  We resolve the real-time electric field of a femtosecond third-order
nonlinear optical signal in the molecular frame. The electric field emitted by
the induced third-order polarization from impulsively pre-aligned gas-phase
molecules at room temperature, in a degenerate four-wave mixing (DFWM) scheme,
is measured using a spectral interferometry technique. We show that by
measuring both the amplitude and phase of the emitted femtosecond pulse,
information related to electronic symmetries can be accessed. The nonlinear
signal is measured around a rotational revival to extract its molecular-frame
angle dependence from pump-probe time delay scans. By comparing these
measurements for two linear molecules, carbon dioxide (CO2) and Nitrogen (N2),
we show that the measured second-order phase parameter (temporal chirp) of the
signal is sensitive to the valence electronic symmetry of the molecules,
whereas the amplitude of the signal does not show such sensitivity. We compare
these measurements to theoretical calculations of the chirp observable in the
molecular frame. This work is an important step towards using field-resolved
nonlinear optical measurements to study ultrafast dynamics in electronically
excited molecules.
