Radiative and Nonradiative Transitions

[fog37]

Hello Forum,
I am trying to get some clarity around the difference between radiative and nonradiative transitions:

CASE 1:
If the incident photons have energy (h*f) that matches that of one of the excited states of the materials, every atom in the material will absorb the photon making a quantum jump to that higher energy level. But in solids and liquids (which are dense), it is very likely that the absorbed excitation energy will not be returned as an emitted photon of the same initial frequency but converted into thermal energy due to random collisions. So the original photon at frequency f vanishes and its energy is converted into thermal energy (infrared photons) which is also a form of electromagnetic energy. The conversion of incident photons to thermal energy process is called resonant dissipative nonelastic absorption.

Do nonradiative transitions correspond to the phenomenon of resonant dissipative nonelastic absorption? Nonradiative transitions still and eventually lead to an emission of radiation, so they are ultimately radiative but at longer wavelengths than the incident wavelengths.

Only in the case of low density gases the incident photons with resonant frequencies that are absorbed are emitted as light (seen in the famous line spectra). In that case, we can talk about resonant radiative processes and resonant radiative emission.

CASE 2:
For solids and liquids, when the incident photons have frequency f that does not match any of the excited states, there is also ground state, non-resonant, non-dissipative elastic scattering which occurs when the incoming light has frequencies which are not resonant. For example, if the incident photon energy is too small to cause an electron excitation to any higher state, the incident photon can still drive the electron cloud into oscillation (without atomic transitions). The electron will remain in its ground state (no electronic transitions) while the cloud vibrates (or rotates) at the frequency of the incident light. I think those vibrations/rotations correspond to the vibrational/rotational excited states. The electron, being accelerated through the mechanism of rotation or vibration, reemits light of the same frequency as the incident light (hence "elastic" scattering). Each atom becomes an omnidirectional scattering center. It is this nonresonant elastic scattering that accounts for the transmission of light through all transparent materials and reflection of light from surfaces.

Glass, for example, is very transparent (which I guess means highly transmissive) at visible wavelengths (so non-resonant elastic scattering takes place in the visible) but presents dissipative resonant absorption at infrared wavelengths.

Is my understanding correct? Do you have any correction?

Thanks!

 

[DrDu]

Nonradiative transitions occur also in the gas phase for relatively small molecules like benzene. So it is not related to the density of the material. Yes, you can call them resonant dissipative nonelastic.

 

[fog37]

I often read how an electronic energy level is "composed" of a band of vibrational energy levels. What does that really mean? This is a possible explanation: say an atom is in the 2nd electronic excited state which means that its electrons have a certain energy above ground level. The molecule may have slightly more energy than that (not enough to go into the 3rd electronic excited state). That extra energy becomes associated to a vibrational energy level and is used by the molecule to vibrate.

I know an overall molecule can translate and/or rotate and/or vibrate and its electrons can either jump (or not ) to higher energy levels while those other types of motion take place. That said, I would imagine that each type of motion has a specific, quantized, associated energy and mode (except for translation whose energy is not quantized).

 

[DrDu]

Yes, in terms of the Born Oppenheimer approximation, each molecular energy level carries an electronic, a vibrational and a rotational quantum number (and maybe more for total angular momentum, nuclear spin ...). After excitation from the electronic and vibrational ground state, usually a state is obtained in which both electronic and vibrational quantum numbers are higher than in the ground state (see Franck-Condon principle). In a molecule, these states are nearly degenerate with highly excited vibrational states of the electronic ground state or lower lying electronic states. By non-adiabatic transitions (i.e. transitions which results from effect beyond the Born-Oppenheimer approximation), the initial excited state can develop in one of these vibrational excited states of lower electronic quantum number. This is then a radiationless transition. It is usually irreversible as the density of excited vibrational states with lower electronic quantum number is much higher than the density of excited vibrational (=electronic + vibrational) states.