FAST SCAN SUBMILLIMETER SPECTROSCOPIC TECHNIQUE FASSST'

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FAST SCAN SUBMILLIMETER SPECTROSCOPIC TECHNIQUE
(FASSST). IVAN R. MEDVEDEV, BRENDA P.
WINNEWISSER, MANFRED WINNEWISSER, FRANK C. DE
LUCIA, DOUGLAS T. PETKIE, MARKUS BEHNKE, RYAN P.
A. BETTENS, and ZBIGNIEW KISIEL
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Overview

• TECHNIQUE
• FAst Scan Submillimeter Spectroscopic Technique
  (FASSST).
• Frequency measurements
• CHALLENGES
• Line intensity measurements
• Spectral predictions without assignment or
  analysis

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FAst Scan Submillimeter Spectroscopic Technique
(FASSST) spectrometer
Magnet
Mylar beam splitter 1
Glass rings used to suppress reflections
Lens
Reference gas cell
InSb detector 1
BWO
Lens
Aluminum cell length 6 m diameter 15 cm
Length 60 cm
Path of microwave radiation
Frequency roll-off preamplifier
Mylar beam splitter 2
Reference channel
InSb detector 2
Preamplifier
Interference fringes
Spectrum
Signal channel
Stepper motor
Data acquisition system Computer
Ring cavity L15 m
Stainless steel rails
Slow wave structure sweeper
Trigger channel /Triangular waveform channel
Filament voltage power supply
High voltage power supply
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• FASSST Attributes
• 1. Can record 10000-100000 resolution
  elements/sec
• Freezes Source Frequency Drift
• Freezes Chemistry Changes
• 2. Frequency measurement accuracy
• Typically 30 kHz ( 1/107 at 300 GHz)
• 3. Locally ( 1 GHz) intensity measurement
  is flat to 1
• A basis for intensity measurement

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Identification of the U lines in the
interstellar molecular spectra. Spectral
predictions without assignment or
analysis. Orion. IRAM 30-m telescope line survey
3 mm 79750-115750 MHz Lines with TAgt0.04 K
2152 504 are U 23 2 mm130000-178000 MHz
Lines with T gt 0.1 K 4031 1606 are U
40 1mm196750-281000 MHz Líneas con Tgt0.1 K
7676 lines 3281 are U 43
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• Large fraction of U lines most likely
  corresponds to isotopologues and vibrational
  excited states of well known molecules like
  CH3OCH3, CH3OH, methyl formate
• To identify lines unambiguously we need to know
  their frequencies and intensities at the
  temperature of the observed interstellar region
• To predict the intensity we need to know the
  lower state energy and the transition dipole
  moment matrix element
• The proposed scheme allows us to make spectral
  predictions without assignment or analysis

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Bootstrap Analysis of A and E Ground State
Lines of Methyl Formate
But this is only about 10 of lines
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Prediction scheme

• Measure spectrum of the molecule at two
  temperatures
• Ratio the intensities of all spectral lines
• Given the lower state energy of a single
  previously assigned transition we can calculate
  lower state energies and relative intensities at
  the desired sample temperature for the frequency
  region where power profile of the spectrometer is
  known

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Absorption Coefficients What You Need to Know
to Simulate Spectra at an Arbitrary Temperature
T3 without Spectral Assignment

• The total number density (chemistry and pressure
  issues).
• But, for an unassigned line, one does not know
• -The matrix element
• -The lower state energy
• -The partition function

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Consider two lines, one assigned and one unknown
at two temperatures T1 and T2
Eqn. 1

• Step 1 With Eqn. 1 for both the known and
  unknown line, we have two equations and two
  unknowns
• 1. The number density and partition function
  ratio for the T1 and T2 lab measurements
• 2. The lower state energy of the unassigned
  line
• Step 2 Solve for the lower state energy of
  unassigned line

Eqn. 2
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Comparison of Energy Levels Calculated from
Experimental and Quantum Calculations for SO2
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Comparison of Energy Levels Calculated from
Experimental and Quantum Calculations for SO2
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The Combined Equation
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Comparison of Intensities Calculated from
Experimental and Quantum Calculations for SO2
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Propagation of Uncertainty (T2 300 K)
Collisional cooling
T1 77 K
T1 77 K
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Summary and Conclusions From experimental
measurements at two temperatures T1 and T2, it is
possible to calculate spectrum (with intensities)
at an arbitrary T3. For low T3, a relatively
low T2 improves the accuracy of the calculated
spectrum. Collisional cooling provides a
general method for achieving this low T2, with 77
K convenient and suitable for all but the lowest
temperatures. FASSST is a means of obtaining
the needed data rapidly and with chemical
concentrations constant over the data collection
period.