  Type III radio bursts are intense radio emissions triggered by beams of
energetic electrons often associated with solar flares. These exciter beams
propagate outwards from the Sun along an open magnetic field line in the corona
and in the interplanetary (IP) medium. We performed a statistical survey of 29
simple and isolated IP type III bursts observed by STEREO/Waves instruments
between January 2013 and September 2014. We investigated their time-frequency
profiles in order to derive the speed and acceleration of exciter electron
beams. We show these beams noticeably decelerate in the IP medium. Obtained
speeds range from $\sim$ 0.02c up to $\sim$ 0.35c depending on initial
assumptions. It corresponds to electron energies between tens of eV and
hundreds of keV, and in order to explain the characteristic energies or speeds
of type III electrons ($\sim 0.1$c) observed simultaneously with Langmuir waves
at 1 au, the emission of type III bursts near the peak should be predominately
at double plasma frequency. Derived properties of electron beams can be used as
input parameters for computer simulations of interactions between the beam and
the plasma in the IP medium.
