FASSST Talk 2006.ppt
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Transcript FASSST Talk 2006.ppt
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
Overview
• TECHNIQUE
FAst Scan Submillimeter Spectroscopic Technique
(FASSST).
Frequency measurements
• CHALLENGES
Line intensity measurements
Spectral predictions without assignment or analysis
FAst Scan Submillimeter Spectroscopic Technique (FASSST) spectrometer
Glass rings used to suppress reflections
Lens
Reference
gas cell
BWO
Lens
Path of microwave
radiation
Aluminum cell: length 6 m;
diameter 15 cm
Mylar beam
splitter 2
InSb
detector 2
InSb detector 1
Length ~60 cm
Reference channel
Preamplifier
Interference fringes
Spectrum
Stepper
motor
Signal channel
Mylar beam
splitter 1
Frequency roll-off
preamplifier
Magnet
Ring cavity: L~15 m
Data acquisition
system
Computer
Stainless steel rails
Slow wave structure
sweeper
Filament voltage
power supply
High voltage
power supply
Trigger channel /Triangular waveform channel
FASSST Attributes
1. Can record 10000-100000 resolution elements/sec
Freezes Source Frequency Drift
Freezes Chemistry Changes
2. Frequency measurement accuracy
Typically ~ 30 kHz ( 1/10^7 at 300 GHz)
3. ‘Locally’ (~ 1 GHz) intensity measurement is flat to ~1%
A basis for intensity measurement
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 TA>0.04 K = 2152; 504 are U = 23%
2 mm:
130000-178000 MHz
Lines with T > 0.1 K = 4031; 1606 are U = 40%
1mm:
196750-281000 MHz
Líneas con T>0.1 K: 7676 lines; 3281 are U = 43%
• 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
Bootstrap Analysis of
A and E Ground State Lines
of Methyl Formate
But this is only about 10% of lines
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
Absorption Coefficients
What You Need to Know to Simulate Spectra at an Arbitrary
Temperature T3 without Spectral Assignment
l u n (1 e
h / kT
8 3
)
3ch
2
i,l u
i x,y,z
g l e El / kT
g
n
e En / kT
n0
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
Consider two lines, one assigned and one
unknown at two temperatures T1 and T2
n(T ) / g e
~
(T )
1 e
f (T )
l u
1
l u (T2 )
1
n 0
E n / kT1
n
n(T2 ) / g n e
En / kT2
h / kT1
1 e h / kT2
e El / k (1/ T1 1/ T2 )
n 0
1
~Eqn. 1
f (T2 )
C - constant for all
transitions in the spectru
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
(T )
k
1
k
E l,u sgn
ln l u,n 1 E l,a sgn
ln
(1/T1 1/T2 ) C l u,n (T2 )
(1/T1 1/T2 )
l u,u sgn (T1)
l u,u sgn (T2 )
l u,a sgn (T1)
l u,a sgn (T2 )
Eqn. 2
Comparison of Energy Levels
Calculated from Experimental and
Quantum Calculations for SO2
Comparison of Energy Levels
Calculated from Experimental and
Quantum Calculations for SO2
The Combined Equation
1/ T3 1/ T1
l u, u sgn(T1 ) 1/ T1 1/ T2
l u, u sgn(T3 )
l u, u sgn(T1) l u, u sgn(T2 )
l u, a sgn(T3 )
l u, a sgn(T1) l u, a sgn(T1 )
l u, a sgn(T2 )
Comparison of Intensities Calculated
from Experimental and Quantum
Calculations for SO2
Propagation of Uncertainty (T2 = 300 K)
Collisional cooling
T1 = 77 K
l u , 2 (T3 )
(
T
)
l u ,1 3 l u
l u
l u , 2 (T3 )
l u ,1 (T3 )
(1 / T3 1 / T2 ) 2 (1 / T3 1 / T1 ) 2
2
(1 / T1 1 / T2 ) 2
T1 = 77 K
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.