Energy measurement after infinite square well suddenly expands to twice its size

[Zong xiaobo]

A particle is in the ground state of the 1D infinite square well (0<x<a). Suddenly the well expands, the right wall moving from a to 2a, leaving the wave function (momentarily) undisturbed. The energy of the particle is now measured. What is the most probable result, and what is its probability? What is the most probable result in the next measurement, and what is its probability?

 

[Feldoh]

What have you tried so far?

You're trying to find a wave function for which the initial state is the wave function in a 1D infinite well, how do you do that?