Fill in the blanks. A metastable state in atoms of the active medium of a laser is an electron energy
level with energy blank that of electrons in the ground state of these atoms. The average lifetime of electrons in a metastable state is blank that for most
excited states of electrons. (A) Greater than, less than; (B) less than, greater than; (C) greater than, greater
than; (D) greater than, equal to; or (E) less than, less than.
To answer this question, we are going to remind ourselves of what we know about the
ground, excited, and metastable states of electrons in a material. The ground state is the lowest energy level of electrons in atoms and is where most
electrons will end up, unless energy is being provided by an external source. If energy is being provided by an external source, then electrons can become
excited. This means that the electron absorbs a photon and moves to a higher energy level.
Electrons in the excited state have a very short lifetime, meaning they don’t stay in
this excited state for very long, only about 10 nanoseconds before they
spontaneously decay down to a lower energy level. This isn’t very helpful. To produce a laser beam, we need stimulated emission to occur, and this is unlikely
to happen if each electron is only out of the ground state for 10 nanoseconds at a
time. To get around this, lasers are built using materials that have a third energy level,
in between the ground and excited states, called the metastable state.
When electrons in the excited state spontaneously decay, they can move to the
metastable state, instead of moving all the way back to the ground state. The metastable state is much more stable than the excited state. So, electrons in this state have a much longer lifetime, long enough for stimulated
emission to occur, which is what we need for a laser beam.
We’re now ready to fill in the blanks in this statement. We know that the metastable state is between the ground and excited states. So, the energy of the electrons in the metastable state must be greater than that of
the electrons in the ground state. We also know that the lifetime of electrons in the metastable state is greater than
that for an excited state.
If we look through our answer options, we see this corresponds to option (C). So, option (C) is the correct answer. And our final statement reads, “A metastable state in atoms of the active medium of a
laser is an electron energy level with energy greater than that of electrons in the
ground state of these atoms. The average lifetime of electrons in a metastable state is greater than that for most
excited states of electrons.”