Hitting MGF

Let $W_t$ be a standard Brownian Motion and define $T_a = \text{inf}\{t > 0: |W_t| > a\}$ for $a > 0$. Find $\mathbb{E}[e^{-\lambda T_a}]$ as a function of $\lambda$ and $a$. Evaluate this function with $\lambda = 4$ and $a = \ln(2)$.