CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER 16.5
the lifetime of the excited state is very short (theelectron only stays in the high
energy level for a very short time). However, there are some excited states where
an electron can remainin the higher energy level for a longer time than usual
before dropping down toa lower level. These excited states are called metastable
states.- Stimulated emission: As the picture above shows, stimulated emission happens
when a photon with anenergy equal to the energy difference between two levels
interacts with an electron in the higher level. This stimulates the electron to emit
an identical photon anddrop down to the lowerenergy level. This process results
in two photons at the end.
DEFINITION: Spontaneous Emission
Spontaneous emission occurs when an atom is in an unstable excited
state and randomly decays to a less energetic state, emitting a photon to
carry off the excess energy. The unstable state decays in a characteristic
time, called the lifetime.DEFINITION: Meta-stable state
A meta-stable state is an excited atomic statethat has an unusually
long lifetime, comparedto the lifetimes of otherexcited states of that
atom. While most excited states have lifetimesmeasured in microsec-
onds and nanoseconds ( 10 −^6 s and 10 −^9 s), meta-stable states canhave
lifetimes of milliseconds( 10 −^3 s) or even seconds.DEFINITION: Stimulated emission
Stimulated emission occurs when a photon interacts with an excited
atom, causing the atomto decay and emit another identical photon.