1.

In a middle of the rotatation-vibration band of emission specturm of `HCI` molecule, where the "Zeroth" line is forbidden by the selcetion rules, the interval between neighbouring lines is `Delta omega= 0.79.10^(13) s^(-1)`. Calculate the distance between the nuclei of an `HCI` molecule.

Answer» In the ratation vibration band the main transition is due to change in vibrational quantum number `v rarr v-1`. Together with this roatational quantum number may change. The "Zeroeth line" `0 rarr 0` is forbidden in this case so the neighbouring lines arise due to `1 rarr 0 or 0 rarr 1` in the rotaional quantum number. Now
`E=E_(v)+(ħ^(2))/(2I)J(J+1)`
Thus `ħ omega=ħ omega_(0)+(ħ^(2))/(2I)(+-2)`
Hence `Delta omega =(2ħ)/(I)=(2V)/(mud^(2))`
so `d= sqrt((2ħ)/(muDelta omega))`
Substitution gives `d= 0.128nm`.


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