In the first part of this work, eleven two-photon transitions originating from the 2p 5 3s[3/2] 2 , 2p 5 3s [1/2] 0 , 2p 5 3s[3/2] 1 , and 2p 5 3s[1/2] 1 states to the 2p 5 4d configuration states of neon have been investigated using optogalvanic spectroscopy in the visible region (570–626 nm) . The two-photon assignments were confirmed by evaluating the temporal evolution, power dependency, and line widths of the optogalvanic signals. In order to study the dynamics of plasma, temporal evolution of two-photo optogalvanic signals of neon was also evaluated. Optogalvanic signals for four transitions from the metastable 2p 5 3s[3/2] 2 state to 2p 5 4d 0 [3/2] 1 , 2p 5 4d 0 [3/2] 2 , 2p 5 4d 0 [5/2] 3 and 2p 5 4d 0 [5/2] 2 states were recorded over a range of discharge currents (3.4–9 mA). It was found that the shape of the optogalvanic signal was strongly dependent on the discharge current. In the second part, synchrotron radiation was used to obtain the spectra of triplet and singlet metastable helium atoms resonantly photoexcited to doubly excited states. The first members of three dipole-allowed 1,3 P series have been observed and their relative photoionization cross sections determined, both in the triplet (from 1s2s 3 S) and singlet (from 1s2s 1 S) manifolds. The intensity ratios were drastically different with respect to transitions from the ground state. When radiation damping is included the results for the singlet are in agreement with theory, while for triplets spin-orbit interaction must also be taken into account. In the final part of this thesis, The photo-dissociation spectroscopy of the protonated benzaldehye was investigated. The spectrum of protonated benzaldehyde was recorded in the 435–385 nm wavelength range. The first excited state is a ??* state, strongly red shifted compared to the ??* state of neutral benzaldehyde. The spectrum presents well resolved vibronic bands in contrast to some other protonated aromatic molecules like benzene or tryptophan in which the excited state dynamics is so fast that no vibrational structure can be observed. The bands can be assigned on the basis of a Franck–Condon analysis using ground and excited state frequencies calculated at the CC2/TZVP level.