Question

Lifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nano second range. If the radiation source has the duration of $2\, ns$ and number of photons emitted during the pulse source is $2.5 \times 10^{15}$, the energy of the source is

• A. $8.28 \times 10^{-10}\, J$
• B. $8 .2 8 \times 10^{-10}\, J$
• C. $5 \times 10^{-10}\, J$
• D. $82.8 \times 10^{-10}\, J$

Answer 1

Answer: (A)

Frequency $(\mathrm{v})=\frac{1}{2 \times 10^{-9}}$
$=0.5 \times 10^{9} \mathrm{~s}^{-1}$
Energy of the source $=N \times h v$
$ =2.5 \times 10^{15} \times 6.626 \times 10^{-34} \times 0.5 \times 10^{9} $
$=8.28 \times 10^{-10} \mathrm{~J}$