The photon Doppler velocimetry (PDV) spectrum is investigated in an attempt to reveal the particle parameters of ejecta from shock-loaded samples in a vacuum.A GPU-accelerated Monte-Carlo algorithm,which considers the multiple-scattering effects of light,is applied to reconstruct the light field of the ejecta and simulate the corresponding PDV spectrum.The influence of the velocity profile,total area mass,and particle size of the ejecta on the simulated spectra is discussed qualitatively.To facilitate a quantitative discussion,a novel theoretical optical model is proposed in which the single-scattering assumption is applied.With this model,the relationships between the particle parameters of ejecta and the peak information of the PDV spectrum are derived,enabling direct extraction of the particle parameters from the PDV spectrum.The values of the ejecta parameters estimated from the experimental spectrum are in good agreement with those measured by a piezoelectric probe.