Bandwidth-controlled Mott transition in $κ-(BEDT-TTF)_2 Cu [N(CN)_2] Br_x Cl_{1-x}$ I. Optical studies of localized charge excitations

Infrared reflection measurements of the half-filled two-dimensional organic conductors $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br$_{x}$Cl$_{1-x}$ were performed as a function of temperature ($5 {\rm K}