Title Page

Contents

1. Introduction 13

1.1. The Carbon Monoxide Molecule 16

1.2. Overview of this Dissertation 18

2. Theory 19

2.1. Thermal Desorption Spectroscopy (TDS) 19

2.1.1. Principle 19

2.1.2. Theory 19

2.2. Photoemission Spectroscopy 24

2.2.1. Back ground 24

2.1.2. Basic Principles of Photoelectron Spectroscopy 27

2.3. Photoemission Studies with Adsorbed CO 27

2.4. Near Edge X-ray Absorption Fine Structure 36

2.4.1. Introduction 36

2.4.2. Principle 36

3. Experimetal Apparatus 44

3.1. Overview 44

3.2. The UHV Chamber for TDS, XPS 44

3.2.1. Sample Holder 46

3.3. Photoemission Spectroscopy Beamline 2B1 at Pohang Light Source 48

3.3.1. Overview 48

3.3.2. Photoemission Spectroscopy Chamber 51

4. Adsorption and Desorption of CO on W(110) 53

4.1. Introduction 53

4.2. Experimental Procedure 56

4.3. Results and Discussion 58

4.3.1. TDS 58

4.3.2. XPS 62

4.3.3. Valence Band Spectra 67

4.4. Conclusion 84

5. Adsorption and Desorption of CO on Mo(110) 86

5.1. Introduction 86

5.2. Experimental Procedure 89

5.3. Results and Discussion 90

5.3.1. TDS 90

5.3.2. Valence band spectra results 94

5.3.3. NEXAFS 111

5.4. Conclusion 116

6. The Adsorpton and Desorption of Carbon Monoxide on Oxygen Modified W(110) and Mo(110) Surfaces. 119

6.1. Introduction 119

6.2. Experimental procedure 119

6.3. Results and Discussion 120

6.4. Conclusion 131

7. Conclusions 134

References 138

Abstract 145

Table 1. Summary of β-state CO at W surface. 55

Table 2. CO/W(110) valence band spectra peak parameters (in eV) 70

Table 3. Energy separation Δ(4σ-1π) between 4σ and 1π peaks of chemisorbed CO. 78

Table 4. Summary of β-state CO at Mo surfaces. 88

Figure 1-1. Tendency of CO to dissociate on the transition metals at 300 K. Rapid dissociation occurs to the left of the heavy line on the clean metal. 15

Figure 1-2. Contour plots of some valence orbitals of the CO molecule. 17

Figure 2-1. An experimental setup for TPD 20

Figure 2-2. Schematic experimental arrangement for photoeletron spectroscopy of solid surfaces. 25

Figure 2-3. The energetics of X-ray & UV photoemission 26

Figure 2-4. Energy level diagram for the XPS process. 28

Figure 2-5. UP spectra (He II radiation) from a) CO adsorbed on a Pd(110) surface; b)Ru6(CO)16; c) Gaseous CO.(이미지참조) 31

Figure 2-6. Blyholder model describing chemisorption of CO 32

Figure 2-7. Structure of hexarhodium-hexadecacarbonyl, Rh6(CO)16.(이미지참조) 33

Figure 2-8. Schematic representation of ionization energies obtained for gaseous and adsorbed CO. 34

Figure 2-9. Ionization potentials (top and bottom) and orbital energies (middle) of gaseous and adsorbed CO. 35

Figure 2-10. XANEX and EXAFS regions of x-ray absorption fine structure. 37

Figure 2-11. X-ray absorption near edge structure spectra shown for acetylene (C₂H₂) 40

Figure 2-12. X-ray absorption near edge structure spectra of solid state N₂O. 41

Figure 2-13. Polarization dependence of resonances for oriented molecules, illustrated for vertically oriented CO on Mo(110). 43

Figure 3-1. Schematic of the UHV chamber for Thermal Desortion Spectroscopy (TDS) and X-ray Photoelectron Spctroscopy (XPS). 45

Figure 3-2. The schematic diagram of sample holder on manipulator. 47

Figure 3-3. 2B1 spherical grating monochromator (SGM) beamline. 50

Figure 3-4. An experimental setup for the photoelectron spectroscopy chamber in 2B1 beamline at Pohang Light Source. 52

Figure 4-1. The thermal desorption spectra of CO adsorbed on W(110) surface with various CO exposures at 300 K. 59

Figure 4-2. The Thermal desorption spectra of CO adsorbed on W(110) surface with various CO exposures at 900 K. 61

Figure 4-3. O(1s) spectra from the CO adsorbed on W(110) surface at 300 K at the photon energy of 770 eV using synchrotron radiation (a) CO 5.0 L adsorbde on W(110) surface at 300 K (b)(c)(d) after annealing at... 63

Figure 4-4. O(1s) spectra of various oxygen adsorbed W(110) surface at R.T. at the photon energy of 770 eV followed by 1500 K flash. 64

Figure 4-5. Valence band spectra of CO adsorbed on W(110) surface with various CO exposures at 120 K at the photon energy of 90 eV using synchrotron radiation. 68

Figure 4-6. Fitting of valence band spectra derived by CO exposure. 71

Figure 4-7. Intensity distributions for CO-induced valence band spectra deconvolution peaks as a function of total CO exposure obtained at the photon energy of 90 eV. 73

Figure 4-8. Valence-band spectra of 3.0 L CO adsorbed W(110) surface recorded after heating to indicated temperatures. 74

Figure 4-9. Difference valence-band spectra of 3.0 L CO adsorbed W(110) surface recorded after heating to indicated temperatures. 76

Figure 4-10. Difference valence-band spectra of 0.8 L CO adsorbed W(110) surface recorded after heating to indicated temperatures. 80

Figure 4-11. Valence band spectra from the stepwise desorption of various exposure CO and O₂. 83

Figure 5-1. The thermal desorption spectra of CO adsorbed on Mo(110) surface with various CO exposures at 300 K. 91

Figure 5-2. The Thermal desorption spectra of CO adsorbed on Mo (110) surface with various CO exposures at 700 K. 93

Figure 5-3. Valence band spectra of cleaned Mo(110) surface followed at 120 K with various photon energy. 95

Figure 5-4. Valence band spectra of the 5.0 L CO adsorbed Mo(110) surface followed at 100 K with various photon energy. 96

Figure 5-5. Valence band spectra of the 5.0 L CO adsorbed Mo(110) surface followed at 400 K with various photon energy. 98

Figure 5-6. Valence band spectra of the 5.0 L CO adsorbed Mo(110) surface followed at 900 K with various photon energy. 99

Figure 5-7. Valence band spectra of CO adsorbed on Mo(110) surface with various CO exposures at 100 K at the photon energy of 80 eV using synchrotron radiation. 101

Figure 5-8. Fitting of the valence band spectra derived by 0.5 L and 3.0 L CO Mo(110) surface. 103

Figure 5-9. Intensity distributions for CO-induced valence band spectra deconvolution peaks as a function of total CO exposure obtained at the photon energy of 80 eV. 104

Figure 5-10. Difference valence-band spectra of 3.0 L CO adsorbed Mo(110) surface recorded after heating to indicated temperatures at the photon energy 80 eV. 106

Figure 5-11. Difference valence-band spectra of 3.0 L CO adsorbed Mo(110) surface recorded after heating to indicated temperatures. At the photon energy of 50 eV. 107

Figure 5-12. Difference valence-band spectra of 0.5 L CO adsorbed Mo(110) surface recorded after heating to indicated temperatures at the photon energy 80 eV. 109

Figure 5-13. Difference valence-band spectra of 0.5 L CO adsorbed Mo(110) surface recorded after heating to indicated temperatures at the photon energy 50 eV. 110

Figure 5-14. NEXAFS spectra above the C K-edge for 3.0 L CO on Mo(110) at 120 K as a function of θ. 112

Figure 5-15. NEXAFS spectra of 3.0 L CO adsorbed Mo(110) surface recorded after heating to indicated temperatures at normal beam incidence (θ＝90˚). 114

Figure 5-16. At normal X-ray incidence the dominant E vector component lies in the surface plane and the π resonance dominates at 120 K, the σ* resonance is predominantly excited at 800 K. 115

Figure 5-17. Schematic of the possible absorption and desorption mechanism of CO on Mo(110). 118

Figure 6-1. Valence band spectra of 0.5 L O₂ preadsorbed at 120 K, with various CO exposures on W(110) at 120 K. 121

Figure 6-2. Fitting of valence band spectra 0.5 L O₂ preadsorbed at 120 K, derived by various CO exposure. 122

Figure 6-3. Valence band spectra of 0.8 L CO preadsorbed at 120 K, with various O₂ exposures on W(110) at 120 K. 125

Figure 6-4. Difference valence band spectra of 0.5 L CO preadsorbed at 120 K, with various O₂ exposures on W(110) at 120 K. 127

Figure 6-5. Difference valence-band spectra of 3.0 L CO adsorption on a 0.8 L O₂ covered Mo(110) surface recorded after heating to indicated temperatures at the photon energy 80 eV. 128

Figure 6-6. The thermal desorption spectra of 3.0 L CO adsorbed at 950 K with various oxygen exposures (0~30 L) preadsorbed on W(110) at 300 K. 130

Figure 6-7. Schematic drawing of the conversions channels for CO adsorbed on Mo (110). 133

Figure 7-1. Schematic of the possible bonding orientations CO on the BCC(110) plane. 137

 The adsorption and desorption of CO on clean and oxygen-modifed W(110) and Mo(110) surfaces have been investigated using thermal desorption spectrometer(TDS), X-ray electron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS) and photoelectron spectroscopy (PES) by synchrotron radiation under ultra high vacuum conditions.

After CO saturation at room temperature, two desorption states, called α and β were observed at about 400 and 1150 K in thermal desorption spectra, respectively, From the thermal desorption spectra analysis, we could see that, at 900 K, the kinetics of the β-CO follows the first order kinetics, indicating molecular CO on W(110) surface. This result is contrary to the previous studies where CO molecule is dissociated on the surface.

According to the valence band spectra, we observed two peaks at near 10.7 eV (4σ) and 7.0 eV (5σ+1π), indicating the molecular adsorption of CO at about 120 K. There are two types of the adsorbed carbon monoxide at low temperatures below 300 K, one of them, called α₁ state is characterized by 4σ bonding energy of 11.4 eV and the other, called α₂ state, by that of 10.6 eV. Comparison of these results with surface vibrational studies anc TDS studies indicates that the α₁ and α₂ states are the strongly inclined species and the conventional end-on species, respectively. The species can be identified by detection of 4σ around 900 K, which provides a direct evidence of non-dissociation, that the adsorbed layer in β state consists of lying-down species. Comparing the energy separation, Δ(4σ-1π), between 4σ and 1π valence band spectra peaks of chemisorbed CO, we found the increased separation that reflects a decreases of C-O bond strength. This can also be explained with the contribution of the 4σ orbital to the CO - metal bond. Also, similar results have been found in the case of CO/Mo(110).

The effect of preadsorbed (postadsorbed) O₂ on the behavior of CO on W(110) and Mo(110) has also been studied. Valence band spectra data have provided unambiguous evidence for the conversion of adsorbed CO from an inclined lying-down species (α₁ state ) to a conventional, terminally bonded chemisorption species (α₂ state) by the subsequent addition of oxygen at low temperatures. Also, the adsorption of oxygen removes electron density from the surface causing electron donation to the π* orbital of inclined carbon monoxide to decrease and resulting in bridged and terminal CO formation.

On the basis of the results, we suggest that β state of CO on W(110) and Mo(110) may not be dissociated and has an adsorption geometry of lying-down species.