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The phase transitions of a three-dimensional complex (dusty) plasma under gravity are studied by means of the dynamic light scattering technique. The thermal energy of the dust component is determined during the phase transition to characterize the phase state. Detailed parametric studies are performed to examine the impact of the main plasma parameters on phase transitions. The results are found to be in good agreement with the predictions of a theoretical description of phase transitions that considers plasma instabilities to be the main heating mechanism. The distribution of energy in the system is strongly anisotropic. In the melted state, the dust particle movement in the vertical direction has much higher thermal energies than in the horizontal plane. The ratio of the two energies is constant and in particular independent of all the plasma parameters studied. Furthermore, the phase transition of the dust component is a gradual process. A transition front moves through the system in the vertical direction until the whole system reaches the new phase state. The velocities of energy fronts for the horizontal and vertical components of particle motion are very different. In particular, the melted state is a strongly inhomogeneous and anisotropic state in terms of energy transport.

The dynamic light scattering (DLS) technique is applied to the dust component of a complex (dusty) plasma, revealing a Gaussian intensity autocorrelation function for scattering angles between 4° and 175°. The Gaussian decay form represents free (ballistic) particle motion and allows determination of the one-dimensional squared particle velocity $\left \langle v_x^2\right \rangle $ . At scattering angles below 1°, the intensity autocorrelation function is shown to be a combination of a Gaussian and an exponential function. This allows determination of the particle velocity and the diffusion constants at the same time. The dust system is fully described by the two components of motion in the horizontal and vertical directions. The two components are simultaneously measured on two scattering paths using only a single incident laser beam. In contrast to standard imaging techniques, the DLS method can be applied even to the disordered phase state where the dust particles have very high kinetic energies. In the ordered phase state, the assumptions of the DLS approach were verified by the independent Charge Coupled Device technique on the fundamental kinetic level. Furthermore, a careful discussion of the standard deviation of the DLS method proves that it can be used to study phase transitions of complex plasmas in detail.