State-specific surface scattering with laser-prepared molecules
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Transcript of State-specific surface scattering with laser-prepared molecules
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State-specific surface scattering with laser-prepared molecules
Dynamics of Molecular Collisions
Asilomar July 2005
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Outline INTRODUCTION: Evidence for breakdown of Born-
Oppenheimer Approximation for molecules at metal interfaces.
EXPERIMENTAL
Vibrationally promoted ejection of electrons from a surface
Relative importance of vibration and translation on trapping at a surface.
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Eyring and Polanyi said…Let there be the Born Oppenheimer Potential Energy Surface Many talented theoreticians
work for 6-7 decades…
Many great experiments
Agreement between experiment and theory leads to “understanding”
Predictive power for new reactions • H. Eyring, J. Walter, G. E. Kimball, Quantum Chemistry
(John Wiley and Sons, New York, 1944).• H. Eyring, H. Gershinowitz, C. E. Sun, J. Chem. Phys. 3, 786 (1935). • J. Hirschfelder, H. Eyring, B. Topely, J. Chem. Phys. 4, 170 (1936).
H + H2 exchange reaction
“Über einfache Gasreaktionen”, H. Eyring and M. Polanyi, Sonderdruck aus Z. Phys. Chem., Abt. B, 12, Heft 4.
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D-atom flux contour map from the H+HD reaction
Experiment TheoryH
H + HD → H2 + D
Rydberg Tagging of reaction products- by X. Yang
Full quantum dynamics on accurate PES - by R. Skodje
“For F+H2, What is the importance of multiple surface scattering?” …Cassavecchia
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What happens when we apply this strategy to reactions at metal surfaces?
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N-N recombination on Ruthenium: 2N(ad) →N2(v)(g)
L. Diekhoner, L. Hornekaer, H. Mortensen, E. Jensen, A. Baurichter, V. V. Petrunin, A. C. Luntz, J. Chem. Phys. 117, 5018-5030 (2002).
Surface by: M. J. Murphy, J. F. Skelly, A. Hodgson, B. Hammer, J. Chem. Phys. 110, 6954-6962 (1999).
Theory
Experiment
Early barrier
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Dissociation of O2 on Aluminum
Experiments show activated process… Indicative of reaction barrier.
Behler, J., Lorenz, S., Reuter, K., Scheffler, M. and Delley, B., 2004
O-O
L.Österlund, I.Zoric,B.Kasemo, PRB 55 (1997) 15452
Born-Oppenheimer PES Calculated with DFT shows no barrier!
eV
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Both results imply: Important energy transfer between the reactive center and the surface N2 reaction on Ruthenium
Many vibrational quanta appear to be lost from the molecule during the 10’s of femto-seconds required for the product to leave the surface
O2 reaction on Aluminum Despite no electronically adiabatic barrier on the potential energy
surface, translation energy is need to induce the reaction
Possible important energy transfer couplings To phonons: vibration of the metal substrate To electrons: creating excited electron hole pairs (EHP’s)
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Born-Oppenheimer Approximation at metal surfaces is less obviously correct The stronger attraction of a molecule to a surface, proportional to the inverse
third power of distance compared to the inverse sixth power in the gas phase, can lower the energies of more polarizable excited electronic states, bringing them nearer in energy to the ground state.
More dramatically, positively or negatively charged molecules at surfaces are
stabilized by an image potential proportional to the first power of the distance, frequently resulting in the crossing or avoided crossing of ionic and neutral potential energy surfaces.
Metals bind electrons more weakly (work functions are generally less than 5 eV) than gas phase molecules (Ionization potentials are generally more than 8 eV).
Finally, metal surfaces exhibit a continuum of electronic states, the conduction band, for which there is no energy separation whatever between electronic states. Electron-hole pair (EHP) transitions between electronic levels in the conduction band can provide a mechanism for energy transfer with an adsorbate molecule and perhaps even call into question the applicability of the concept of motion evolving on a PES.
Excellent review: J. C. Tully, Ann. Rev. Phys. Chem. 51, 153 (2000).
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Of greatest interest: How is energy exchanged between nuclear motion and metal electrons near the Transition state?
Products (B-C)weak coupling to metallic electrons
Transition Statestrong coupling to metallic electrons!
RB-C
RA-B
Reactants (A-B)weak coupling to metallic electrons
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Experimental Approach
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Experimental Setup: NO(v=15) + Au(111)
Au(111)
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Optical Pumping on Nitric Oxide
Capabilities Franck-Condon Pumping of low
vibrational states (v=0-10) Stimulated Emission Pumping of
high vibrational states (v<27, 5.1 eV) Laser induced fluorescence
detection Resonance enhanced multiphoton
Ionization detection A few percent of sample may be
excited.
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Large amplitude vibration a mimic of the kind of motion near the transition state!
1.2 1.4 1.6
2
4
6
8
10
v=15
v=0
31%
11%
Numerical Solution to the Vibrational Schrödinger Equation for NO
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The Stark Effect: Quantum Broom to Sweep Away the Ground State
)1()1(
JJ
MEE
EEE 5
3
)12/3(*2/3
2/3*2/3
23/2
21/2
ESO~120 cm1
EEE 3
1
)12/1(*2/1
2/1*2/1
NO: ~0.15 D
Ground Spin-Orbit State has weak Stark Effect
Excited Spin-Orbit State is unpopulated in a beam
SEP can prepare high v-state with large Stark effect
D. Matsiev, J. Chen, M. Murphy, A.M. Wodtke J. of Chem. Phys. 118 9477-9480 (2003);Chem. Phys. 301(2-3) 161-172, (2004)
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Hexapole acts to focus highly vibrationally excited molecules selectively v=18 21/2
v=18 23/2
v=0 21/2
Fields of 150 kV/cm now routine
D. Matsiev, J.Chen, M. Murphy, A.M. Wodtke J. of Chem. Phys. 118 9477-9480 (2003)
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Beam is re-focused in two dimensions to about 1-mm
75 cm downstream sample returns to ~1-mm size
observed simulated
Three-dimensional refocusing will be possible in the future.
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When it is all put together, a machine that can…
…transport optically prepared molecules to UHV surface science chamber and refocus them.
…enrich beam in concentration of high-v molecules.
…select M-states for orientation studies.
…Provide vibrational-state specific dipole moments.
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Scattering from a simple inert metal surface
NO(v=15) from Au(111) and comparison to LiF insulator
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Multiquantum Vibrational Relaxation on Au(111)
15 14 13 12 11 10 9 8 7 6 50.0
0.2
0.4
0.6
0.8
1.0
1.2
17.4
0 -17.
8
-36.
1
-54.
3
-73.
2
-92.
1
-111
.7
-131
.3
-151
.6
-171
.9
-192
.8
-213
.7
Probabili
ty (ar
b. un
its)
Evib (kJ/mole)
Final Vibrational State (v) Important Points <3% survival in v=15 (by summing histogram above) 150 kJ/mol most likely vibrational exchange Vibrational energy is transferred to the metal surface
Conditions Prepare NO(v=15) with SEP Ei= 0.05 eV
Ts = 300K Clean Au(111)
Y. Huang, S.J. Gulding, C.T. Rettner, D.J. Auerbach A.M. Wodtke, Science 290, 111 (2000)
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Sub-picosecond time-scale for energy exchange
50
6070
8090100110
120
130
v=11 Ei=0.05
v=8 Ei=0.30
v=10 Ei=0.30
v=11 Ei=0.30
v=13 Ei=0.30
Cos14
Strongly peaked angular distributions independent of Eincidence
Independent of v Time-Scale of the Interaction is ~10-13 Sec
Prepared NO(v=15) + Au(111)
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Quite different dynamics observed on insulators.
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Without the Electrons: Scattering from cleaved LiF insulator surface Scattering of NO(v=12)
from LiF Limited vibrational
relaxation No Detectable vibrational
excitation
Phonon frequencies higher on LiF than Au
9 10 11 12 130.0
0.2
0.4
0.6
0.8
1.0
Po
pu
lati
on
Vibrational State
Vibrational Population distribution after scattering
Ts=480K and Ein=72 kJoule/Mol
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Survival probability vs. incidence energy on an insulator NO(v=12) survives LiF
Low Energy: Trapping Desorption
High Energy: direct scattering
0 10 20 30 40 50 60 70 80
0.6
0.8
1.0
1.2
Sur
viva
l Pro
babi
lity
Energy (KJ/mole)
0
30
60
90
120
150
180 0
30
60
90
120
150
180
A.M. Wodtke and Y. Huang, D.J. Auerbach, J. Chem. Phys.118(17) 8033-41 (2003)
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Some ideas about mechanisms
Electron transfer appears to play an important role.
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1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5-7.3
-6.2
-5.2
-4.1
-3.1
-2.1
-1.0
0.0
1.0N + O + e
N + O
RN-O
Electro
nic Energy
(eV)
1.0 1.5 2.0 2.5
-2.1
-1.0
0.0
1.0
2.1
r >(v=1
5)
rer< (v=15)
Electron Binding Energy (eV)
RN-O
NO’s ability to bind an electron depends strongly on bond length
v=15
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Image charge interaction allows charge transfer events
Newns, Surf. Science 171, 600 (1986) 1 2 3 4 8 10
0
2
4
6
Au) EA(NO)
NO
NO
RNO-M
(A)
Energ
y (eV
)
1.1 eV
Image Charge
interaction ~1/R
The critical factors here are: Surface work functionElectron affinity
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A picture is worth a thousand words
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Monte Carlo Wave Packet Study of Negative Ion Mediated Vibrationally Inelastic Scattering of NO from Metal Surface Shenmin Li and
Hua Guo, J. Chem. Phys,
v117 2002
Qualitative features of this picture appear in theory.
Outer turning point electron transfer to NO
Electron re-transfer to metal at smaller bond lengths
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Does this mean multiquantum relaxation on Au excites a single electron? If, for example, transient
NO is formed Dipole derivative for NO
during large amplitude vibration is quite large ~8 Debye/Å
Dipole function is quite linear. Might couple one vibration
at a time to many electrons as the anion equilibrates to the surface.
1.0 1.2 1.4 1.6
Dip
ole
Mom
ent (
D)
Inernuclear Separation (A)
NO Dipole moment
ccsdtqcisdt
Can we observe vibrationally induced electron emission from a low work function surface?
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If you can convert such large amounts of vibrational energy to electronic excitation……Can you induce electron emission
form a low work-function surface?
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Details of the Apparatus
Photoemission probe of Cs-doseBeam exposure to low work-function surface
View from the molecular beam
View looking down from above
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Photoemission vs. Cs Dosing of Au(111)
0 10 20 30 40 50 60
0
100
200
300
400
500
HeN
e la
ser
phot
oem
issi
on
Cs Dose (Seconds)
Working here~1.3-1.6 eV
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Experimental observation of vibration induced electron emission from a solid Spectroscopic identification of the signal
Fluorescence Dip’s indicate the resonances where, NO(A-state) population is moved back to NO(X-state) (v=18)
Emission of electrons from the surface are also shown
Two pump lines indicated Q21(0.5) Q11(0.5)
461.5 462.0 464.0 464.5
-1.0
-0.5
0.0
0.5
1.0
Q11
(0.5)
Wavelength of Dump Laser (nm)
-1.0
-0.5
0.0
0.5
1.0
Fluorescence D
epletion
Exo
-ele
ctro
n em
issi
on
Q21
(0.5)
NO(v=18) scattered from Cs/Au at Ein=29 meV
NOA2(v=3)→X2(v=18) Resonances observed in two ways
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Vibrational quantum number dependence of electron emission: Preliminary results
Emission probability high Previous reports of
electron emission due to exothermic surface reactions ~10-6-10-8
Threshold coincident with work-function
J. White, J. Chen, D. Matsiev, D.J. Auerbach and A.M. Wodtke, Nature 433(7025),503-505, (2005).
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Oxygen Coverage Dependence: Indirect evidence for work function dependence
0.0 0.5 1.0 1.5 2.0 2.50
50
100
150
200
250
300
Exo
-ele
ctro
n E
mis
sio
n S
ign
al
Oxygen Exposure (L)
Electron Emission on Oxygen Free Surface
Low Coverage maximum
Possible evidence of signal scaling with surface work function
J. White, J. Chen, D. Matsiev, D.J. Auerbach and A.M. Wodtke, J.V.S.T. 23, 1085-1089 (2005).
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Vibrational and Translational Influence on Trapping
Few experimental measurements
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v = 2
v = 6
v = 12
vibrational quantum number of NO(v)
z = distance of NO from Au(111)
r = interatomic distance of NO
Sharani Roy and John Tully: A new potential energy surfacee.g. plots for NO approaching an On-top Site
Charge on NO (a.u.)
z (Å)
r (Å
)
r (Å
)
Energy (a.u.)
z (Å)
Binding Energy = 0.72 eV
N-Metal distance ~ 3.0 Å
Charge on NO ~ -0.6 (z = 2.4 - 2.8Å r = 1.6 Å)
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Experimental Logic
Calculated well depth is ~-0.5 - 0.7 eV. This means if trapping occurs, residence times exceed 30 s.
If trapping of vibrationally excited molecules occurs, thermal accommodation and desorption in ground vibrational state can be assumed.
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0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.800.0
0.2
0.4
0.6
0.8
1.0
Incidence Kinetic Energy (electron-Volt)
Sur
viva
l Pro
babi
litie
s
State Specific Survival Probabilities
v=2
v=0
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Why don’t survival of v=2 and 0 look identical at high incidence energy?
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.800.0
0.2
0.4
0.6
0.8
1.0
Incidence Kinetic Energy (electron-Volt)
Sur
viva
l Pro
babi
litie
s
Because v=2-1 relaxation increases in probability with incidence energy
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After correcting for this…
One can obtain trapping probability vs. translational energy for v=0 and 2
Insensitivity of trapping at surfaces to molecular vibration
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Tra
pp
ing
Pro
ba
bili
ty
Incidence Kinetic Energy (electron-Volt)