4.1: Spectroscopy- Basic Elements and Principles In this Chapter, we will introduce a general notion of spectroscopy as a method and of its most basic type of data, a spectrum. We will also introduce the most basic features of each spectroscopic signal (spectral line or resonance): position, intensity and width/lineshape. Lastly, we will take a look at a relationship between quantum chemistry features of a sample (energy levels of electrons and other quantized particles) and spectral properties of such a sample. 4.2: “Two Masses on a Spring” Model and Infrared (IR) Spectroscopy his Chapter presents a basic view of a classical physics model of ???two masses on a spring??? and shows how this model can help understand and interpret infrared (IR) spectroscopy data. This will be one of our examples of some theoretical foundation for a certain type of spectroscopy. This theory-experiment connection is important for every type of spectroscopy because spectroscopic data always need to be interpreted and it is not possible without a sufficiently accurate theory. 4.3: Quantum Mechanics and Quantum Oscillator Model This Chapter describes a basic model which allows to explain the phenomenon of quantization of energy levels for a system of electrons in atoms (that is the quantum mechanical models of levels available for a sub-atomic particle). The model presented here ("particle in a box") is much more basic than the ones used by quantum chemists today, yet this model allows understanding of the basics of quantum mechanics as a guiding theory vital for modern experimental physics and chemistry. 4.4: Fluorescence and Phosphorescence This chapter explains fluorescent spectroscopy using quantum mechanics, focusing on electron energy levels that determine absorption and emission spectra. It differentiates between fluorescence, which is quick, and phosphorescence, which is slower due to spin flips. The Jablonski diagram illustrates these processes, showing energy transitions within molecules.