Why decarbonate soda for spectroscopy experiment?
The primary reason is that when soda warms up, the $\ce{CO2}$ would escape slowly as bubbles. These bubbles would significantly scatter light affecting the absorption spectra. Usually, the effect of such scattering is quite pronounced and the absorption bands of individual molecules will be (might be) buried below the broadened feature in the lower wavelength region.

In colloidal and suspension systems light is scattered by individual particles. These particles are larger in size than the wavelength of light and such scattering phenomena is termed as Tyndal scattering or Tyndall effect.  For such a process, the scattering coefficient has $wavenumber^{4}$ dependence which means that for lower wavelengths the scattering increases significantly. Thus when the sample has particles then absorption spectra is skewed and we see an increasing absorption peak when going to a lower wavelength. (You can yourself try this with milk which is a suspension).

About the absorption spectrum of $\ce{CO2}$:

$\ce{CO2}$ being a simple molecule is expected to have electronic absorption bands below 300nm. It does have electronic absorption between 200 to 270 nm. It is more difficult to find on the internet since usual absorption of $\ce{CO2}$ is referred to the vibrational absorption observed in the in the infrared region. Hence, 'UV absorption of $\ce{CO2}$' is a better search term.

For your reference: T. Joutsenoja, A. D'Anna, A. D'Alessio, and M. I. Nazzaro, "Ultraviolet Absorption Spectra of Carbon Dioxide and Oxygen at Elevated Temperatures," Appl. Spectrosc. 55, 130-135 (2001)