Why is the absorption value of some solid powder negative in the ultraviolet-visible diffuse reflectance spectrum? How to solve it?

I have measured DRS spectra of DPA (9,10-diphenylanthracene) solid powder from 300 to 800 nm. What puzzles me is that its absorption value begins to decrease when the wavelength is less than 440...

30 April 2021 8,301 3 View