A new technique is presented that makes it possible, with a single laser pulse, to determine the three-dimensional spatial distribution of state-selected photoproducts. Initially, absorption of a photon from a laser beam causes fragmentation of a molecule. Multiphoton ionization is used to select the internal state of a desired fragment without perturbing its velocity. Following a short delay, the three-dimensional spatial distribution caused by the fragment velocities is projected onto two dimensions by accelerating the state-selected fragment ions into the surface of a channel plate particle multiplier. Electrons emerging from the multiplier are imaged onto a phosphorescent screen for analysis by a digital-image processing device such as a two-dimensional optical multichannel analyzer. The three-dimensional spatial distribution is reconstructed by taking the Hankel transform of the Fourier transform of the projection. The technique is illustrated by recording the spatial distribution of methyl fragments produced in their vibrational ground state by the 266 nm photodissociation of CH3I. From this study it is determined that the fraction of CH3(v=0) formed in coincidence with I(2P1/2) is greater than 0.95, the rest being formed in coincidence with I(2P3/2) ground state.

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We report the cooling of nitric oxide using a single collision between an argon atom and a molecule of NO. We have produced significant numbers (108 to 109 molecules per cubic centimeter per quantum state) of translationally cold NO molecules in a specific quantum state with an upper-limit root mean square laboratory velocity of 15 plus or minus 1 meters per second, corresponding to a 406 plus or minus 23 millikelvin upper limit of temperature, in a crossed molecular beam apparatus. The technique, which relies on a kinematic collapse of the velocity distributions of the molecular beams for the scattering events that produce cold molecules, is general and independent of the energy of the colliding partner.

The first measurements of differential inelastic collision cross sections of fully state-selected NO ͑j =1/2, ⍀ =1/2, ⑀ =−1͒ with He are presented. Full state selection is achieved by a 2 m long hexapole, which allows for a systematic study of the effect of parity conservation and breaking on the differential cross section. The collisionally excited NO molecules are detected using a resonant ͑1+1Ј͒ REMPI ionization scheme in combination with the velocity-mapped, ion-imaging technique. The current experimental configuration minimizes the contribution of noncolliding NO molecules in other rotational states j , ⍀ , ⑀ -that contaminates images-and allows for study of the collision process at an unprecedented level of detail. A simple method to correct ion images for collision-induced alignment is presented as well and its performance is demonstrated. The present results show a significant difference between differential cross sections for scattering into the upper and lower component of the ⌳-doublet of NO. This result cannot be due to the energy splitting between these components.