Quantum-state-specific investigation of NO2 and NO3 is of paramount importance in atmospheric chemistry and fundamental molecular physics. High-resolution laser spectroscopy investigations of these two free radicals can unravel the interplay mechanism between vibronic (vibrational-electronic) interactions, including the Renner-Teller, Jahn-Teller, and pseudo-Jahn-Teller effects. Also, vibronic interactions are often coupled to other intramolecular interactions such as the spin-orbit interaction and (pre-)dissociation. Therefore, a comprehensive investigation of the molecular energy level structure is desired for a quantitative understanding of intermolecular dynamics. However, selection rules dictate the ro-vibronic transitions so that there exist “dark states” that cannot be accessed in one-photon spectroscopy measurements of ground-state molecules. Two-photon cavity-enhanced spectroscopy can access and resolve these dark states and provide the much-desired information for understanding vibronic interactions. In our lab, two cavity-enhanced spectroscopy techniques are being developed: the double-resonance spectroscopy and the stimulated-emission pumping. Both techniques are based on the highly sensitive cavity ring-down technique and have the advantages of being two-photon spectroscopy techniques, including high quantum-state-selectivity, narrow linewidth, and simplified spectra. Doppler-free spectroscopy is a pre-requisite for the two-photon spectroscopy measurement. A Doppler-free saturation absorption spectra of CH$ will be presented. We will also discuss the energy level structure and vibronic interaction mechanisms of NO2 and NO3, and the planned two-photon excitation scheme to access their dark states.