Spectral projections of the complex cubic oscillator

We prove the spectral instability of the complex cubic oscillator $-\frac{d^2}{dx^2}+ix^3+i\alpha x$ for non-negative values of the parameter $\alpha$, by getting the exponential growth rate of $\|\Pi_n(\alpha)\|$, where $\Pi_n(\alpha)$ is the spectral projection associated with the $n$-th eigenvalue of the operator. More precisely, we show that for all non-negative $\alpha$ $$\lim\limits_{n\to+\infty}\frac{1}{n}\log\|\Pi_n(\alpha)\| = \frac{\pi}{\sqrt{3}}.$$