Tussenelements

Tussenelements are a proposed class of short-lived intermediate quantum states that appear in processes where energy or information is transferred between components of a larger system. The term has appeared in the physics and materials literature as a way of describing states that are not permanent eigenstates of the system but act as temporary bridges between initial and final configurations.

In electronic or vibronic systems, tussenelements are characterized by lifetimes on ultrafast timescales and by their ability to couple simultaneously to multiple manifolds of states. They are often described using effective models that include a bridging state in addition to donor and acceptor states, with the bridging state's properties determined by electronic coupling, vibronic interactions, and environmental fluctuations.

Evidence for tussenelements comes from ultrafast spectroscopy and multidimensional electronic spectroscopy, where decay dynamics or cross-peak features indicate intermediate reservoirs that mediate transfer. They are particularly discussed in the context of organic semiconductors, perovskites, and photosynthetic-inspired energy transfer.

There is ongoing debate about whether tussenelements correspond to real, discrete physical states or emerge as a convenient description of complex continua and coherent dynamics. Some researchers view them as essential organizing concepts for rate-based models, while others urge caution about overinterpreting spectroscopic signatures.

Possible applications include guiding design principles for energy-transfer materials, by tuning the energy and coupling of tussenelements to optimize transfer efficiency or suppress loss pathways. The concept remains active and somewhat informal, without universal consensus on definition or measurement standard.

See also: transient state, intermediate, vibronic coupling, energy transfer, exciton.