Gamow Vectors in a Periodically Perturbed Quantum System

We analyze the behavior of the wave function $\psi(x,t)$ for one dimensional time-dependent Hamiltonian $H=-\partial_x^2\pm2\delta(x)(1+2r\cos\omega t)$ where $\psi(x,0)$ is compactly supported. We show that $\psi(x,t)$ has a Borel summable expansion containing finitely many terms of the form $\sum_{n=-\infty}^{\infty} e^{i^{3/2}\sqrt{-\lambda_{k}+n\omegai}|x|} A_{k,n} e^{-\lambda_{k}t+n\omega it}$, where $\lambda_k$ represents the associated resonance. This expression defines Gamow vectors and resonances in a rigorous and physically relevant way for all frequencies and amplitudes in a time-dependent model. For small amplitude ($|r|\ll 1$) there is one resonance for generic initial conditions. We calculate the position of the resonance and discuss its physical meaning as related to multiphoton ionization. We give qualitative theoretical results as well as numerical calculations in the general case.