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Particle in a Well (PIW) (14.5)PowerPoint Presentation

Particle in a Well (PIW) (14.5)

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Particle in a Well (PIW) (14.5)

- A more realistic scenario for a particle is it being in a box with walls of finite depth (I like to call it a well)
- Particles can escape the well by having enough energy, and then behave like free particles
- When a free particle passes by a well, it is still influenced by the well though it is not trapped

- The problem is now divided into three regions and the wavefunctions (and their first derivatives) in the three regions must match at the boundaries
- Regions I and III have a non-zero, but constant, potential energy V0
- Region II is the well and has no potential energy (length is from –a/2 to a/2)

- Since it is possible for the particle to exist in regions I and III above the well, it is also possible for the particle to exist there “below” the well
- The wavefunctions extend beyond the walls of the well into classically forbidden regions
- The wavefunctions MUST approach zero as one moves deeper into the well walls

Quantum Mechanical Tunneling (14.5)

- Inside classically forbidden regions, the wavefunction must decay to zero and do so quickly
- For PIW, the wavefunctions beyond the well wall decay exponentially

- How quickly the particle decays outside the well depends on the parameter κ
- Larger value of κ means faster decay
- Heavy particles have a more difficult time tunneling into well wall
- Particles closer to the top of the well (i.e., in higher energy states) have an easier time penetrating the walls

- Tunneling into a well wall is possible, but leads to the eventual decay of the particle
- What if the wall had a finite length?

Tunneling Through a Barrier (14.9)

- If the PIW model is inverted, we now have a barrier
- The barrier has a certain width (a) and height (V0), and the potential everywhere else is zero
- Classically, a particle can only get from one side of the barrier to the other by going over it (e.g., passing through transition states)

- Since the wavefunction is nonzero inside the barrier, it is possible for the particle to completely pass through the barrier
- The width of the barrier dictates whether the particle can pass or not
- The decay parameter κ also determines whether particles can pass through the barrier
- The wavefunctions must “connect” between all three regions

- When tunneling occurs, processes occur faster than one expects
- Classically, reaction rates depend on the size of the activation barrier
- Tunneling may make the activation barrier appear smaller
- Tunneling occurs most often with electron and proton transfer processes

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