  Free-electron laser hard X-ray light sources can provide high fluence,
femtosecond pulses, enabling the time-resolved probing of structural dynamics
and elementary relaxation processes in molecules. Traditional X-ray elastic
scattering from crystals in the ground state consists of sharp Bragg
diffraction peaks that arise from pairs of molecules and reveal the ground
state charge density. Scattering of ultrashort X-ray pulses from gases,
liquids, and even single molecules is more complex and involves both single-
and two- molecule contributions, diffuse (non-Bragg) features, elastic and
inelastic components, contributions of electronic coherences in nonstationary
states, and interferences between scattering off different states (heterodyne
detection). We present a unified description that covers all these processes
and discuss their relative magnitudes for gas-phase NaI. Conditions for the
observation of holographic (heterodyne) interference, which has been recently
discussed [1], are clarified.
