Laser‐induced gratings in free jets. II. Photodissociation dynamics via photofragment transient gratings

We describe the use of photofragment transient gratings to measure populations, velocities, and translational anisotropies of nascent, state‐selected photofragments. This technique is a variant of two‐color laser‐induced grating spectroscopy (LIGS); scanning the probe laser gives spectra that reveal fragment‐state populations and scanning the grating laser, with the probe laser monitoring a specific fragment state, produces photofragment excitation (PHOFEX) spectra. The temporal behavior of the photofragment grating is used to determine speeds and translational anisotropies of the photofragments. We have demonstrated the photofragment grating method on the NO(²Π_1/2, v″=0, J″) fragments produced from the near‐threshold photolysis of jet‐cooled NO₂. The grating PHOFEX spectrum yields the threshold for production of NO(²Π_1/2, v″=0, J″=0.5, e)+O(³P₂) from NO₂ (N″=0) as 25 128.5±0.2 cm⁻¹. For photolysis at 126 cm⁻¹ above threshold, the NO rotational distribution obtained from the probe spectrum is in good agreement with that determined from a LIF spectrum recorded simultaneously. In addition, analysis of the grating decays for state‐selected NO fragments gives their speeds and anisotropies; the speeds are in excellent agreement with the known values, even for fragments with as little as 7 cm⁻¹ of translational energy.