Cu nanoparticles shows UV-vis absorption spectrum exhibited an absorption band at 572-582 nm.

Check the following references

1)Synthesis of high-concentration Cu nanoparticles in aqueous CTAB solutions

http://www.sciencedirect.com/science/article/pii/S0021979704001432

2) Synthesis, Characterization, and Nonlinear Optical Properties of Copper Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/la9605495

Cu nanoparticles shows UV-vis absorption spectrum exhibited an absorption band at 572-582 nm.

Check the following references

1)Synthesis of high-concentration Cu nanoparticles in aqueous CTAB solutions

http://www.sciencedirect.com/science/article/pii/S0021979704001432

2) Synthesis, Characterization, and Nonlinear Optical Properties of Copper Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/la9605495