Experimental Measurements of Collisional Cross Sections and Rates at Astrophysical and Quantum Colli

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Experimental Measurements of Collisional Cross
Sections and Rates at Astrophysical and Quantum
Collisional Temperatures Frank C. De
Lucia Department of Physics Ohio State
University Leiden Center on Herschel
Preparatory Science Leiden December 5 - 7, 2006
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An Experimentalists History and
Perspective Pioneering Theory of Green and
Thaddeus Explore New Experimental Regimes
What is the physics in the regime where kT
hnr Vwell?
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Energy Level vs Collisional Spectroscopy The
Relation between Experiment and Theory
Collisional Spectroscopy ab initio 1
uncertainty no practical equivalent Transition
probabilities are a strong function of
temperature because collision energy provides the
electromagnetic radiation which causes the
transitions. The transition probabilities are
much more complex because they are not action at
a distance and the whole collisional problem
must be quantized. There is not an efficient
parameterizable relation between experimental
measurements and predictions, so We must use
computational methods to make our catalogues,
which we very sparsely check with a measurement,
but we dont need 10-7 accuracy.
Energy Level Spectroscopy ab initio 1
uncertainty parameterized angular momentum
fitting lt 10-7 uncertainty Transition
frequencies and transition probabilities are not
a function of temperature, but intensities are
because of population effects. Transition
probabilities are easy because the only molecular
moment they depend upon is the electric dipole,
which is easy to measure to high accuracy
Action-at-a-distance uses photons to decouple
the QM of the source and that of the
molecules For many simple molecules measure a
subset of lines and predict a large number to
high accuracy, or Quickly measure them all with
modern techniques
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COLLISION COOLING AN APPROACH TO GAS PHASE
STUDIES AT VERY LOW TEMPERATURES
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Typical Spectra - HCN
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Other Systems
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INELASTIC CROSS SECTIONS
Probe Source
Pump Source
Although the measurement of inelastic rates is
much harder than the measurement of pressure
broadening, the inelastic rates agree much better
with theory below 10K
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CROSS SECTIONS FOR CO-He COLLISIONS
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CO-He CROSS SECTIONS
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Doppler Width Are the molecules cooled to the
same temperature as the walls of the cell?
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HCN
1?0 Elastic Cross Section
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What Underlies the Difference between Experiment
and Theory? The Theory Quantum Scattering
Calculations Impact Approximation
Intermolecular Potential ab initio from
Quantum Chemistry Inversion of bound
state energy levels The Experiment The
Pressure - Transpiration The Frequency
Measurements The Temperature Measurements
THE JOURNAL OF CHEMICAL PHYSICS 105, 4005
(1996) Linewidths and shift of very low
temperature CO in He A challenge for theory or
experiment Mark Thachuk, Claudio
E. Chuaqui, and Robert J. Le Roy
Department of Chemistry, The University of
Waterloo
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QUANTUM COLLISIONS
300 K 1 K ________________________
__________
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.
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CH3Cl SEMICLASSICAL ENERGETICS AND ANGULAR
MOMENTUM
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CH3Cl EXPERIMENTAL SEMICLASSICAL CROSS SECTIONS
Initial overpopulation of low J
Relaxation to thermal population
Relaxation to larger, higher J pool of states at
higher temperature
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Final Remarks 1. There is a very different
relation between experiment and theory in
collisional spectroscopy vs energy level
spectroscopy. 2. This is exasperated at low
temperature because of vapor pressure limits on
experiment, but 3. Collisional Cooling provides
an experimental method for the validation of
theoretical results at low temperature. 4. Below
about 10 K there gets to be a significant
difference between experiment and theory
(especially for the lowest J lines) for pressure
broadening. 5. This difference if much less or
missing for inelastic rates. 6. Is there a
transition temperature above which the classical
averaging makes possible more empirical
approaches?