Title Unimolecular Decomposition of Chemically Activated ...
Rate C onstants and Kinetic Energy Releases in Unimolecular Processes , Detailed Balance Results
description
Transcript of Rate C onstants and Kinetic Energy Releases in Unimolecular Processes , Detailed Balance Results
Rate Constants and Kinetic Energy Releases in Unimolecular Processes,
Detailed Balance Results
Klavs HansenGöteborg University and
Chalmers University of TechnologyIgls, march 2003
Realistic theories:RRKM, treated elsewhereDetailed BalanceV. Weisskopf, Phys. Rev. 52, 295-303 (1937)
Same physicsDifferent formulae Same numbers?
Yes (if you do it right)
Physical assumptions for application of detailed balance to statistical processes
1) Time reversal,2) Statistical mixing, compound cluster/molecule: all memory of creation is forgotten at decay
General theory, requires input: Reaction cross section, Thermal properties of product and precursor
Detailed balance equation
Number of states (parent)
Evaporation rate constant
Number of states (product)
Formation rate constant
Density of state of parent, product
Detailed balance (continued)
(single atom evaporation)
Important point: Sustains thermal equilibrium,Extra benefit: Works for all types of emitted particles.
D = dissociation energy = energy needed to remove fragment,OBS, does not include reverse activation barrier. Can be incorporated (see remark on cross section later, read Weisskopf)
Ingredients1) Cross section2) Level densities of parent3) Level density of product cluster4) Level density of evaporated atom
ObservableObservableObservableKnown
Angular momentum not considered here.
Microcanonical temperatureTotal rates require integration over kinetic energy releases
Define
OBS: Tm is daughter temperature
Total rate constants, example
Geometrical cross section:
Numerical examples
Evaporated atom Au = geometric cross section = 10Å2
Evaporated atom C = geometric cross section = 10Å2
(Monomer evaporation)
g = 2
g = 1
Dimer evaporationReplace the free atom density of states with the dimer density of states (and cross section)
Integrations over vibrational and rotational degreesof freedom of dimer give rot and vib partition function:
Kinetic energy releaseGiven excitation energy, what is the distribution of the kinetic energies released in the decay?
Depends crucially on the capture cross section for the inverse process,
Stating the cross section in detailed balance theory is equivalent to specifying the transition state in RRKM
Measure or guess
Kinetic energy release
General (spherical symmetry):
Geometric cross section:
Langevin cross section:
Capture in Coulomb potential:
Simple examples:
Kinetic energy releaseSpecial cases: Motion in spherical symetric external potentials. Capture on contact.
0 1 2 3 4 5
geometriccross section
Langevin cross section
Coulombpotential
KE
R d
istri
butio
ns (a
rb. u
nits
)
Kinetic energy release (Tm)
If no reverse activation barrier,values between 1 and 2 kBTm:
Geometric cross section: 2 kBTm Langevin cross section: 3/2 kBTm
Capture in Coulomb potential: 1 kBTm
OBS: The finite size of the cluster will often change cross sections and introduce different dependences.
Average kinetic energy releases
No reverse activation barrier
Reverse activation barrier
Barriers and cross sections
Reaction coordinate Reaction coordinate
= 0 for < EB
EB
Level densitiesVibrational degrees of freedom dominates
Calculated as collection of harmonic oscillators.Typically quantum energy << evaporative activation energy
At high E/N:
(E0 = sum of zero point energies)
More precise use Beyer-Swinehart algorithm,but frequencies normally unknown
Warning: clusters may not consist of harmonic oscillators
Level densities
Examples of bulk heat capacities:
0 50 100 150 200 250 3000
1
2
3
4
5
Cp /
R
T (K)
Level DensitiesHeat capacity of bulk water
What did we forget?
Oh yes, the electronic degrees of freedom.
Not as important as the vibrational d.o.f.s but occasionally still relevant for precise numbers or special cases (electronic shells, supershells)
Easily included by convolution with vib. d.o.f.s (if levels known), or with microcanonical temperature