Question

why do we use energy in per cm in microwave spectroscopy

Answer 1

To make this more accessible to more people; Im gunna broaden/expand you're question to be "why are the size units [per cm] used in spectroscopy class when we are studying the section on microwave radiation? Why not feet or or meters themselves?"

To Answer this question set
-The wavelength of microwaves is in the range of 1 mm (size of a grain of sand) to 1 meter (the diameter of snoop dogs rims in the music video "Nuthin' But A G Thang") Therefore, it would make sense to pick centimeters (width of human male fingernail) as a fair compromise to keep it balanced, son

Answer 2

This may be (read as 'definitely is') one of my old professors talking, but firstly, it is important to understand that a 'wavenumber' is not a unit of energy, but the concepts are still closely related enough that spectroscopists are justified in 'replacing' energies (E) with wavenumbers (v). This is most easliy shown via the planck formula for the energy of a transition:

E = h * c * (v2 - v1)

I.e. the energy of a photon required to cause a transition between two states is proportional to the difference between the 'wavenumber' of the initial and final states. Spectroscopists find it 'prettier' to throw away a factor of hc, rather than carry it round as a redundancy and interpret the wavenumbers directly. This justifies why spectroscopists choose dimensions of inverse distance rather than energy, but why the unit of inverse CENTIMETRES of all things?! Well, traditionally, spectrophotometric measurements on chemical samples are carried out with 'cuvettes' (sample chambers) with a radiation path length of 1 cm. So, there's your answer - history has a funny way of messing with stuff.

Hope this helps!

Answer 3

;)