Fig. 1: Formation of the alkali trimer coordinated with a CO₂^δ- radical anion on copper.

a Constant-current STM image of Cs/Cu(111) surface. b, c STM images of 3Cs⁺∙CO₂^δ- clusters on Cu(111) (b) and Cu(100) (c) surfaces. d Top and side views of the geometric model of 3Cs⁺∙CO₂^δ- cluster on Cu(100). Cs, Cu, C, O atoms are denoted by green, yellow, grey and red spheres, respectively. e–g Magnified STM image (e), constant-height AFM images (\(\Delta f\)) (f) and simulated AFM images (g) of the 3Cs⁺∙CO₂^δ- cluster on Cu(100). The black arrows in d, f, g denote the lower Cs⁺ at the bridge site of Cu(100) surface. Set point of STM images: −300 mV, 20 pA (a); 100 mV, 10 pA (b); 300 mV, 5 pA (c, e). The tip heights of experimental and simulated AFM images are marked on the images. The tip heights in (f) are referenced to the STM set point on the Cu substrate (100 mV, 10 pA). The tip heights in (g) are defined as the vertical distance between the apex atom of the metal tip and the outermost atom of the Cu substrate. The oscillation amplitude of the experimental and simulated AFM images is 100 pm. All the STM/AFM images were obtained at 5 K.