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The first-principal simulations are applied to study a photo-inducedmetastability in amorphous selenium (a-Se) and the contribution of thevalence-alteration pair (VAP) defects in this process. The VAP defect isconfirmed to be the equilibrium defect; it minimizes the destabilizinginteraction between adjacent Se chains induced by dis-orientation of thelone-pair (LP) electrons, and thus relieves a tension in a system. Theexcitation of LP electrons is proposed to be described by two coexistingprocesses, namely, single and double electron excitations. Both processes havebeen found to form defect states in the band gap and to cause theexperimentally observed photo-darkening and photo-volume expansion, however,only double electron excitation is capable to trigger bond rearrangement andstructural transformation. Lattice relaxation, which follows bond rearrangementoccurs with characteristic energy of -0.9$\pm$0.3 eV and promotes formation ofenergetically favorable VAP defects or crystalline inclusions thus ultimatelystimulating the photo-induced crystallization. In addition, photo-inducedcrystallization has been directly simulated in a system with an increasedcrystalline order.

The two-step photoexcitation mechanism in a-Se