Desorption mechanism of hydrogen isotope from metal oxides Contents 1.Background 2.Experimental system and Mechanism 3.Results and discussion 4.Conclusions Yasuhisa Oya, Ryushi Jinzenji, Takuji Oda and Satoru Tanaka The University of Tokyo

Background （ tritium decontamination) Methods:surface washing, thermal desorption, light irradiation etc. For the safety of a fusion reactor ， effective decontamination methods from construction materials of a reactor （ such as piping materials, vacuum chamber and so on ） are required. Advantage of light irradiation Effective for strongly bonded OH ， non-contact ， selectivity of reaction products. To understand a desorption behavior of hydrogen isotope from metal oxides and construct desorption models of desorbates from those surfaces. To estimate the effect of electron transition between components （ Fe 2 O 3 -TiO 2 ) on the behavior of desorbates ． Purpose Current situation ： Desorption process by light irradiation has not understood yet.

Different species of H 2 O adsorption on metal oxides There are several chemical form of H 2 O adsorbed on metal oxides. M M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O M H O H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H Molecular adsorption [Hydrogen bonding] M H O M H O M H O M H O M H O M H O M H O M H O M M M M H O H Dissociative adsorption （ chemical form:OH ） M M M H O H H O H O O Molecular adsorption （ intermolecular force ） Light irradiation H O Thin metal oxide film Desorption

Characteristic of the two main desorption process Thermal desorption Photon-stimulated desorption (PSD) M H O M H O H O H Negative charging M H O M H O Vertical vibration e-e- e-e- H O H OH - TiO 2 Energy 4σ 1π 3σ occupied Unoccupied Vertical vibration 1-photon process Higher translational energy than that via thermal process Random vibration Thermal equilibrium

To understand a desorption behavior of hydrogen isotopes which are adsorbed on metal oxides in several chemical forms. Methods In order to construct the desorption model of hydrogen isotope, Analysis of desorption behavior Velocity distribution Desorption amount Laser power dependency Wave length dependency Adsorption state By using TOF technique, we can obtain information about Velocity distribution of desorbed species from the surface Analysis of chemical form and desorption amount. Desorption behavior Adsorption state etc. Time of Flight Mass Spectroscopy (TOF-MS)

Correspondence of experiments to adsorption states Experimental conditions ２ ４℃ Sample temperature H 2 O exposure in the chamber Estimated adsorption state （ chemical form ： H 2 O ） ○ physical （ multilayer ） △ dissociative （ chemical form:OH ） ◎ physical （ a few layers ） △ dissociative (chemical form:OH ） ◎ dissociative (chemical form:OH ） １５ ０℃ TiO 2 Fe 2 O 3 ◎ physical （ a few layers ） △ dissociative (chemical form:OH ） Exposure （ 2x10 -5 Pa) No exposure (4x10 -6 Pa ）

Detector and MCP Sample and Anode Flight tube Laser for desorption Probe laser Time of Flight Mass Spectroscopy Ionization laser:266nm,43mJ Laser for desorption:355and 430nm, mJ Total pressure in the chamber : 6~8x10 -6 Pa Partial pressure of H 2 O : 3~4x10 -6 Pa

Repulsion between electrons by space- charge effect in MCP ⇒ Causing the width of TOF spectrum v [m/s] Desorption amount at each delay time is estimated as follows; MCP Detector Diffusion(Thermal and concentration gradient ） Velocity error at the ionization point is supposed to be zero t Area ： Q (charge ） I [A] flux jacobian Principle of measurement of velocity distributions acceleration ( v=s/τ)

Velocity distribution Desorption amount of desorbates Desorption amount can be estimated as the area of velocity distribution. By calculating the desorption amount, we can estimate the laser intensity dependence for desorption amount obtained in various experimental conditions.

Sample preparation of TiO 2, Fe 2 O 3, Fe-Ti(5at%) oxide. TiFeFe-Ti(5at%) Purity [%] Thickness [mm] Oxidation time [hour]222 Partial pressure of O 2 [Pa] 10 Ti, Fe and Fe-Ti alloy(5at%) were used as samples. These samples were oxidized in the vacuum chamber and thin oxide film was formed on the surface of these metals.

Result 1 TiO 2 rutile (Band:3.0eV) ~Laser power dependency of desorption amount of H 2 O 430 nm(2.89eV) < Bandgap(3.0eV) 355 nm(3.49eV) > Bandgap In the case of 355 nm(>bandgap), Exponentially increasing →Hot electron does not transit efficiently from conduction band to unoccupied orbital of OH. Desorption via both thermal and PSD process occurs. Bandgap OH - TiO 2 E 4σ E BG 3.0eV

Result 2 Fe 2 O 3 (Band:2.2eV) ~Laser power dependency of desorption amount of H 2 O In the case of 430 nm, because of easy transition of hot electron, most H 2 O desorbed from surface via 1-photon process so that the desorption amount of H 2 O increases linearly with laser power. 4σ E BG 2.2eV OH - Fe 2 O 3 E 4σ E BG 2.2eV OH - Fe 2 O 3 E

Result 3 Fe-Ti (5at%) oxide ~Laser power dependence for the desorption amount of H 2 O

Sample temperature 24 ℃ 150 ℃ Sample  H 2 O exposure Non H 2 O exposure Fe 2 O 3 (E BG :2.2 eV) 430nm (>E BG ) PSD (MMB, linear) PSD (MMB, linear) PSD (MB+MMB,exp) 355nm (>E BG ) PSD+Thermal (MB?, exp.) TiO 2 (E BG :3.0 eV) 430nm (<E BG ) Thermal (MB, exp.) No or little desorption 355nm (>E BG ) PSD+Thermal (MB?, exp.) PSD +Thermal (OH >> H 2 O) Fe-Ti (5at%) (E BG :??) 430nm (>E BG ) PSD (MMB?, linear) PSD (MMB?, linear) PSD (MMB?,linear) 355nm (>E BG ) PSD (MMB?, linear) PSD (MMB?, linear) PSD (MMB?,exp) Desorption process (Fitting results, Laser intensity dependence)

Desorption mechanism (Now under consideration) 2.89eV 3.49eV 430nm 355nm In case of a transition of hot electron, the conduction band of Fe plays an important role. In case of 430nm, only one path can be considered. On the other hand, in case of 355nm, two paths in which hot electrons transit between components of bulk can be considered. 4σ Conduction band E BG 2.2eV 95%5% OH - Fe 2 O 3 E TiO 2 Valence band λ:430 nm E OH - Fe 2 O 3 TiO 2 5% 95% 4σ E BG 3.0eV λ:355 nm

Conclusions  The efficiency of the electron transfer from the substrate to the adsorbed species makes a large influence on the water desorption.  To achieve high efficiency of water desorption, an overlap between the conduction band of substrate and the unoccupied orbital plays an important role in the electron transfer from the substrate to absorption species.  For Fe-Ti(5at%) oxide, the desorption process caused by the effect of electron transfer between Fe and Ti oxides would exist.