  Tunnel spectroscopy is used to probe the electronic structure in GaAs quantum
well of resonant tunnel junction over wide range of energies and magnetic
fields normal to layers. Spin degenerated high Landau levels ($N=2\div7$) are
found to be drastically renormalised near energies when the longitudinal
optical-phonon ($\hbar\omega_{LO}$) and cyclotron energy ($\hbar\omega_{C}$)
are satisfied condition $\hbar\omega_{LO}=m\hbar\omega_{C}$, where $m=1,2,3$.
This renormalisation is attributed to formation of resonant magnetopolarons,
i.e. mixing of high index Landau levels by strong interaction of electrons at
Landau level states with LO-phonons.
