Optical phase manipulation of nonresonant frequencies is investigated as a method of achieving optimal population transfer during resonant impulsive stimulated Raman scattering. Wave packets containing quantum beats between an initially prepared rovibrational level in the A((1)Sigma(u)(+)) electronic state of Li-2 and states populated via a resonance-enhanced rotational Raman process are created using a shaped ultrafast pulse centered near 800 nm. Study of these wave packets allows a quantitative comparison of population transfer as a function of applied phases in the ultrafast pulse. Two cases are explored to determine the ability to enhance population transfer: one with a wide state spacing [A(nu(A)=11, J(A)=28)-A(11,30) at 50.1 cm(-1)] and one with a narrow spacing [A(11,8)-A(11,10) at 16.6 cm(-1)]. In both cases, several different phase masks are applied to the wave packet preparation pulse to enhance the population transferred to the newly formed state of interest. One phase mask involves the application of a -90degrees phase shift to the nonresonant optical frequencies that lie between the resonant transition frequencies, resulting in an optimal phase relationship between pairs of nonresonant frequencies contributing to the Stokes-Raman excitation. Another extends the phase modification to the nonresonant frequencies lying outside the two resonant transitions to allow constructive enhancement from a larger range of frequencies. Significant population enhancements, up to a factor of similar to12, of the newly formed A(11,30) and A(11,10) states are demonstrated. In addition, the dependence on the state spacing and therefore the extent to which nonresonant frequencies affect the population transferred in the stimulated Raman process are demonstrated.