE (electron) >> cm -1  E (1 D) ~ 1 cm -1  B (electronic) ~ 1 cm -1  B (nuclear) ~ 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 the same as closing it The Energetics Atoms and Molecules E (electronic) ~ 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 cm -1 FIR cm -1 THz cm -1 SMM 10 – 100 cm -1 MMW cm -1 RF/MW < 1 cm

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 h  vs IMP Technical Detectors/background: The THz is very quiet: 1 mW in 100 Hz ~10 18 K Sources: lasers vs classical sources – size scales Applications – why the SMM? Astrophysical Atmospheric Spectroscopy Sensors: Remote and Local

Jmax  18 Jmax  30 Jmax  55 Jmax  96 Jmax  305 Spectra as a Function of Molecular Size Population of levels

Absorption Coefficients Number Boltzmann Einstein Photon Density Factor Coefficient Size (in long wavelength limit)  Effect: Degeneracy/rotational partition function Emission vs. Absorption Photon Size

Frequency and Temperature Factors (Partition function and degeneracy) (Pressure broadening = Doppler broadening) 10 GHz GHz: K - 3 K: K - 1 K: 3 x 10 7

Collisional Spectroscopy Classical at Ambient Temperature Quantum 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 channels for a source with these characteristics leads to 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 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. h rot ~ kT ~ E well

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

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] K K K K K K K K K In 1 MHz 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 10 9

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.

The Energetics of HNO 3 kT  a = 1.98 D  b = 0.88 D a-type  Ka = 0,  2  Kc =  1,  3 b-type  Ka =  1,  3  Kc =  1,  3 N OO O H

Perturbations in 2 9 in ClONO 2 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 MA2. EIGHT-FIVE YEARS OF SUBMILLIMETER WAVES min. Frank C. DeLucia, Department of Physics, Ohio State University, Columbus, OH, 43210