Need help from Fluther's grammarians. Can you help me to determine if I should use "subsequent" or "subsequently" in this sentence?

I have poked around on a couple of websites but cannot figure out the correct answer. Which sentence below is correct:

Subsequent arriving employees should look for the signal.
or
Subsequently arriving employees should look for the signal.

They both sound right to me and I cannot make a decision. Lack of sleep and depression do not help in the making of grammar decisions.

Thank you.