thanks Sir for your inputs. these are quite useful 

I would like to add a special case to Mr Van der Zwan's answer, where it is indeed possible to calculate the (radiative) lifetime of an excited state.

In an optically excited semiconductor, the photoluminescence is determined via the so-called semiconductor luminescence equations. This is a set of integro-differential equations for microscopic quantities, namely the carrier occupations, the photon-assisted polarization and the photon number (the latter stemming from a fully quantized light field). The solution will yield the complete dynamics of all these quantities. From the carrier occupation, you can of course derive a recombination rate, see Eq. 7 in the link I provided. And if you have the rate, you can calculate the lifetime (basically the reciprocal).

https://www.osapublishing.org/view_article.cfm?gotourl=https%3A%2F%2Fwww%2Eosapublishing%2Eorg%2FDirectPDFAccess%2FCD6D268C%2DFBA1%2DDD97%2D5E31D47D3EAB3EA5%5F63410%2Foe%2D4%2D13%2D512%2Epdf%3Fda%3D1%26id%3D63410%26seq%3D0%26mobile%3Dno&org=

Thanks for your input. I will go through it and let you know