The spectroscopy method helps to monitor the evolution of the spin in radical pairs

The change that occurs between the singlet and triplet states of electron pairs in charge-separated states plays a crucial role in nature. The compass of migrating birds could even potentially be explained by the impact of the geomagnetic field on the magnetic interaction between these two spin states.

Basic scheme of the experiment. The pair of radicals separated by charges (CSRP, black curve) decays in about 1000 ns by recombination of the electrons into a singlet or triplet product. Here, the dynamic alternation of the CSRP between the singlet (S) and triplet (T) state is recorded only as an average over the total reaction time. Using the pump-push-pulse technique, the singlet and triplet character of SRFC can be read at any time. Image credit: Christoph Lambert / Universität Würzburg.

Until now, this quantum process could not be followed directly optically and was only summarily evaluated in the final result. In this issue of the journal Science, research collaboration, with Professor Ulrich Steiner from the University of Constance and scientists from the universities of Würzburg and Novosibirsk (RUS), presents the pump-push-pulse method. This allows researchers to optically identify the time course of the singlet or triplet ratio for the first time. This opens up new avenues, for example in the field of organic solar cells, as well as for qubits in quantum computers.

Usually, the electrons in a molecule occupy the theoretically permissible quantum orbits in pairs. The intrinsic property of the angular momentum of electrons, their spin, is of paramount importance in this context. According to the Pauli exclusion principle of quantum mechanics, two electrons only have the ability to travel along the same orbit if their spins are antiparallel. If one electron tends to rotate clockwise, the other must rotate counterclockwise.

In the molecular ground state, generally all electronic spins are paired. Through light excitation, a single electron is separated from the paired constellation and raised to a higher energy level, where it alone holds a free orbit.

From there, it could still jump into a free orbit in a suitable neighboring molecule. This leads to photo-induced electron transfer. The two lone electrons can currently change their spin parameters independently via magnetic interaction with their corresponding environment, as they are no longer forced by the Pauli principle.

The two lone electrons form a radical pair

In addition, such photo-induced electron transfer charge separation occurs, as in photosynthesis. There is only a slight decrease in the energy of the electron transferred during this step, so the majority of the electronic energy initially absorbed via light excitation is still maintained. Therefore, this original excitation energy is stored in chemical form.

In chemistry, the charge-separated state with the two electrons isolated is also called a radical pair. If the spins of the two isolated electrons are arranged in parallel, we tend to speak of a triplet state and if their alignment seems antiparallel, it is a question of a singlet state of the radical pair.

Due to the free individual evolutions of the two spins, the spin state of the pair of radicals alternates between the singlet and triplet state. As there is not much variation between these spin alignments regarding energy, so far they have not been directly distinguishable optically.

Stabilization of the energy of the radical pair could be achieved by returning the radical electron from the acceptor molecule to the donor molecule, whereby the original singlet state was restored, thereby discharging energy in the form of heat.

But in order to be able to pair again with the original partner electron, its spin should have remained opposite to that of the latter, which is not essential, because a spin reorientation could have taken place in the meantime.

If its current alignment appears to be different, it cannot return to its original orbit, but alternatively, it can discharge energy by passing into another lower orbit that is still free. This results in a triplet product which can be optically differentiated from the singlet product.

Radical pair as a template for qubits and migrating birds magnetic field sensor

For several aspects, the phase in which the pairs of radicals tend to oscillate between the singlet and triplet states is of particular interest. Since it is a coherent movement controlled by quantum mechanics, it can be regulated, for example by an external magnetic field. These movements are used in physics to be implemented in quantum computers.

Our radical pair can serve as a model for qubits, since they exist as elements in quantum computers., or to understand the function of the radical pairs in the migratory bird biological compass model mentioned above. For such reasons, it is interesting how is the spin currently positioned in this process.

Ulrich Steiner, Photokinetics and Spin Chemistry, University of Constance

“Pump-Push-Pulse” technique to read the singlet / triplet ratio

With the “pump-push-pulse” technique, the team developed a procedure that allows for the first time to read the singlet / triplet ratio at specific times. First, the transfer of electrons from the donor to the acceptor molecule is started with a pump laser pulse.

This, in turn, gives rise to the charge-separated state with spin singlet. Now the spins of uncoupled electrons can evolve. After a while, a second laser pulse is followed.

This pushing laser pulse in turn transfers an electron from the acceptor to the donor, whereby the second laser pulse forces the system to immediately make the decision between triplet or singlet product formation, for which the radical pair would take normally several periods of spin oscillation..

Ulrich Steiner, Photokinetics and Spin Chemistry, University of Constance

Ulrich Steiner and his Russian collaborator verified the interpretation of the experiments through model calculations based on quantum theory. By adopting this method, it is possible to take snapshots of the spin state of the pair of radicals at different times.

Journal reference:

Mims, D., et al. (2021) Reading quantum spin beats in a pair of charge-separated radicals by pump-push spectroscopy. Science.


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