  We investigate the scattering of a wave packet describing low-energy
electrons in graphene by a time-dependent finite step potential barrier. Our
results demonstrate that, after Klein tunneling through the barrier, the
electron acquires an extra energy which depends on the rate of change the
barrier height in time. If such a rate is negative, the electron loses energy
and ends up as a valence band state after leaving the barrier, which
effectively behaves as a positively charged quasi-particle
