It is known, that the fluorescence lifetime of an ensemble of molecules being in close vicinity can change, as intermolecular processes between excited chromophores (e.g. superradiance, exciton-exciton-annihilation) or one excited and one ground-state chromophore (e.g. singlet fission) can occur. These phenomena depend on the excitation density in different ways.
My question is now: How can the lifetime of a single isolated molecule's excitation be influenced by a variation in excitation density?
Dear Matthias Bohlen,
The first phenomenon that you have mentioned "super radiance" is due to the emission of a photon induced by the presence of photons that matches in energy with the fluorescence emission. This is the term B21 of the Einstein coefficients for the induced emission. If the wavelength of the excitation laser is in the blue part of the fluorescence band of a single molecule, the lifetime will reduce when increasing the excitation power. Is this your case ?
Dear Robert Pansu,
thank you very much for your reply. I have 2 questions regarding your reply.
First, why would superradiance a useful model to be considered here, since we are talking about single molecules? From my understanding, SR is a collective phenomenon, where you would need at least 2 atoms or molecules involved.
My second question is, I don't understand yet, why the B-coefficient would explain the findings. Yes, we see a lifetime reduction with higher excitation power. But I would argue, that first, the laser pulse length is way shorter than the lifetime, so there must be a large part of the emission, that could not be related to stimulated emission, but only to the spontaneous emission. I took laser induced fluorescence (LIF)-spectra (so, kind of absorption) beforehand, so I know, that for the lifetime measurements, the excitation is at the 0-0 transition (in the sense of LIF-spectra). I did not record emission spectra, but there might be a red-shift of the 0-0 emission (Here is a study about neon clusters, but they didn't discuss the shift of the LIF- to emission-spectra: Article Optical signatures of pentacene in soft rare-gas environments
)I would consider, that from the LIF spectra, we can safely assume, that we are in the vibrational ground state of the electronic excited state, and that the only way for a red-shift in the emission (that would be a Stokes-shift then, right?) is a relaxation to a vibrationally excited electronic ground state. Still I did not understand, why that would lead to an excitation power dependence. Could you elaborate more on that?
Thank you again and best regards,
Matthias Bohlen
Dear Matthias Bohlen,
The B-coefficient is responsible for light induced emission that is behind superradiance and lasing.
Since you are measuring lifetimes I should have guess that the excitation laser was pulsed. I guess that the fluorescence lifetime is small compared to the repetition time of the laser. Then there should be no effect of the laser power on the lifetime through B-coefficient.
Upon increasing the laser power, you favour absorption to higher excited states. Most of them will relax to the fluorescent state. This relaxation will release heat around the molecule. How fast is the heat propagation in your matrice ?
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