Hexapole state selection of OH molecules and subsequent orientation in an electric field is performed to study orientational effects in rotational excitation of OH in molecular collisions. Laser-induced fluorescence spectroscopy of OH is used to determine the orientational probability distribution function and to measure the cross sections for excitation. For the collisionally induced transitions of OH in the rotational ground state the steric asymmetry is determined for collisions with He, Ar, n-H2, and p-U2 (n = normal, p = para). The results show that for He excitation is preferential at the H-end of the molecule, whereas for Ar and H2 a preference for the O-end is shown in transitions to the lowest rotational states.

Parity resolved state-to-state cross sections for rotational excitation ofOH(X 2n) colliding with He and Ar, have been obtained in a crossed molecular beam experiment. The OH radicals were produced in a pulsed dc discharge in a H 2 0/ Ar gas mixture. Adiabatic expansion into vacuum resulted in an effective rotational cooling yielding a 94% population of the lowest A-doublet (X 2n3/2' J=~). Further state preparation could be achieved via electrostatic state selection in a hexapole electric field, resulting in a 93.5% population of the upper A-doublet component (f, + ). Experiments were performed both with and without the state selector to provide detailed information about the rotational excitation from both A-doublet states. The OH rotational state distribution was probed, before and after the collision event, by means of laser-induced fluorescence (LIF) spectroscopy of the A -X electronic band at 308 nm. The OH-Ar and OH-He scattering behave very similar with the exception of the J=~ A-doublet transition which is induced much weaker by the He collisions.

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Dissociative chemisorption of H2 on Cu(100): A fourdimensional study of the effect of parallel translational motion on the reaction dynamics In this paper we report the measurement of ⌳-doublet resolved state-to-state cross sections for inelastic collisions of OH by H 2 at a translational energy of 595 cm Ϫ1 . Experimental values are obtained for transitions from both the upper ( f ) and the lower (e) ⌳-doublet substates of the lowest rotational state (J ϭ 3 2 ) of OH( 2 ⌸ 3/2 ) to almost all other states within the range of the collision energy. Cross sections for scattering by both para-and normal-H 2 have been determined. The main difference between para-and normal-H 2 scattering is seen in the ⌳-doublet cross section. The results are compared to He-scattering which shows that H 2 (J ϭ 0) scattering behaves similar to He scattering. When averaged over the ⌳-doublet states, the cross sections are in good agreement with the measurements of Andresen et al. ͓J. Chem. Phys. 81, 571 ͑1984͔͒, although the conclusions with regard to collisional pumping of interstellar OH masers are different. The quantum calculations of Offer et al. ͓J. Chem. Phys. 100, 362 ͑1994͔͒ show a surprisingly good correspondence with the measured values. In this comparison the full initial state distribution of the OH and H 2 beams has been taken into account.

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