Using the dual laser-produced plasma technique photoabsorption studies of Mg, A1 and Si plasmas have been carried out. The objective was to measure the 2p-subshell absoiption in neutral and ionised species of the Na I, Mg I, Al I and Si I sequences prepared both in ground and valence-excited states. The spectra were recorded on a grazing incidence 2m Hilger stigmatic spectrograph and a 2.2m McPherson spectrometer equipped with MCP/PDA multichannel detection. In an attempt to distinguish the origin of the various absorption features observed, and to record almost pure spectra of the species of interest, a number of time and space-resolved studies were carried out. These studies which exploit the temporal and spatial evolution of absorbing species in a laser-generated plasma plume were very successful, particularly, for neutral and singly ionised species. To facilitate the analyses of the spectra a range of ab initio atomic structure configuration-interaction Hartree-Fock (CIHF) calculations were carried out. The Mg sequence in ground state absorption has been studied for atomic Mg and Mg-like ions Al+ and Si2+. The effects of configuration-interaction in the ground and excited states have been examined in detail. Interpretation has been provided for the distribution of oscillator strength and the relative positions of observed transitions. The Mg sequence in excited state absorption is reported for Mg, Al+ and Si2+. In order to provide insight into the dynamics of excited levels a comparison is made with the ejected-electron spectra of the corresponding atomic and ionic species arising from the autoionising and Auger transitions following electron- and photon-impact excitation/ionization. The 2p-subshell photoabsorption study along the A1 sequence in ground state has been carried out for atomic A1 and Al-like ionic Si+. In this work a complete analysis of these three electron systems is reported. The 2p absorption study of Al-like Si+ in their excited states has also been carried out. The 2p-subshell photoabsorption spectrum of ground state atomic Si has been investigated. In addition to providing interpretation for the observed spectrum, a comparison is made with molecular spectra. Photoabsorption spectra of the isoelectronic species Mgr1-, Al2+ and Si3+ both in ground and valence-excited states are presented and await analysis. The overall results show that 2p photoabsorption of atoms and ions along the Mg I, Al I and Si I sequences in ground and excited states is dominated by the 2p63sa3pb -» 2p53sa3pb[(n+l)s + nd] + 2p53sa_13pb+1np, for n > 3, and 2p63sa*13pb+1 -» 2p53sa3pb+1 transitions respectively. In ground state absorption spectra the 2p53sa3pb[(n+l)s + nd] Rydberg series members are strongly perturbed by the 2p53sa_13pb+1np doubly-excited states. In moving from neutral to ionic absorption along the sequences dramatic changes are observed. However, in the valence-excited spectra the absorption structures corresponding to the 2p53sa3pb+1 type configurations are observed to be unperturbed and same in appearance along the sequences.