We present a study of two-beam coupling energy transfer arising from electr
onic and nuclear (i.e., Raman) contributions to the third-order refractive
nonlinearity. Calculations of transient two-beam coupling in fused silica w
ith no free parameters agree reasonably well with experiments. Numerical ca
lculations of two-beam coupling in a medium with a purely electronic nonlin
earity modeled by a Debye-type response are presented. The calculations ind
icate that it should be possible, although challenging, to measure electron
ic two-beam coupling for response times as short as similar to 0.1 fs. Expe
riments designed to isolate the electronic response fail to produce two-bea
m coupling signals but do indicate that the time scale of the electronic re
sponse is well below ifs. (C) 2000 Optical Society of America [S0740-3224(0
0)02109-3].