This thesis is composed of two parts. The objective of the first part was to investigate the influence of the intermediately coupled plasma environment on the radiating hydrogen-like aluminium ions by using high resolution, high dispersion vertical geometry Johann spectrometer. Benefiting from the high spectral and spatial resolution of the spectrometer as well as from optimized sandwich-target geometry, the influence of the ionization potential lowering and the influence of electron-plasma waves in the emission spectra are observed. These results have both practical and theoretical importance. Practical, because some signatures observed in the emission spectrum lead to a new density diagnostic, and, theoretical, because they can be used to test different theoretical formulations of radiator-plasma interactions. The second part of the thesis reports about the quantitative investigations of atomic motions in semiconducting samarium mono-sulphide (SmS) using optical pumping and X-ray probing. Coherent acoustic phonon propagation in semiconducting SmS is observed. The observed acoustic phonons have frequencies up to about 0.08 THz. The time resolved diffraction data are in good agreement with theoretical predictions for coherent phonon excitation, demonstrating that, up to the laser fluence of about 10 mJ/cm2 SmS behaves as a standard semiconductor, i.e., a semiconductor which (at the laser fluences used) shows no interplay between valence fluctuations and lattice vibrations.