Dear Mirza,

The beta electron transfer mechanism is not fully understood yet (the first link in the bottom of this message deals with certain system for understanding the BET mechanism). For the other part of the question, please read the following text:

Back electron transfer

A term often used to indicate thermal reversal of excited state electron transfer restoring the donor and acceptor in their original oxidation level. In using this term one should also specify the resulting electronic state of the donor and acceptor.

Back electron transfer (BET) is one of the important processes that govern the decay of generated ion pairs in intermolecular photoinduced electron transfer reactions. Unfortunately, a detailed mechanism of BET reactions remains largely unknown in spite of their importance for the development of molecular photovoltaic structures.

Photoinduced electron transfer (PET) is an excited state electron transfer process by which excited electron is transferred from donor to acceptor. Due to PET a charge separation is generated, i.e., redoxreaction takes place in excited state (this phenomenon is not observed in Dexter electron transfer).

Photoinduced Oxidation

[MLn]2+ + hν → [MLn]2+*

[MLn]2+* + donor → [MLn]+ + donor+

Photoinduced Reduction

[MLn]2+ + hν → [MLn]2+*

[MLn]2+* + acceptor → [MLn]3+ + acceptor−

The end result of both reactions is that an electron is delivered to an orbital that is higher in energy than where it previously resided. This is often described as a charge separated electron-hole pair when working with semiconductors. In the absence of a proper electron donor or acceptor it is possible for such molecules to undergo ordinary fluorescence emission. The electron transfer is one form  of photo quenching.

Subsequent Processes

In many photo-productive systems this charge separation is kinetically isolated by delivery of the electron to a lower energy conductor attached to the p/n junction or into an electron transport chain. In this case some of the energy can be captured to do work. If the electron is not kinetically isolated thermodynamics will take over and the products will react with each other to regenerate the ground state starting material. This process is called recombination and the photon's energy is released as heat.

For more details in this topic, please use the following two links which describe in details the indicated process. 

http://www.ncbi.nlm.nih.gov/pubmed/25044892

https://www.google.ps/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0ahUKEwiMs8Lppt_LAhXGMhoKHS1eD9wQIAg3MAQ&url=http%3A%2F%2Fwebcache.googleusercontent.com%2Fsearch%3Fq%3Dcache%3AIMqrgZNffN4J%3Awww.springer.com%2Fcda%2Fcontent%2Fdocument%2Fcda_downloaddocument%2F9789400720411-c2.pdf%253FSGWID%253D0-0-45-1269328-p174129377%2B%26cd%3D5%26hl%3Den%26ct%3Dclnk%26gl%3Dps&usg=AFQjCNEDuGyj68b5iSZaBCmC00vxPPKs6w&sig2=5_a0Wm1CHv-mTVHtUSxRhg

Hoping this will be helpful,

Rafik

Thank you very much Prof. Rafik, your explanations are very helpful.

To add to Prof. Karaman's answer; sometime it is also possible that, you excite the molecule and it goes to an excited state and then ET happens. But the CT state that is created after the ET, may use back ET to go back to the excited or to the ground state directly. In several cases; I only saw the BET to excited state is operational but not the other one. The reason might be that the free energy of that transition (CT state --> ground state) is so negative that, it lies in the inverted marcus region. So if you compute rate of that ET it will be very small.