Answer
$3.47\times 10^{18}~~$ atoms undergo stimulated emission.
Work Step by Step
We can find the energy of each photon:
$E = \frac{hc}{\lambda}$
$E = \frac{(6.63\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{690\times 10^{-9}~m}$
$E = 2.8826\times 10^{-19}~J$
We can find $E_p$, the energy emitted during the pulse:
$E_p = (100\times 10^6~W)(10\times 10^{-9}~s) = 1.0~J$
We can find the number of photons emitted during this pulse:
$\frac{1.0~J}{2.8826\times 10^{-19}~J} = 3.47\times 10^{18}$
$3.47\times 10^{18}~~$ atoms undergo stimulated emission.
