As i know,

The 2.7 eV is indicating the optical band gap

and 2.0 eV is representing the second optical absorption band gap.

For more info.

DOI: 10.1021/acs.jpcc.5b03875

J. Phys. Chem. C 2015, 119, 14443−14449

Dear Prof. Md. Saiduzzaman,

Both are energy gaps for the prepared film

The absorption spectrum of the element arises when a white light beam passes through that element or element vapor and produces a spectrum with black lines at specific and characteristic frequencies of the element. The absorption spectra is the opposite of the spectrum of the atomic version. The spectrum generally arises when atoms of an element are induced by the heat, for example, making the atom electrons leave their low-level orbits and move to a higher energy level. But the electron can not stay long in this excited state, and it quickly jumps from the high-energy orbit to the low-energy orbit. The electron emits the difference of the high and low-orbit energy in the form of a photon.

Thank you so much for your kind information.

Could you kindly tell me why second optical absorption showed?

Does it possible due to the two types of oxidation states (Tb3+ and Tb4+).

color of sample is yellow. @ Mallikarjuna Reddy Kesama Sir

This is not thin film, but powder and I am a Phd student not Prof. @ Khalid Haneen Sir.

Thanks.

We hope to complete your PhD soon and become a professor.

There may be more than one type of electronic transition in the same material

How you confirmed that this material is a single phase material?

Single phase was confirmed by the crystal structure refinement from the XRD data.

Dear Md. Saiduzzaman ,

I deduce from your tags that this single crystal belongs to the metal oxide family (it would have been easier if you mentioned the complete name of the material). Such behavior is not new (look for absorption spectra or Tauc plots for beta Gallium Oxide for example), it usually indicates different band gaps that are close in energy, for your case 0.7 eV apart. This means that it's possible, for example, that your material has a principle direct (or indirect) band gap and another band gap as well that it indirect (direct) but lower in energy, thus absorption increases at lower energy than that expected if you treated you material as if it has one band gap only.

To construct Tauc plot, you may use the principal band-gap treatment, however you should notice that this doesn't mean that it is the only band-gap.

Such behavior can originates from different oxidation/reduction states, non-symmetry in crystal structure, or in general from different electron states. To gain more insight, electronic structure calculations and simulations would be of great help, next to plenty of electronic structure experimental characterization as well!

Assuming that your sample is Terbium Oxide, yes, this may indicate presence of Tb+4, especially that the energies that you got fall between 400 and 600 nm (see Article Structural and Optical Properties of Terbium Oxide Nanoparticles

Article Optical absorption of terbium oxide

Article Structural and Optical Properties of Terbium Oxide Nanoparticles

Article Tb2O3 thin films: An alternative candidate for high-k dielec...

These articles might shed some light on the reason behind the abs shoulder you have:

Article First principles study of rare-earth oxides

Article Preparation, characterization and optical properties of terb...

Hope that was useful.

Good luck!

single phase give only on fitted straight line which is nearly perpendicular on x-axis (hu) eV intern you can estimate only one energy gap with very high absorption

That's usually true, but there are many special cases.

Article Optical Absorption and Photoconductivity in the Band Edge of...

Article Anisotropy of Electrical and Optical Properties in β-Ga2O3 S...

Article First-principles calculations of the near-edge optical prope...

please try to read the first reference which you have presented:

1-Optical Absorption and Photoconductivity in the Band Edge of β − Ga 2 O 3

((( This is due to the low symmetry of the Ga203 lattice )))

add to that the author have been related the smaller edge to the induced conductivity due to the utilized beam; so still we have one transition from band to band !!!!

I think the higher value is (Eg+Ephonon) and the lower one is (Eg-Ephonon) , by solving the two equations you will obtain the phonon energy and Eg

Dear Ibrahim Hany Sir,

Thanks a lot for your valuable information.Its really very much helpful.

Thanks to all also.

You're welcome Md. Saiduzzaman! Good luck in your research.