Absolute Level-to-Level Rate Constant Distributions in 7 Li + 7 Li 2 * Inelastic Collisions and Exchange Reactions Steve Coppage Wesleyan University Physics Department Middletown, CT Spring 2009 •Foundations •The Experiment •The Data •Modeling
Absolute Level-to-Level Rate Constant
Distributions in 7Li + 7Li2*
Inelastic Collisions and Exchange
Reactions
Steve Coppage
Wesleyan University
Physics Department
Middletown, CT
Spring 2009
•Foundations
•The Experiment
•The Data
•Modeling
Li + 7Li2* Collisions – Foundations
Rate constants are a measure of the level-to-level molecular dynamics of collisions and reactions. They provide the experimental underpinning for modeling molecular energy transfer and testing potentials of physical systems.
Laser-induced fluorescence is a technique to measure the relative populations of excited –state levels populated by collisions with a single excited state level.
7Li nuclei have spin 3/2. They behave as fermions. In the vapor phase, this results in an ortho-para mixture of odd and even rotational levels.
Assuming the nuclear state does not change in a collision, the molecular rotational level can only change by an even j.
Odd-j change is permitted in an exchange reaction. Observation of odd-j rotational change is observation of a change in the nuclear spin state or an exchange reaction.
Ground State
A State
14,903 cm-1
Ratcliff, Fish, and Konowalow,
J. Mol. Spec.,122, 293 (1987)
7Li2 Molecular
Potentials
Parent Lines are the result of Elastic Collisions or no Collision
and appear across many vibrational bands.
Red laser light
at
14477.26 cm-1
excites
v=0, j=18
in the
ground state
to v=2, j=19
in the
excited state.
Satellite Lines are the results of Inelastic Collisions or
Exchange Reactions and also appear across vibrational bands.
What is a Rate Constant?
Second, solve for the ratios of the densities of the final and initial levels, which is
directly proportional to the ratio of the satellite to parent line intensities.
First, write a rate equation and set it to 0 to represent a steady state.
The lifetimes and densities of Li and Li2 are known and tabulated. For v=2, j=11, the
inverse lifetime is ~5.6*107 Hz. The number density, nLi , of lithium at 660ºC ~
2·1015 cm-3. The first term in parenthesis is ~ 3·10-8 cm3/s
The ratio of the densities of the final and initial levels is calculated from our
measurements.
The rate constant has units of cm3/s. When divided by an average velocity, one can
get a thermally averaged cross section, cm2. <v> at 660ºC ~ 1900 m/s. A rate
constant of 10-12 cm3/s gives a thermally averaged cross section ~ .05 2.
n f
n i
k if n Li
f k Q n Li
tn f
d
dk if n i n Li k Q n f n Li f n f 0
k if
n f
n i
f
n Li
k Q
Experimental Setup
Schematic of the Apparatus. LPC Laser Power Controller, A 10nm FWHM Band pass
Filter, B-Polarization Rotator (~54.7º off-horizontal), C-Polarization Analyzer (set to
horizontal polarization), D-Beam Stop
The Cell
Photo Courtesy of
B. Stewart
12440-12550 11/14/06 660ºC .200 Torr
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
12440 12450 12460 12470 12480 12490 12500 12510 12520 12530 12540 12550 12560
WaveNo(cm-1
)
Co
un
t
v=2 j=19
A Typical Measurement
1.E-13
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69
kif
[cm
3/s
]
jf
ji = 3
ji = 19ji = 11
ECS Paameters
a=5.673e-9lc=6.399e-8
γ=1.279j*=57.835
ji = 3
Ne ji = 30
ji = 11
0.0E+00
2.0E-12
4.0E-12
6.0E-12
8.0E-12
1.0E-11
1.2E-11
1.4E-11
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
kif
[cm
3/s
]
jf
Vibrationally Inelastic Collisions (VI) vi = 2, vf = 1
ji = 3
ji = 11
ji = 19
Ne ji = 30
0.0E+00
2.0E-12
4.0E-12
6.0E-12
8.0E-12
1.0E-11
1.2E-11
1.4E-11
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
kif
[cm
3/s
]
jf
Vibrationally Inelastic Collisions (VI) vi = 2, vf = 0
ji = 3
ji = 11
ji = 19
Ne ji = 30
0.0E+00
1.0E-12
2.0E-12
3.0E-12
4.0E-12
5.0E-12
6.0E-12
7.0E-12
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
kif
[cm
3/s
]
jf
Vibrationally Inelastic Collisions (VI) vi = 2, vf = 3
ji = 3
ji = 11
ji = 19
Ne ji = 30
Exchange Reactions
0.E+00
1.E-12
2.E-12
3.E-12
4.E-12
5.E-12
6.E-12
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
kif
[cm
3/s
]
jf
Exchanges (E) vf = 2
ji = 3
ji = 11
ji = 19
0.0E+00
5.0E-13
1.0E-12
1.5E-12
2.0E-12
2.5E-12
3.0E-12
3.5E-12
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
kif
[cm
3/s
]
jf
Exchanges (E) vf = 1
ji = 3
ji = 11
ji = 19
0.0E+00
5.0E-13
1.0E-12
1.5E-12
2.0E-12
2.5E-12
3.0E-12
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
kif
[cm
3/s
]
jf
Exchanges (E) vf = 0
ji = 3
ji = 11
ji = 19
Exchange Reactions with Prior/Statistical Distributions
0.0
0.5
1.0
1.5
2.0
2.5
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80
kif
[1
0-1
2cm
3/s
]
jf
v'=2,j'=11
vf =2
0.0
0.5
1.0
1.5
2.0
2.5
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80
kif
[1
0-1
2cm
3/s
]
jf
vf =1
v'=2, j'=11
0.0
0.5
1.0
1.5
2.0
2.5
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80
kif
[1
0-1
2cm
3/s
]
jf
vf=0
v'=2, j'=11
Modeling with QCT
QCT stands for quasi-classical trajectories, a Monte Carlo simulation of
many collisions over a random sampling of initial conditions.
LEPS Potential
LEPS stands for London, Eyring, Polanyi, and
Sato
First used with H3, it is a mixture of an
attractive singlet state with a repulsive triplet
state. Whitehead used one to model the
ground state Li exchange reaction.
We use a fit to the A-state singlet to assure
correct asymptotic behavior and vary the
triplet parameters to find the best QCT fit.
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0
Cro
ss S
ectio
n (Å
2)
Energy (kcal/mole)
Whitehead Trajectory Study 1976/Wesleyan 2008 Total Reaction Cross Sections, v=0 j=10
1976 2008
1.E-14
1.E-13
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
kif
[cm
3/s
]
jf
Rotationally Inelastic Collisions (RI) with QCT
vi = vf = 2ji = 3
ji = 11
ji = 19
0.0E+00
2.0E-12
4.0E-12
6.0E-12
8.0E-12
1.0E-11
1.2E-11
1.4E-11
1.6E-11
1.8E-11
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
kif
[cm
3/s
]
jf
Vibrationally Inelastic Collisions (VI) with QCT
vi = 2, vf = 1
0.0E+00
2.0E-12
4.0E-12
6.0E-12
8.0E-12
1.0E-11
1.2E-11
1.4E-11
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
kif
[cm
3/s
]
jf
Vibrationally Inelastic Collisions (VI) vi = 2, vf = 0
-200
0
200
400
600
800
1,000
1,200
1,400
1,600
1,800
2,000
01
23
45
67
89
10
1,800-2,000
1,600-1,800
1,400-1,600
1,200-1,400
1,000-1,200
800-1,000
600-800
400-600
200-400
0-200
-200-0
0.E+00
1.E-04
2.E-04
3.E-04
4.E-04
5.E-04
0 1000 2000 3000 4000 5000v (m/s)
Laser-prepared relative velocity distribution (929K)
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10
1800-2000
1600-1800
1400-1600
1200-1400
1000-1200
800-1000
600-800
400-600
200-400
0-200
-200-0
0.0E+00
5.0E-13
1.0E-12
1.5E-12
2.0E-12
2.5E-12
3.0E-12
3.5E-12
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68
kif
[cm
3/s
]
jf
Exchanges – vf = 2
Exchanges – vf = 1
Acknowledgements
Brian Stewart, Thesis Advisor
Paula Matei, Ramesh Marhatta
Lutz Hüwel and Reinhold Blümel, Committee Members
The Wesleyan Chemical-Physics Community
References
S. Coppage, P. Matei, B. Stewart, J. Chem. Phys., 128, 241103 (2008)
Levine Raphael D. , Molecular Reaction Dynamics, Clarendon Press,
N.Y. (2005)
Whitehead, J. C. , Molecular Physics, 1, 177 (1975)