Ultrahigh spatial resolution of two-photon photoelectron images (as high as
3 nm, which is the best value that has been achieved to date in photoelect
ron microscopy with spatial resolution) is obtained when silicon nanotips a
re irradiated by the second harmonic of a pulsed femtosecond Ti : sapphire
laser. In addition, the absolute value of the two-photon external photoeffe
ct coefficient is measured. (C) 1999 American Institute of Physics. [S1063-
7761(99)01105-1].