An electron of mass m with an initial velocity $\overrightarrow{v} = v_{0} \hat{i} \left ( v_{0}> 0 \right )$ enters an electric field $\overrightarrow{E} = -E_{0} \hat{i} \left ( E_{0}= constant > 0 \right )$ at t = 0 . If $\lambda _{0}$ is its de-Broglie wavelength initially, then its de-Broglie wavelength at time t is