An electron of mass m is moving in an electric field $\vec{E}=-2 E_{\mathrm{o}} \hat{i}\left(\mathrm{E}_0=\right.$ constant $\left.>0\right)$, with an initial velocity $\vec{V}=v_{\mathrm{o}} \hat{i} \quad\left(v_0=\right.$ constant $\left.>0\right)$. If $\lambda_{\mathrm{o}}=\frac{h}{4 m v_{\mathrm{o}}}$, its de Broglie wavelength at time t is $\_\_\_\_$ . (e =