Zero Kinetic Energy Photofragment Spectroscopy:  The Threshold Dissociation of NO₂

A new type of photofragment spectroscopy has been developed and applied to the photodissociation of NO2. Photofragments with zero recoil kinetic energy are selectively detected as the total energy of the dissociating system is varied. The resulting spectra provide information about the transitions that occur between energy levels of the transition state and energy levels of the final products. With proper calibration of the laser wavelength, the method may be useful in the accurate identification of the dissociation threshold. The technique is demonstrated by the detection of O atoms from the near-threshold photodissociation of NO2. A sharp onset of signal occurs at the dissociation threshold of, while for higher energies, a new peak occurs as the energy threshold is reached for each possible NO internal state. An analysis of the intensity distribution of the spectral peaks suggests that the maximum orbital angular momentum allowed by the 0.5 cm-1 resolution of the instrument is near 5ℏ, implying that the centrifugal barrier for that value of the angular momentum is less than or equal to 0.5 cm-1.