# Laser - Einstein's Coefficients

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?

Redbelly98
Staff Emeritus
Homework Helper
Hi all,