  We consider the superposition of plane waves and localized wave packets. This
kind of wave function can result from a local excitation of a particle
described by a plane wave. For charged particles, the wave packet means a
current, the time integral of which results in an extra charge, additionally to
the one carried by the plane wave. When the duration of the wave packet is too
short for its details to be resolved experimentally, it is the extra charge
that can be determined and analyzed for information regarding the nature of the
interaction that created the wave packet. Assuming that the wave function is
known initially, we show an analytic method for the calculation of the extra
charge by the aid of Fourier transform. Our approach is verified by comparing
to both the well-known case of a Gaussian wave packet and numerically obtained
results. As an important physical example, finally we consider the case of
local excitation by laser pulses.
