In laser-irradiated plasmas, the Langdon effect alters the electron energy distribution function (EEDF) from a Maxwellian to a super-Gaussian profile^[1], which in turn affects the dynamics of backward stimulated Brillouin scattering (SBS)^[2]. This study examines the influence of the Langdon effect on the nonlinear evolution of SBS in Au plasmas using Vlasov-Maxwell simulations^[3], focusing specifically on electron-ion collisions. In the absence of electron-ion collisions, the super-Gaussian EEDF results in a higher ion acoustic wave (IAW) frequency, which slows the initial growth rate of SBS compared to the Maxwellian EEDF. As the IAW amplitude increases, higher-order harmonics and ion trapping occur, which cause nonlinear frequency shifts and spectral broadening. These effects contribute to the saturation of SBS, maintaining it at a relatively low reflectivity in a quasi-steady state. Under the super-Gaussian EEDF, the reduction in IAW nonlinearity and Landau damping leads to a higher saturation level of SBS. However, when electron-ion collisions are considered, they dampen the rapid growth of the IAW, suppress frequency shifts, and limit spectral broadening. This results in a higher average reflectivity for SBS, even with the same initial EEDF. In this case, the Langdon effect enhances SBS further by lowering IAW nonlinearity and inverse bremsstrahlung (IB) absorption. The reduced IAW nonlinearity leads to weaker harmonics, while the decrease in IB absorption allows the pump wave to propagate with a higher amplitude, significantly enhancing the SBS reflectivity.
 

laser plasma instability,stimulated Brillouin scattering,Langdon effect