Near-field spectroscopy of nanoparticles was performed with a subnanom
eter probe size, using a scanning transmission electron microscope. at
nonintersecting beam-particle configuration, surface collective modes
are isolated from the entire excitation spectrum. The relativistic na
rrow beam generates a new type of spatial dispersion and may induce an
additional spectral quantization. Line intensity variations during th
e mapping can be accounted for quantitatively by using a relatively si
mple theoretical model. This method is also suggested as a new way for
transmission electron microscopy studies of beam sensitive samples.