the good news is that you were promptly hired by another company and hopefully a competitor. you might consult with a PA attorney but my recommendation would be to avoid getting into a very costly, time consuming and unproductive venture. you are justifiably angry but the best revenge is to be a great employee for the new company. eventually your old company will recognize that and your old boss may find himself the victim of his own dishonesty. i say move forward in a positive way and avoid lawsuits when you can.
Defamation lawyers typically want a $5000.00 retainer to begin, so unless you have sustained a significant monetary loss, the case could cost thousands more than would be recovered, but discuss with a local Irwin defamation lawyer