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.
