Direct visualization of hydrogen-transfer intermediate states by scanning tunneling microscopy*

Hydrogen atoms bonded within molecular cavities often undergo tunneling or thermal-transfer processes that play major roles in diverse physical phenomena. Such transfers may or may not entail intermediate states. The existence of such fleeting states is typically determined by indirect means, while their direct visualization has not been achieved. Here we use density-functional-theory (DFT) calculations and scanning-tunneling-microscopy (STM) simulations to predict a unique STM-image signature of thermal-transfer double-C morphology for the cis-intermediate of the two-hydrogen transfer process that occurs in metal-free naphthalocyanine (NPc) molecules adsorbed on Ag(111) surfaces. As guided by the theoretical predictions, at elaborately adjusted scanning temperature and bias, STM experiments achieve a direct visualization of the cis-intermediate. This work demonstrates a practical way to directly visualize elusive intermediates, which enhances understanding of the quantum dynamics of hydrogen atoms.