[texhax] How do write in two or more lines using overbraces/underbraces?

[Darío]

Hello, in the next code I would like to that in the overbrace appear in 
two lines the words "es 0 porque $\Psi_1$ cumple la ecuación de 
Schrödinger", because when compile the constant a_1 appears very far, 
the same with the sign = on the right.

\begin{align}
-\frac{\hbar^2}{2m}a_1\frac{\partial^2\Psi_1}{\partial x^2}+Va_1-i\hbar 
a_1\frac{\partial\Psi_1}{\partial t}&=0\nonumber\\
a_1\underbrace{\left(-\frac{\hbar^2}{2m}\frac{\partial^2\Psi_1}{\partial 
x^2}+V-i\hbar\frac{\partial\Psi_1}{\partial t}\right)}_{\text{es 0 
porque $\Psi_1$ cumple la ecuación de Schrödinger}}&=0\nonumber
\end{align}

I would like to appear something like this (then of compile):
"es 0 porque $\Psi_1$ cumple
la ecuación de Schrödinger"

I tried with \\ but do not works

Thanks a lot!
Darío