Theoretical study of hydrogen adsorption on quaternary alloy Ti-Zr-V-Hf non-evaporable getter surface applied in vacuum system of particle accelerators

Non-evaporable getter (NEG) films are extensively employed in particle accelerators to attain and sustain ultra-high vacuum (UHV) and extremely high vacuum (XHV) conditions. This study primarily focused on studying the adsorption characteristics of the main residual gas-H₂ molecules in the vacuum system on the Ti-Zr-V-Hf (110) NEG surface. To carry out this analysis, we employed the first-principles Density Functional Theory (DFT). The adsorption energies of 23 distinct adsorption sites, and particularly the electronic structure of the adsorption sites with the highest absolute adsorption energies (Zr, Ti-V, Hf-V, and Ti-Hf-V), were analyzed by partial density of states (PDOS) and Mulliken charge and bond overlap population calculations. The results of our investigation revealed that the order of effectiveness of adsorption sites is as follows: bridge > hollow > top. Based on the adsorption energy results, it can be inferred that there are strong chemical interactions between H atoms and Ti-Zr-V-Hf (110) surface metal atoms at Zr, Ti-V, Hf-V, and Ti-Hf-V adsorption sites. The results of PDOS calculations also indicate that there is strong hybridization between the H₂ molecule and the Ti-Zr-V-Hf (110) surface at each studied adsorption site, demonstrating the formation of strong chemical bonds between them. The Mulliken charge and bond overlap population and electron density difference analyses show significant changes in charge distribution between H atoms and Ti-Zr-V-Hf (110) surface metal atoms before and after H₂ adsorption at each studied adsorption site, suggesting the occurrence of chemisorption. Additionally, covalent bonds are formed between H atoms and Ti-Zr-V-Hf (110) surface metal atoms at these sites. This study uncovered specific adsorption sites where the H₂ molecule interacts most efficiently on the Ti-Zr-V-Hf (110) surface and where high-energy bonds are formed. These findings can provide a potential pathway to improve the adsorption efficiency of quaternary Ti-Zr-V-Hf NEG films.