Is there any circumstance when I can record someone without their knowing?

I am wondering if it is possible to record someone without their knowing. I will be in Illinois at the time. Someone at work has been sexually harassing me and perusing sex with me non-stop. They have even offered me money for it. They have rank on me and I am hoping to record this so all of a sudden if things start going bad for me it will be obvious to the higher-ups and others in my field.