popdistributionsdiatmollect18me501f2011
DESCRIPTION
PopDistributionsDiatMolLect18ME501F2011TRANSCRIPT
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Purdue University
School of Mechanical Engineering
ME 501: Statistical ThermodynamicsLecture 18: Population Distributions
for Diatomic Molecules
Prof. Robert P. Lucht
Room 2204, Mechanical Engineering BuildingSchool of Mechanical Engineering
Purdue UniversityWest Lafayette, Indiana
[email protected], 765-494-5623 (Phone)
October 17, 2011
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Lecture Topics
• The nuclear partition function.
• Rotational distributions for heteronuclear molecules.
• Vibrational distributions.
• Electronic distributions.
• Rotational distributions for homonuclear molecules.
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The Nuclear Partition Function
• Let IA and IB be the spins of the ground states of nuclei A and B (don't have to consider excited nuclear states very often):
, ,
, ,
2 1 2 1
2 1 2 1
o A A o B B
nuc o A o B A B
g I g I
Z g g I I
• The nuclear partition function is often grouped with the rotational partition function because of the strong influence of nuclear spin on the relative intensities of even and odd J lines for homonuclear molecules,
1 2 1 2 1rot nuc A Brot
Z Z I I
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Rotational Distribution for a Heteronuclear Molecule
• Boltzmann distribution law is applied separately to each mode: for the translational mode this leads to the Maxwell velocity distribution that will be discussed in detail later.
• Rotational distribution: Assume that T >> rot, heteronuclear molecule, electronic level with electronic degeneracy =1. Then:
v
v
2 1 exp 1 /exp //
rotJ J Bl J
l rot rot
J J J Tg k TNN Z T
• Note that the level population is proportional to (2J+1).
• Determination of the rotational level with the maximum population. Assume that J varies continuously:
vmax
102 2
l J
rot
dN Tfor JdJ
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Rotational Distribution for the OH Molecule• For OH in the ground electronic state, Be = 18.911 cm-1, rot = 27.2 K.
T (K) Jmax
500 2.5
1000 3.5
1500 4.5
2000 5.5
2500 6.5
3000 7.5
Jmax rounded to
nearest half-
integer
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Vibrational Distribution
• Include zero-point energy (ZPE):
Note: ZPE contribution divides out.
vib vibv 1vib2
vib
exp / 1 exp /exp v /
exp / 2B vibl
l vib
g k T TN TN Z T
v exp v / 1 exp /lvib vib
l
N T TN
• Do not include zero-point energy (ZPE):
vib vibvvib
vib
exp /exp v / 1 exp /Bl
vibl
g k TN T TN Z
Note: You get the same answer as long as you are consistent in your treatment of ZPE.
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Electronic Distribution
• Not much simplification because we don’t have an analytical expression for the energy of different electronic levels in a diatomic molecule:
exp /elecl l Bl
elec
g k TNN Z
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Rotational Distributions for Homonuclear Molecules: Hydrogen
• For nuclei of odd mass number (fermions) the total wavefunction tot = rot
vib elec nuc must be antisymmetric with respect to exchange of nuclei:
, , , ,
, , , ,
, , , ,
, , , , v
tot tot
rot rot
rot rot
vib vib
x y z x y z
x y z x y z J odd
x y z x y z J even
x y z x y z all
• Diatomic Hydrogen: Diatomic hydrogen has a symmetric ground electronic level. We construct the following table:
1g
1H2 rot vib elec nuc tot
J even + + + - -J odd - + + + -
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Rotational Distributions for Homonuclear Molecules: Hydrogen
• For H2 molecules with a symmetric nuclear spin wavefunction, only rotational states with odd J are possible. For H2 molecules with an antisymmetric nuclear spin wavefunction, only rotational states with even Jare possible.
• There are a total of (2I + 1)2 = (go)2 nuclear spin states, and of the nuclear spin states are go (go+1)/2 are symmetric, and go (go-1)/2 are antisymmetric.
• The population of the rotational levels is thus given by:
( , )
( , )
1 2 1exp 1 /
2
1 2 1exp 1 /
2
o oJrot
rot nuc
o oJrot
rot nuc
g g JN J J T J oddN Z
g g JN J J T J evenN Z
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Rotational Distributions for Homonuclear Molecules: Hydrogen
• For H2 molecules, I = 1/2 because the proton has a spin of 1/2. The nuclear degeneracy is thus g0 = (2I+1) = 2. For H2:
2( , )
2( , )
2 13 exp 1 / 1,3,5,...
2 11 exp 1 / 0,2,4,...
Jrot
rot nuc
Jrot
rot nuc
JN J J T J ortho HN Z
JN J J T J para HN Z
22( , ) 0
1 1 2 12 2rot nuc
rot rot
T TZ g I
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Rotational Distributions for Homonuclear Molecules: Hydrogen
• CARS spectrum of diatomic hydrogen clearly shows the effect of nuclear spin on the rotational level populations:
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Rotational Distributions for Homonuclear Molecules: Nitrogen
• Diatomic nitrogen also has a symmetric ground electronic level .
• However, the nuclear spin of the 14N nucleus is I = 1, and therefore diatomic nitrogen obeys boson statistics. Nuclei of even mass number are bosons because the nuclear spin will be integral and the total wavefunction tot = rot vib elec nuc must be symmetric with respect to exchange of nuclei :
, , , ,
, , , ,
, , , ,
, , , , v
tot tot
rot rot
rot rot
vib vib
x y z x y z
x y z x y z J odd
x y z x y z J even
x y z x y z all
1g
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Rotational Distributions for Homonuclear Molecules: Nitrogen
• Therefore we develop the following table:
14N2 rot vib elec nuc tot
J even + + + + +J odd - + + - +
• For N2 molecules with a symmetric nuclear spin wavefunction, only rotational states with even J are possible. For N2 molecules with an antisymmetric nuclear spin wavefunction, only rotational states with odd Jare possible.
• For N2 molecules, the nuclear spin I = 1 and the nuclear degeneracy is thus g0 = (2I+1) = 3.
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Rotational Distributions for Homonuclear Molecules: Nitrogen
( , )
( , )
( , )
( , )
1 2 1 1exp
2
2 1 16 exp 0,2, 4,
1 2 1 1exp
2
2 1 13 exp 1,3,5,
o o rotJ
rot nuc
rot
rot nuc
o o rotJ
rot nuc
rot
rot nuc
g g J J JNN Z T
J J Jeven J
Z T
g g J J JNN Z T
J J Jodd J
Z T
• For N2 therefore:
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Rotational Distributions for Homonuclear Molecules: Nitrogen
• The N2 CARS spectrum also shows the effects of nuclear spin coupling with the rotational levels:
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Rotational Distributions for Homonuclear Molecules: Oxygen
• Diatomic oxygen also has an antisymmetric ground electronic level . For diatomic oxygen the nuclei are bosons and the total wavefunction must be symmetric with respect to exchange of nuclei.
• We develop the following table:
3g
16O2 rot vib elec nuc tot
J even + + - - +J odd - + - + +
• For O2 molecules with a symmetric nuclear spin wavefunction, only rotational states with odd J are possible. For O2 molecules with an antisymmetric nuclear spin wavefunction, only rotational states with even Jare possible.
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Rotational Distributions for Homonuclear Molecules: Oxygen
• For 16O, I = 0, go = 1. The populations of the rotational levels are given by
( , )
( , )
( , )
( , )
1 2 1 1exp
2
2 1 11 exp 1,3,5,
1 2 1 1exp
2
2 1 10 exp 0 0,2, 4,
o o rotJ
rot nuc
rot
rot nuc
o o rotJ
rot nuc
rot
rot nuc
g g J J JNN Z T
J J Jodd J
Z T
g g J J JNN Z T
J J Jeven J
Z T
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Rotational Distributions for Homonuclear Molecules: Oxygen
• The even rotational levels are missing for the oxygen molecules, at least for the most common isotopic species 16O2. The isotope 16O18O has both even and odd rotational levels, however; this was how the isotope was first discovered.