The two-step photoexcitation mechanism in a-Se
  The first-principal simulations are applied to study a photo-induced metastability in amorphous selenium (a-Se) and the contribution of the valence-alteration pair (VAP) defects in this process. The VAP defect is confirmed to be the equilibrium defect; it minimizes the destabilizing interaction between adjacent Se chains induced by dis-orientation of the lone-pair (LP) electrons, and thus relieves a tension in a system. The excitation of LP electrons is proposed to be described by two coexisting processes, namely, single and double electron excitations. Both processes have been found to form defect states in the band gap and to cause the experimentally observed photo-darkening and photo-volume expansion, however, only double electron excitation is capable to trigger bond rearrangement and structural transformation. Lattice relaxation, which follows bond rearrangement occurs with characteristic energy of -0.9$\pm$0.3 eV and promotes formation of energetically favorable VAP defects or crystalline inclusions thus ultimately stimulating the photo-induced crystallization. In addition, photo-induced crystallization has been directly simulated in a system with an increased crystalline order. 
J. Berashevich, A. Mishchenko, and A. Reznik
cond-mat.mtrl-sci