An electron of mass m with an initial velocity  $\overrightarrow{\text{V}}=,V_{0}i\left(V_0>0\right)$enters an electric field  $\overrightarrow{\text{E}}=-E_0\hat{i}$ = (E0 = constant > 0) at t = 0. If  ${\lambda }_0$ is its de-Broglie wavelength initially, then its de-Broglie wavelength at time t is