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Photo physics and photo chemistry of ice films on graphite. Department of Applied Physics Chalmers and Göteborg University. Dinko Chakarov Johan Bergeld Michael Gleeson Bengt Kasemo ……………. Photon induced processes (UV - visible – IR) Water ice (amorphous, crystalline, reactions) - PowerPoint PPT Presentation
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Photo physics and photo chemistry of ice films on graphite
Department of Applied PhysicsChalmers and Göteborg University
Dinko ChakarovJohan Bergeld
Michael GleesonBengt Kasemo
……………
Key words:
• Photon induced processes• (UV - visible – IR)• Water ice (amorphous, crystalline, reactions)• Surfaces• Graphite • Spectroscopy
Experimental conditions:
° Atomically clean and ordered surfacesIce (H2O/D2O) on HOPG (XYA)
° UHV (below 10-10 torr)
° Temperature range 25 - 1500 K
° Photon fluxes (cw and pulsed)1012 - 1029 photons.s-1
° Arc lamps + filters/monochromator and/or Nd:YAG based OPO (220-1600 nm)
Experimental methods:
° HREELS ° TDS/ITS
° PID ° QCM
Ice - Optical / Electronic properties
Graphite: Electronic/Optical Propertieselectronic configuration: 2-4 or
1s22s22p2
a
Energy (eV)-20-15-10-505σππ∗σ∗EF
νhπsp2120o
Photon Energy (eV)
Ext
inct
ion
coef
fici
ent
J. Chem. Phys. 106 (3), 982, 1997
PHOTO DESORPTION OF ALKALI ATOMS
Graphite
Phenomena/Examples:
•Photoinduced structural changes in amorphous ice
•Photoejection of water molecules from amorphous ice
•Photoreactions: H2O & coadsorbates on graphite
HREELS; energeticsorientation
ITD; structrecoverages
TPD; binding energylateral interactions
0 100 200 300 400 500
0
Energy Loss, meV
x100
x333
Water on Graphite(0001) T=85 K
0
0.5
1
0 200 400 600 800
Time, s
2.2 MLTiso= 134 K
120 135 150 165 180
Temperature (K)
H2O/Graphite (0001)
(UHV and Low Temperatures)
Structure of Ice (Ic and Ih)Hydrogen bonding:
Ideal ice structures obey the so-called Bernal-Fowler rules:
each hydrogen atom (or proton) is situated on the line joining each pair of oxygens; each oxygen atom has two hydrogen atoms attached to it at distances of about 1Å, thereby forming a water molecule H2O.
Photoinduced crystallization• Experimental observations
0
5
10
15
0 50 100 150 200
Energy Loss, meV
x 50
x 1
D
C
B
A
9 meV
ν ννν2R 3R 4RT
2DH2Ogrowth 3DH2Ogrowth
low-coordinated watermoleculeshigher-coordinated watermolecules
hω
hω0
0.2
0.4
0.6
0.8
1
0 1 2 3 4 5 6 7
nonirradiatedirradiated
Coverage, ML
0
0.5
1
1.5
2
0 1 2 3 4 5
crystalline amorphous
Photon dose x(1019
photons.cm-2
)
θH
2O= 2.2 ML
a
0-101b23a11b1
4a12b2 EFEVEg,iceΦGrVBiceVBGr
EF ΔΦCBGrEdππ∗12341/2
hωhω( )D E ( )D E
E ECBiceσGraphiteIce
EV
1. - .Photoexcitation of electron hole pairs in graphite2. T .unneling of the electron into unoccupied defect states near but below the CB edge of ice3. V .ibrational excitation4. .Return of the electron after the local ice structure has relaxed into higher coordination
Re-crystallization Mechanism
PR
L, 8
1 , 5
181
(19 9
9 )
Defects annealing
a
D-defectL-defect
N. Bjerrum, Structure and properties of ice,Science, 115, 385, 1951
Structural changes of ice films
0
0.5
1
0 200 400 600 800
Time, s
T = 134 K
Importance and Consequences
• Balance between adsorbed and gas phase water
• Reactivity of the ice surface
0 5 10 15 20Time [s]
H2O photoejection
2.4 ML film on GraphiteT=85 K
650 nm, 3.7 mJ
0
2 104
4 104
6 104
8 104
0 5 1016 1 1017 1.5 1017 2 1017
Photo peak vs. number of photons H2O/HOPG, ~5MLIllumination position between wavelengths might differ
l = 355 nmλ = 532 nm
= 1.7404 -47 * ^(3.1042) = 0.96769 y e x R
= 4.5075 -68 * ^(4.1701) = 0.99993 y e x R
, .Photon Flux Number of Photons cm2.s-1
0
1 104
2 104
3 104
4 104
5 104
0 5 10 15 20
Photoejection peak intensity for different
water coverages at ~50 K
Coverage [ML]
λ=355 , 1.2 .nm mJ cm2
Photoreactions with ice; investigated systems:
• Substrates: • Graphite, Si(001), Pt(111), …• Coadsorbates: • Metal ions and clusters: Na, K, Cs, Ag, Au
• Simple molecules: CO, NO, H2S
• Observed products:
• H2, CO, CO2, CH4, NH3, …
AM-stabilized H2O : TPD
Photoreactions of water and carbon at 90 K
J. Chem. Phys. 115 (20) 2001
Carbon/Graphite
- C
- O
- H
-25 0 25 50 75 100 125 150 175 200 2250
2000
4000
6000
8000
10000
12000
14000
16000
18000
Irradiation: 355 nm / 5.3 mJ (10x10 sec exposures)
NO
de
sorp
tion
Time (sec)
NO exposure ~12 LAg coverage
0,0 ML 0,3 ML 1,3 ML 2,2 ML
Photo-desorption as a function of Ag coverage
Summary:
•Photoinduced structural changes in amorphous ice
•Photoejection of water molecules from amorphous ice
•Photoreactions: H2O & coadsorbates on graphite
• Demonstrate the richness of the processes and phenomena;• Knowing the conditions is possible to identify the predominant process.
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