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Fundamentals Of Physics
Found in: Page 1246

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Short Answer

Figure 40-21 shows partial energy-level diagrams for the helium and neon atoms that are involved in the operation of a helium–neon laser. It is said that a helium atom in state E3 can collide with a neon atom in its ground state and raise the neon atom to state E2. The energy of helium state E3(20.61 eV)is close to, but not exactly equal to, the energy of neon state role="math" localid="1661494292758" E2(20.66 eV). How can the energy transfer take place if these energies are not exactly equal?

The energy transfer takes place due to the excitation caused by the current in the helium atoms by collisions (not the more massive neon atoms).

See the step by step solution

Step by Step Solution

Step 1: The given data

  1. Figure 40-21 show partial energy-level diagrams for the helium and neon atoms that are involved in the operation of the helium-neon laser is given.
  2. The energy of the helium state E3(20.61 eV) is very close to the energy of the neon state E3(20.66 eV).

Step 2: Understanding the concept of laser action

Due to the laser action, there occurs a potential difference that gives rise to the current passing through the helium-neon gas mixture serving—through collisions between helium atoms and electrons of the current—to raise many helium atoms to state E3, which is metastable with a mean life of 1 microsec. (The neon atoms are too massive to be excited by collisions with the (low-mass) electrons.)

Step 3: Calculation of the reason for energy transfer

According to the given data and concept, when a metastable E3 helium atom and a ground state E0 neon atom collide, the excitation energy of the helium atom is often transferred to the neon atom, which then moves to the state E2 . In this manner, neon level E2 (with a mean life of 170 ns) can become more heavily populated than neon level role="math" localid="1661494432656" E1 (which, with a mean life of only 10 ns, is almost empty).

This population inversion is relatively easy to set up because (1) initially, there are essentially no neon atoms in the state E1, and (2) the long mean life of helium level E3 means that there is always a good chance that collisions will excite neon atoms to their level, E2 and (3) once those neon atoms undergo stimulated emission and fall to their E1 level, they almost immediately fall down to their ground state (via intermediate levels not shown) and are then ready to be re-excited by collisions.

Hence, the energy transfer occurs even if the energy levels of helium and neon are not the same.

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