  Electronic properties and lattice vibrations are supposed to be strongly
correlated in metal-halide perovskites, due to the "soft" fluctuating nature of
their crystal lattice. Thus, unveiling electron-phonon coupling dynamics upon
ultra-fast photoexcitation is necessary for understanding the optoelectronic
behaviour of the semiconductor. Here, we use impulsive vibrational spectroscopy
to reveal ground and excited state vibrational modes of methylammonium
lead-bromide perovskite. We observe a coherent phonon at 106 cm-1 (13 meV),
pertaining to the inorganic octahedral, which is peculiar of the electronic
excited state and generated via displacive excitation mechanism. This indicates
the formation of a new geometry, reached after a quarter of the phonon period
T/4=80 fs, and fully equilibrated within the phonon lifetime of about 1 ps. Our
observations unambiguously prove that this mode drives the crystalline
distortion occurring upon carrier generation, implying the presence of
polaronic effects.
