The transient tunneling current induced by pulsed laser irradiation of a scanning tunneling microscope (STM) tunneling gap was observed to occur over a 100 &mgr;s time scale range in response to a 20 ns duration of the laser pulse. The amplitude of the transient current varies exponentially with laser power, confirming our previous suggestion that thermal expansion of the STM tip is the main source of the transient increase of tunneling current. This thermal expansion mechanism is also supported by the observation of a qualitatively similar variation of the tunneling current during the piezo-driven decrease of the tip-sample separation. ©1997 American Institute of Physics.