Hey! I have classes of physical chemistry (also had physics in high school) and have some misconceptions about how quantum mechanics work. The question I want to ask is: what's the difference between luminescence (fluorescence/phosphorescence) and (not sure about terminology here) concepts of the Bohr's model, lasers. I know that the question is a mess, so I want to clarify some things. In school I learned that a specific atom can absorb a photon that has a specific amount of energy that will excite an electron to elevate to a specific energy level which is noted as n = 0, 1, 2... . Also I learned that the electron can jump between different energy levels and emit photons e.g. go from level 4 to 3 an emit a photon of some energy(frequency) and basically produce light (visible or not). Now in my physical chemistry classes I learned about fluorescence and that an electron emits a photon only when it goes from S1(excited first singlet state) to S0(ground state) and can absorb any kind of photon (the electron non-radiatively relaxes to S1 state). So which one is it? Is it possible to jump just from S1 to S0 or between any two energy levels? I am aware that I'm most likely confusing completely different concepts, but what should I read to understand it all? Another small question: How come fluorescence produces an emission spectrum(!) when the electron changes states only from S1 to S0 (shouldn't the ΔE be the same every time, thus the frequency of the emitted light would be the same).