Confused about emission/absorbtion of photons

[tadasbar]

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).

 

[hilbert2]

A liquid or solid sample doesn't have sharp spectral lines like a dilute gaseous sample, because the intermolecular interactions affect the spectrum.

 

[PeterDonis]

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

Actually it's the other way around. The electron can only absorb a photon of one particular wavelength--the one corresponding to the difference in energy between the S0 and S1 states. But it can (depending on the situation, see below) emit photons of different, longer wavelengths. When the absorbed photon is not visible (too short wavelength, usually in the UV range) and the emitted photon is visible, we call this "fluorescence".

Now for that "depending on the situation" part. If the electron were in a single atom, not part of a molecule and not interacting with any other atoms, then in order to get back to the ground (S0) state from the S1 state, it would have to emit a photon of the same energy that it absorbed to get from the S0 to the S1 state--because there are no other states available with energies in between. But if the atom containing the electron is part of a molecule, or is otherwise able to interact with other neighboring atoms, then there are other states that the molecule (or group of neighboring atoms) can transition to that have energies in between the S0 and S1 states. So the energies of the photons emitted in these transitions can be smaller than the energy of the initial photon that got absorbed--hence the wavelength can be longer. There are also ways for the electron to give up energy that don't require emitting a photon at all--see below.

(the electron non-radiatively relaxes to S1 state)

No. Non-radiative relaxation is a process that takes the electron back to the S0 state, from some intermediate state of the molecule (or group of neighboring atoms) in which the electron has some energy higher than that of the S0 state, but lower than the S1 state. In other words, it's one of the ways the electron can lose energy without emitting a photon--instead the energy goes into other degrees of freedom in the molecule.