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Anyway on p284 of P&S, there is a formula inbetween eqns (9.17) and (9.18) that reads:

[itex]e^{-iHT} | \phi_a \rangle = \sum_n e^{-i E_n T} | n \rangle \langle n | \phi_a \rangle \rightarrow \langle \Omega | \phi_a \rangle e^{-i E_0 \cdot \infty (1-i \epsilon)} | \Omega \rangle[/itex] as [itex]T \rightarrow \infty ( 1 - i \epsilon)[/itex]

I can follow the equality on the left fair enough but I don't understand what happens when we take the limit? Apparently, this is quite a common trick in QFT so can anybody explain to me what is going on here?

Thanks!