[Physics] When solving the Schrodinger equation, how do we know what functions to use (exponential and sine or cosine) for the solution (wavefunction)

quantum mechanicsschroedinger equationwavefunction

In quantum mechanics when we have simple potentials (infinite square well, oscillator harmonic, free particle, delta-Dirac, and finite square barrier) the solution eigenfunctions are different. In infinite potential well the eigenfunctions are sine and cosine. Then, the solutions in finite potential barrier are exponential functions.

When solving the Schrodinger equation, how do we know what functions to use (exponential and sine or cosine) for the solution (wavefunction)?

Best Answer

The solutions are like sines and cosines (oscillating) when the energy of the particle is greater than the energy of the potential. Those regions are regions where a classical particle can exist. The solutions are like exponentials when the the energy of the particle is lower than the potential, regions where a classical particle cannot exist.

For example, if a particle with energy $E$ is in a region of no potential $V(x)=0$, the wave function will be sine-like. Suppose that particle encounters a barrier of height $V>E$. A classical particle does not have enough energy to pass by that barrier. The region of higher potential is classically forbidden. The particle will bounce off the barrier. But in quantum mechanics, the wave function extends into the barrier $\ldots$ into the forbidden region. In the forbidden region the wave function will fall exponentially.