Photoinduced phase transition and associated timescales in the excitonic insulator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ta</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>Ni</mml:mi><mml:msub><mml:mi>Se</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math>

We investigate the non-equilibrium electronic structure and characteristic time scales in a candidate excitonic insulator, Ta_2iSe_5 using time- and angle-resolved photoemission spectroscopy with a temporal resolution of 50 fs. Following a strong photoexcitation, the band gap closes transiently within 100 fs, i.e., on a time scale faster than the typical lattice vibrational period. Furthermore, we find that the characteristic time...