Statistically three triplet excitons will be formed for each singlet exciton. An intermediate case between Frenkel and Wannier excitons is the "charge-transfer (CT) exciton". The THz field generated a coherent quantum superposition of an absorbing and a nonabsorbing exciton. Subsequent exciton recombination results in electroluminescence (EL). Exciton recombination is undesirable and higher levels lead to an inefficient photocatalyst. Such a photogenerated electron-hole pair is termed an "exciton". Exciton condensates have allegedly been seen in a double quantum well systems. An electron-hole pair is often referred to as an exciton. The electron and the hole can bind to each other to form an exciton. In simpler terms, this means a local deformation of the crystal lattice around the exciton. The result is a "Wannier-Mott exciton", which has a radius larger than the lattice spacing. Experimentally, the fundamental exciton for AgF lies at 4.63 eV. Similar effects exist for optical transitions between exciton and biexciton states. They also have high extinction coefficients and have shown the possibility of multiple exciton generation. The spins are coupled by the exchange interaction, giving rise to exciton fine structure.