First we must determine the electron configuration in the ground state. There are 8 p-bonded Cs so there are 8p electrons. These are distributed over the electronic levels as follows:

The first excited state of this configuration is:

So the n value of the lowest completely empty level of the 1st excited state is 6.

 The length of the molecule = 7 X 0.15 = 1.05 nm

 The energy is given by:

E = 62(6.63X10-34)2/8(9.11X10-31)(1.05X10-9)2
= 2.0X10-18 J

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