In this dissertation we present studies of two Coulomb bound systems of very different nature: triplet excitons which are neutral strongly bound states possesing a long lived spin which can addressed by magnetic field and electrons on helium bound to their image charge inside liquid helium owning large dipolar momentum and which are thus sensitive to electric field. In the sense these two systems are complementary and we will show that in both cases simple quantum mechanical models allow to gain a deep insight in the system structure. This Dissertation is divided in two parts. In the first part we investigate bi-exciton fine structure and geometrical aspects of their formation in organic semiconductors. In the second part we demonstrate a new way to realise a quantum electrodynamics system of an atom interacting with an oscillator. Thus we start with rather applied research and then continue with fundamental physics. Despite of very different physical systems explored, both subjects are linked by the experimental spectroscopic methods which we developed.Singlet fission materials exhibit great richness of such phenomena and combine processes and effects occurring in many various physical systems, like photoluminescence, spectral hole burning, excitons formation, diffusion and annihilation, dipolar and exchange coupling, photon up- and down-conversion etc.We present here careful studies of such an intrinsic spin property of TIPS-tetracene as the bi-exciton formation following singlet fission. We focused on two aspects of spin composition in TIPS-tetracene. First, we will present intra-triplet coupling studies. For that we used optically detected magnetic resonance, to probe different spin species and follow their evolution in varying conditions. From those measurements we obtain dipolar coupling parameters for triplet and quintet excitations, examine samples’ morphology and determine not only triplet excitons orientations in the samples, but also establish inner geometry of bi-exciton formation by two exchange-coupled triplet excitons - identify specific molecular sites on which quintet (bi-exciton) states reside via correlating experimentally extracted spin structure with the molecular crystal structure.Then another aspect concerns the coupling actually forming a quintet state of two triplets - we present collaborative results on inter-triplet exchange coupling investigated by means of high field magneto-photoluminescence measurements. They allow us to assign certain exchange-coupling strength values to various triplet pairs and specify photoluminescence spectra corresponding to each pair.2 dimensional electron gas (2DEG) on the liquid helium surface is interesting for fundamental research and applied science at the same time. For both interests one needs to be able to manipulate the system and to tune it finely. We profit from extraordinary characteristics of this system to realise a fundamental quantum electrodynamics (QED) system, in which an atom interacts with an electromagnetic field.Using high purity 2DEG is indeed an original approach to QED with its’ proper advantages and difficulties we managed. We applied perpendicular electric field to confine the electrons, thus inducing Rydberg energy levels series, which then plays role of a hydrogen-like atom. Then perpendicular magnetic field quantized the electrons’ in-plane motion, resulting in equidistant Landau levels - an oscillator for the QED system. Further to couple the atom to the oscillator we mixed perpendicular and in-plane motion degrees of freedom by applying parallel magnetic field. Energy levels are monitored using Stark spectroscopy measurements. In order to describe this coupling we derived Jaynes-Cummings type Hamiltonian with no fitting parameters and it showed perfect agreement with the experimental results. Thus we realised experimentally coupling between quantum objects easily tunable by the parallel magnetic field strength.