# Laser - Einstein's Coefficients

• cupid.callin
In summary, the conversation discusses the concept of spontaneous emission in relation to the rate of decay of electrons from a high-energy state to a low energy state. The probability of this process, denoted as P21, is defined as equal to B21 times the frequency of the transition, and B21 is referred to as Einstein's Coefficients of spontaneous emission. However, in some sources, B21 is also described as the probability of spontaneous emission per unit time, causing confusion for the individuals involved in the conversation. The conversation also mentions the use of dimensional analysis to determine the units of P21 and mentions that spontaneous emission is usually denoted as A21 instead of B21.
cupid.callin
Hi all,

for spontaneous emission, the rate of decay of electrons in high-energy state to low energy state is given by:

$\frac{dN_2}{dt} = P_{21}N_2$

where P21 is equal to the probability of electron to drop from higher energy state to lower energy state.

Now, P21 was defined as $P_{21} = B_{21}u(\nu)$

And $B_{21}$ is called Einstein's Coefficients of spontaneous emission

then in some other topic, they wrote B21 as probability of spontaneous emission per unit time.

Are they really same thing? ... I guess not ... B21 should be something like probability of spontaneous emission per-unit Energy density (:p i know this doesn't make any sense ... ) ...

Spontaneous emission is nearly always written as A21, not B21 - B is used for absorption and stimulated emission. You can use dimensional analysis to find the units in the equation you wrote - what units must P21 have?

cupid.callin said:
Hi all,
By what author? Siegman? Milloni & Eberly? Other?

JeffKoch said:
Spontaneous emission is nearly always written as A21, not B21 - B is used for absorption and stimulated emission. You can use dimensional analysis to find the units in the equation you wrote - what units must P21 have?

Oh yes, sorry i wasn't careful enough while typing ..

Redbelly98 said:
By what author? Siegman? Milloni & Eberly? Other?

Its not from any book ... i pasted it from some website into MS word ... so i don't even remember the site ... :p

## What is the concept of "laser - Einstein's Coefficients"?

"Laser - Einstein's Coefficients" refers to the scientific principles behind the functioning of a laser, which were first proposed by Albert Einstein in 1917.

## What are the three types of Einstein's Coefficients?

The three types of Einstein's Coefficients are stimulated emission, spontaneous emission, and absorption. These coefficients describe the different ways in which atoms interact with photons in a laser.

## What is stimulated emission?

Stimulated emission is a process in which an atom is stimulated by a photon to release a second photon that is identical in properties, such as wavelength, phase, and direction of travel. This process is essential for the amplification of light in a laser.

## What is spontaneous emission?

Spontaneous emission is a process in which an excited atom releases a photon without any external stimulation. This process is responsible for the initial light emission in a laser, but it is not as efficient as stimulated emission for producing a laser beam.

## Why are Einstein's Coefficients important for lasers?

Einstein's Coefficients are crucial for understanding the fundamental principles behind the functioning of a laser. They explain how atoms interact with photons to produce a coherent, amplified beam of light. Without these coefficients, the development and improvement of laser technology would not have been possible.

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