Fluorescence Rate

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abregman
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Fluorescence Rate

Post by abregman » 04 May 2015, 16:11

Hi guys. I was hoping for some help.
I know that there is a method for calculating Phosphorescence Rates and lifetimes in DALTON, and I am pretty sure that there is a way to do Inter System Crossing Rates (not 100% sure, if somebody caould actually direct me to that, that would be awesome) however, I have been looking around and I have not seen any kind of documentation or indication that there is a method for calculating fluorescence lifetimes or emission rates. Is it possible to do this in DALTN? If not, do people have advice for how to do it using other softwares such as g09?
Thank you

Joanna
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Re: Fluorescence Rate

Post by Joanna » 04 May 2015, 16:16


abregman
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Re: Fluorescence Rate

Post by abregman » 04 May 2015, 16:19

Yes I have read the manual. There is no mention of singlet to ground state transition rate calculations. Maybe I missed it though, if you could direct me to the appropriate section, that would be great.

taylor
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Re: Fluorescence Rate

Post by taylor » 04 May 2015, 16:32

I am not sure what you want as a "fluorescence rate". It would seem to me that there are at least two aspects of fluorescence in a molecule: vibrational relaxation of an excited electronic state to a lower vibrational level of that state, and then emission from that lower vibrational level to (depending on Franck-Condon considerations) some vibrational level in the ground state. What do you want? And how do you want to calculate it? What is the vibrational relaxation process in the excited state? If it is nonradiative (say, collisional) what is the excited molecule colliding with, and how would one compute a rate for this process when the program considers electronic structure, not collisional dynamics?

If what you want is the knowledge of how to calculate the electronic relaxation from the vibrationally relaxed excited state to the ground state, this lifetime is related to the intensity of the electronic transition and this can be computed in Dalton, and is described (in various places) in the manual. In addition, if by intersystem crossing you mean spin-orbit coupling between a singlet (say) and triplet excited state, this can also be calculated, and is also described in the manual. In addition, I think among the test jobs there are several that do this, both in a mean-field (AMFI) approximation to the spin-orbit terms and in a full-bore approach that uses both one- and two-electron integrals.

Best regards
Pete
P.S. If you want information on other programs, you might want to post to their forums (if any). After all, this is the Dalton forum...

abregman
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Re: Fluorescence Rate

Post by abregman » 04 May 2015, 16:38

Haha, yeah I will keep other software out of the DALTON forum in the future.
I was looking for relaxation from lowest lying singlet state to ground state. Would you mind directing me to the relevant part of the manual?
THank you so much for your help

P.S. Is there a way to calculate rate of relaxation from excited electronic state to lower vibrational level of same state or to lower excited states?

taylor
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Re: Fluorescence Rate

Post by taylor » 05 May 2015, 14:12

As far as the manual is concerned, there are multiple sections concerning excitation energies and transition moments: since you do not indicate what level of calculation you wish to perform it is difficult to point you to a particular one. But looking at linear response and the calculation of excitation energies for the various levels should get you pointed in the right direction.

With respect to vibrational relaxation, I tried to make this clear in my earlier response. Most "internal conversion" in Nature takes place by the loss of vibrational energy, in the solid state to lattice vibrations or to energy exchange with other moieties (such as other instances of the same molecule in a pure crystal). In the liquid state, probably most common for experiments, to loss of vibrational energy to a solvent, and in the gas phase to other molecules, again for experiment most commonly to a bath gas. For radiative decay of excited vibrational states it is possible to calculate lifetimes from the usual expressions based on transition moments. But this is rarely the commonest or preferred decay channel, and for those I listed above, all can or should be regarded as "collisional". That is, they involve some interaction and exchange of energy between your target molecule and a bath, or lattice, or other external species. This is not simply an electronic structure problem, it is a problem in dynamics involving "collisions" with other species. While it is possible to treat such problems (for example, by constructing a potential energy surface for the interaction between the target molecule and a particular collision partner, and then solving, classically or quantum-mechanically, the dynamical equations that govern these collisions) and to extract lifetimes from the collisional cross-sections, this is (a) far beyond what an electronic structure program, like Dalton, can do, and (b) very difficult to program for a general case. Most dynamics codes, at least beyond the level of classical trajectories, address particular problems, such as rotational excitation of a diatomic molecule by collision with an atom. As far as I am aware there is no equivalent of Dalton --- or any analogous quantum chemistry code --- that offers a general treatment of (quantum-mechanical) collisions in the way Dalton or its analogs treat the general quantum-mechanical electronic problem. As a result, it is difficult to know how one might calculate the vibrational relaxation rate of an arbitrary excited state of an arbitrary system.

Best regards
Pete

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