Effect of orbital symmetry on probing the single-molecule Kondo effect

The low-energy excitation spectrum of a metal-free phthalocyanine molecule on Ag(111) is probed in scanning tunneling spectroscopy experiments. The use of functionalized $p$-orbital and $s$-orbital tips leads to markedly different results. While CO-terminated $p$-wave tips probe the zero-energy Abrikosov-Suhl resonance induced by the molecular Kondo effect, Ag-coated $s$-wave tips---in strong contrast---feign the absence of the Kondo effect due to a missing Abrikosov-Suhl resonance. Reducing the vertical distance between the $s$-orbital tip and the molecule progressively unveils the resonance, compatible with findings for the $p$-orbital tip in the far tunneling range. A mechanism based on orbital overlap is suggested as the tentative origin of the observations. The CO-functionalized tip is then used to explore the altered Kondo effect of the tautomerized phthalocyanine.