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.