Laser illumination of the junction of a scanning tunneling microscope (STM) modulates the barrier height by adding the electric field of light. Numerical simulations suggest that the dependence of tunneling current on the magnitude and frequency of the modulation can serve as a basis for determining the duration of barrier traversal. Modulation increases the mean value of the current when the frequency is much less than the reciprocal of the traversal time. Our simulations also suggest that a 670 nm laser diode at a power density of 100 W/cm2 will reduce the tunneling current. The latter effect is contrary to most competing phenomena such as current rectification, photo-assisted tunneling, thermal-assisted tunneling, and thermal expansion. We are completing a novel STM which has decreased noise and increased stability in order to make the laser/STM measurements. © 1993 John Wiley & Sons, Inc.