Investigating a polycrystalline gold laver on glass bq a scanning tunn
eling microscope in air, tunneling current oscillations were found. wh
ich are excited by the DC voltage across the gap. The oscillation ampl
itude is dependent on the place on the surface of the sample and corre
lates with its topography. The frequency spectra of these oscillations
are influenced by resonances of the mechanical system (z-piezo/sample
holder/transducer/sample). A piezoelectric transducer is able to dete
ct alternating forces originating from the tunnel junction. The resona
nces in the spectrum of the AC tunneling current and the mechanical re
sonances of the STM system seem to be related. Trapping and subsequent
delayed desorption of charge carriers at localized surface states cou
ld play a role in generating the observed time-dependent forces across
the gap and thereby creating tunneling current oscillations.