A set of atom in an excited state decays

Options

A. in general to any of the states with lower energy
B. into a lower state only when excited by an external by an external electric field
C. all together simultaneously into a lower state
D. to emit photons only when they collide

Explanation

### Step-by-Step Text Solution:

1. **Understanding Excited State**:
- An atom in an excited state has absorbed energy and is at a higher energy level than its ground state. This state is unstable.

2. **Stability and Energy**:
- Atoms tend to seek stability, which is associated with lower energy states. Therefore, an atom in an excited state will naturally want to decay to a lower energy state to achieve stability.

3. **Decay Process**:
- The process of decay involves the atom releasing energy, often in the form of photons (light particles), as it transitions from a higher energy state to a lower energy state.

4. **Options Analysis**:
- The question provides several options regarding how the decay occurs:
- **Option A**: Decays only when excited by an external electric field.
- **Option B**: Decays altogether simultaneously into a lower state.
- **Option C**: Emits photons only when they collide.
- **Option D**: Decays in general to any of the states with lower energy.

5. **Evaluating Options**:
- **Option A**: Incorrect, as decay can happen spontaneously without external excitation.
- **Option B**: Incorrect, as decay is a gradual process rather than simultaneous.
- **Option C**: Incorrect, as decay does not require collisions; it can occur independently.
- **Option D**: Correct, as it accurately describes that atoms decay to any lower energy state to achieve stability.

6. **Conclusion**:
- Therefore, the correct answer is that a set of atoms in an excited state decays in general to any of the states with lower energy.