  The scattering of conduction electrons by crystalline electric field (CEF)
excitations may enhance their effective quasiparticle mass similar to
scattering from phonons. A wellknown example is Pr metal where the isotropic
exchange scattering from inelastic singlet-singlet excitations causes the mass
enhancement. An analogous mechanism may be at work in the skutterudite
compounds Pr_{1-x}La_xOs_4Sb_12 where close to x=1 the compound develops heavy
quasiparticles with a large linear specific heat coefficient. There the low
lying CEF states are singlet ground state and a triplet at 8 K. Due to the
tetrahedral CEF the main scattering mechanism must be the aspherical Coulomb
scattering. We derive the expression for mass enhancement in this model
including also the case of dispersive excitations. We show that for small to
moderate dispersion there is a strongly field dependent mass enhancement due to
the field induced triplet splitting. It is suggested that this effect may be
seen in Pr_{1-x}La_xOs_4Sb_12 with suitably large x when the dispersion is
small.
