How specific should my research interest be

[HCY_]

I am applying chemistry PhD for the next fall.

I know that I want to do something that's related to spectroscopy. But I have experience using different spectroscopies (ultrafast, 2D) and look at different system (nanomaterials, protein) and honestly, I dont mind exploring a new area relating to spectroscopy. I dont know if should be honest in SOP and say "My interest is spectroscopy, and I dont mind the exact project"? I'm afraid that gives an impression that I dont have a passion for a beneficial cause (such as energy conservation/curing diseases) or that I'm not totally clear what project I want to do, which in reality I'm not.... 

Thanks in advance for any advice!