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In quantum physics and quantum chemistry, an avoided crossing (AC, sometimes called intended crossing, [1] non-crossing or anticrossing) is the phenomenon where two eigenvalues of a Hermitian matrix representing a quantum observable and depending on continuous real parameters cannot become equal in value ("cross") except on a manifold of dimension . [2]
The effect is named after Wilhelm Hanle, who was the first to explain the effect, in terms of classical physics, in Zeitschrift für Physik in 1924. [ 8 ] [ 9 ] Initially, the causes of the effect were controversial, and many theorists mistakenly thought it was a version of the Faraday effect .
The Landau–Zener formula is an analytic solution to the equations of motion governing the transition dynamics of a two-state quantum system, with a time-dependent Hamiltonian varying such that the energy separation of the two states is a linear function of time. The formula, giving the probability of a diabatic (not adiabatic) transition ...
The time-independent Schrödinger equation states that | = | ; substituting for | in terms of the basis states from above, and multiplying both sides by | or | produces a system of two linear equations that can be written in matrix form, () = (), or = which is a 2×2 matrix eigenvalues and eigenvectors problem. As mentioned above, this equation ...
In quantum field theory, a branch of theoretical physics, crossing is the property of scattering amplitudes that allows antiparticles to be interpreted as particles going backwards in time. Crossing states that the same formula that determines the S-matrix elements and scattering amplitudes for particle to scatter with and produce particle and ...
This is particularly useful in laser physics, where one is interested in the populations of different atomic states in a gas when a time-dependent electric field is applied. These probabilities are also useful for calculating the "quantum broadening" of spectral lines (see line broadening ) and particle decay in particle physics and nuclear ...
Intersystem crossing (ISC) is an isoenergetic radiationless process involving a transition between the two electronic states with different spin multiplicity. [ 1 ] Excited electrons can undergo intersystem crossing to a degenerate state with a different spin multiplicity.
From the equations, the power series must start with at least an order of to satisfy the real part of the equation; for a good classical limit starting with the highest power of the Planck constant possible is preferable, which leads to = = and = = (), with the following constraints on the lowest order terms, () = (()) and () =