The photo-acoustic (PA) spectrum of nitrogen dioxide (NO2) in the range of 420-520 nm with a Nd:YAG pumped optical parametric generator and amplifier as radiation source is presented. The spectrum has a characteristic of banded structure superimposed on continuum. The banded structure of the spectrum can be assigned to NO2 B2B1<--X2A1 transition. While the continual one comes from the chaos states of the first excited state A2B2 and the high vibration levels of the ground state X2A1. The relatio
