Off-stoichiometric surface reconstruction in the altermagnetic candidate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>RuO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>(100)/Ru(<mml:math xmlns:mml="http://www.w3.org/19

Recently, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>RuO</a:mi><a:mn>2</a:mn></a:msub></a:math> has gained significant attention as a promising altermagnetic candidate. However, the presence of magnetism in <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>RuO</b:mi><b:mn>2</b:mn></b:msub></b:math> has been a topic of ongoing debate, with stoichiometry identified as a key factor by Smolyanyuk []. To explore the stoichiometric effects in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>RuO</c:mi><c:mn>2</c:mn></c:msub></c:math>, we oxidized pure single-element Ru at several temperatures. The band structure modulation near Fermi level by stoichiometric variation was clearly observed by angle-resolved photoemission spectroscopy. First-principles calculations revealed that this evolution was relevant to surface reconstruction induced by excessive O rather than altermagnetism. Additionally, our results indicate that the experimentally measured band structure of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>RuO</d:mi><d:mn>2</d:mn></d:msub></d:math>(100)/Ru(<e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mn>10</e:mn><e:mover accent="true"><e:mn>1</e:mn><e:mo>¯</e:mo></e:mover><e:mn>0</e:mn></e:mrow></e:math>) aligns well with the nonmagnetic band calculations. Our findings not only provide a clue to resolve the current experimental discrepancy but also support the nonmagnetic ground state of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msub><g:mi>RuO</g:mi><g:mn>2</g:mn></g:msub></g:math> films grown on Ru metal.