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The electrostatic potential mapped on the electron density contour of gas phase molecules is used to identify several molecules physisorbed on transition metals imaged by scanning tunnelling microscopy within the molecular HOMO–LUMO gap. It is rationalized that the inverted of this potential, representing the potential energy felt by a negative test charge, is a good concept to interpret changes in the dielectricity constant through the influence of physisorbed molecules and thus to understand their submolecular resolution images. The validity of the concept is demonstrated on dichlorobenzene, chloronitrobenzene, and two azobenzene derivates on the surfaces of Au(111), Ag(111), and Cu(111).

Fig. 1. (colour online) meta-Dichlorobenzene: (a) LUMO, HOMO, and ESP as calculated with ArgusLab 4.0.1, Mark A. Thompson, Planaria Software LLC, Seattle, WA, http://www.arguslab.com; in ESP the purple end of the spectrum shows regions corresponding to decreased electron density and green shows regions of increased electron density; (b) STM image on Au(1 1 1), 17 pA, 85 mV, the line depicts the distance between the two chlorine atoms; it has same length as the line in the ball-and-stick model; (c) STM image on Au(1 1 1) with modified tip, 60 mV, 410 pA, 9 K, lines as in (b).