When an alpha particle is emitted during alpha decay, it does not accelerate due to repulsion from the nucleus. In fact, the kinetic energy of the alpha particle remains constant throughout its trajectory, as you have observed in the energy diagrams.
During alpha decay, the parent nucleus undergoes a spontaneous transformation, emitting an alpha particle. This emission is a quantum mechanical process governed by the laws of conservation of energy and momentum. The alpha particle is already formed within the nucleus before it is emitted.
Inside the nucleus, the alpha particle exists as a bound state due to the strong nuclear force, which is responsible for holding the protons and neutrons together. The potential energy of the alpha particle within the nucleus is lower than its potential energy outside the nucleus. This potential energy barrier prevents the alpha particle from escaping spontaneously.
However, through a quantum tunneling effect, the alpha particle can overcome this potential barrier and "tunnel" through it, emerging as a free particle outside the nucleus. Tunneling is a probabilistic phenomenon, and its occurrence allows the alpha particle to be emitted even though its energy is lower than the potential barrier.
Once the alpha particle is emitted, it continues to move away from the parent nucleus with a constant kinetic energy. The repulsive electrostatic force between the alpha particle and the positively charged parent nucleus does exist, but it is offset by the initial kinetic energy of the alpha particle. As a result, the alpha particle does not experience acceleration or deceleration due to this electrostatic repulsion.
The constant kinetic energy of the alpha particle can be understood by considering the conservation of energy. The energy released during alpha decay is primarily in the form of the kinetic energy of the alpha particle itself. Therefore, as the alpha particle moves away from the nucleus, it maintains a constant energy, corresponding to the energy released during the decay process.
In summary, the alpha particle does not accelerate due to repulsion from the nucleus upon emission. It emerges with a constant kinetic energy, which remains unchanged as it moves away from the parent nucleus.