The value of $a$, for which the points A, B, C with position vectors $2 \hat{i} - \hat{j} + \hat{k}, \; \hat{i} - 3 \hat{j} - 5 \hat{k}$ and $a \hat {i} - 3 \hat {j} - \hat{k}$ respectively are the vertices of a right angled triangle with $C = \frac{\pi}{2}$ are :