Question

In a salt solution, which species (cation/anion) is responsible for the absorbance spectrum?

Answer 1

see below

Explanation:

I presume you speak about UV-Vis spectrum , because IR spectrum is generally given by organic molecules while atomic spectrum is given often by cation.
In UV spectrum you can see the answer of the molecular bonds of a salt dissolved in water if the anion (often) or cation (rarely ) are formed by more than one atom. You can see double bonds expecially if they have risonance structure (like`CrO_4^-; MnO_4^-, NO_3^-, NO_2^-` and so on).

But you can also see the presence of coloured metallic cations that have an excitation of the electrons of "d" orbitals (like `Cu^2+; Fe^3+; Cr^3+`; and so on) or molecular colored compounds (like`I_2`)