  The coherent interaction with ultrashort light pulses is a powerful strategy
for monitoring and controlling the dynamics of wave packets in all states of
matter. As light presents an oscillation period of a few femtoseconds
($T=2.6$~fs in the near infrared spectral range), the fundamental light-matter
interaction occurs on the sub-cycle timescale, i.e. in a few hundred
attoseconds. In this work, we resolve the dynamics of autoionizing states on
the femtosecond timescale and observe the sub-cycle evolution of a coherent
electronic wave packet in a diatomic molecule, exploiting a tunable ultrashort
extreme ultraviolet pulse and a synchronized infrared field. The experimental
observations are based on measuring the variations of the extreme ultraviolet
radiation transmitted through the molecular gas. The different mechanisms
contributing to the wave packet dynamics are investigated through theoretical
simulations and a simple three level model. The method is general and can be
extended to the investigation of more complex systems.
