Given that \(\mathbf { u } = \left( \begin{array} { r } - 2 1 2 \end{array} \right) , \mathbf { v } = \left( \begin{array} { l } a 0 1 \end{array} \right)\) and \(\mathbf { u } \times \mathbf { v } = \left( \begin{array} { l } 1 b 3 \end{array} \right)\), find \(a\) and \(b\).
Using \(\mathbf { u } \times \mathbf { v }\), determine the angle between the vectors \(\mathbf { u }\) and \(\mathbf { v }\), given that this angle is acute.