FIRST YEAR PROGRESS REPORT

RESEARCH RESULTS

    The Joint Programme of work set out the proposed research under several distinct, but overlapping tasks which fall under two main headings: Experimental Studies of Reaction Dynamics, and Theoretical Studies of Reaction Dynamics. The various teams have fulfilled all the commitments set out in the Joint Programme of work for the first year. In particular, strong, synergistic collaborations have been established and brought to fruition between the Perugia, Madrid, Stuttgart, Oxford (Expt), Oxford (Theory), and Nijmegen teams, that have already led to seven joint publications - five printed and two in press (to appear in the summer 2001). In addition, the various teams have also published several other papers reporting work related to the network and containing acknowledgment to it.
    We describe now the scientific highlights obtained during the first year period of the Network.
 
The principal targets of the Perugia group (P1) during the early stages of the Network were to start measurements of differential cross sections (DCSs) for the prototype 3-atom abstraction, Cl(²P)+H₂, and insertion, O(¹D)+H₂, N(²D)+H₂, and C(¹D)+H₂ , reactions. These objectives have been largely achieved. DCSs for Cl(²P)+H₂ have been measured at low collision energies (Ec) and the results, together with those obtained previously at higher Ec, have been compared with the results of both quasiclassical trajectory (QCT) and quantum mechanical (QM) dynamics calculations carried out in Madrid (P5) and in Stuttgart (P7) on a new potential energy surface (PES) developed by ab initio means in Stuttgart. The role of the rotational and spin-orbit excitation of the reagents was addressed. This has led to two joint papers, published in Chem. Phys. Lett. and in the J. Chem. Phys. DCS data for the N(²D)+D₂ reaction have been compared with the results of QCT calculations on an ab initio PES in a collaboration with researchers outside the Network (Northwestern Univ., USA) and published in J. Phys. Chem. A; new measurements on the isotopic variant N(²D)+H₂ have also been undertaken and their analysis will involve collaboration with also a Network team (Madrid, P5). Some measurements of DCS for the other prototype insertion reaction C(¹D)+H₂ have been anticipated to this first year and have already been published in Chem. Phys. Lett.; these results will soon be compared with those of QM and QCT calculations on a new ab initio CH₂ PES in collaboration with Madrid (P5) and with French researchers in Rennes. DCSs for O(¹D)+D₂ at high Ec have also been measured; the results are being analyzed together with complementary results from Oxford (Expt) (P2) and will be compared with the results of QCT calculations on a new ab initio PES in collaboration with Madrid (P5). In addition, a comprehensive review of the field, setting the status at the beginning of this Network, has been published in Reports on Progress in Physics 63, 355-414 (2000).

In Oxford (Expt) (P2), significant progress on the O(¹D) + H₂ reaction, which was a primary target for the first year of the programme, has been made. The principal concern of the research has been to elucidate the role of excited electronic states and to assess the reliability of state-of-the-art quantum mechanical dynamical calculations. The reaction has been studied as part of a collaborative programme between the Oxford experimental group and the Madrid group (P5), but also involves researchers outside the Network in Rennes (France) and Birmingham (UK), and our first report on this important reaction has been published in Physical Review Letters: preparation of more detailed collaborative papers is in progress. A joint QCT study of O(¹D)+HD with Madrid (P5) has been published in J. Chem. Phys. In addition to these studies we have made significant progress with the H + H₂O reaction, which was one of the goals for year two of the Network: some of this work has been accepted for publication in the J. Chem. Phys. Further collaborative work on this system with the groups of Madrid (P5) and Perugia (P1) are anticipated in the next year.

At Nijmegen (P3) the research was aimed at the study of orientation effects in non-reactive scattering of OH radicals. Simultaneously preparations were made for reactive scattering experiments. The experiments on non-reactive scattering culminated in the following highlights: (1) Measurement of molecular reorientation in rotationally elastic scattering of OH by Ar. Determination of differential and integral cross sections for reorientation of the molecular axis of OH as a function of the initial orientation (M.C. van Beek et al., Phys. Rev. Lett., in press). (2) Determination of inelastic cross sections for rotationally inelastic scattering of OH by N₂ and CO and the measurement of the effects of OH orientation on these inelastic collisions (M.C. van Beek et al., J. Chem. Phys., in press). (3) Characterization of an intense pulsed molecular beam source of cold OH radicals (M.C. van Beek et al., Chem. Phys. Lett., in press).

At Bielefeld (P4), photodissociation processes for the production of OH radicals have been studied, to be applied in crossed molecular beam experiments on the OH+H₂/D₂ reactions. The OH production process was monitored using laser induced fluorescence (LIF) spectroscopy. Photolysis of a pulsed beam of HNO₃ at 193 nm yielded unsatisfactory results; an order of magnitude higher intensity was obtained using an effusive beam of 3 mbar H₂O₂ (the vapour pressure at room temperature), which should be sufficient for the desired experiment. The experimental apparatus, which has been designed for the H-atom Rydberg tagging time-of-flight (TOF) method, has been adjusted to make velocity map imaging (VMI) also possible, which can be used for the detection of the atomic product (H/D) as well as the molecular product (H₂O/HOD) from  OH+H₂/D₂.

At Madrid (P5),  the QCT code has been extended to study non-adiabatic processes using the surface hopping methodology and the new code has been applied to study non-adiabatic effects in the O(¹D)+H₂,HD reactions. The Cl(²P)+D₂ reaction has been studied by using the QCT and QM methods on a new PES developed by the Stuttgart group (P7) and the results have been compared with the experiments of the Perugia group (P1). In addition, a new QCT code has been developed to treat four-atom reactions and first results concerning the H+H₂O and H+N₂O reactions have been obtained. All these developments have permitted the publication of five papers in which four groups within the Network have participated (Madrid (P5), Oxford (Expt) (P2), Stuttgart (P7) and Perugia (P1)). From the experimental side, the REMPI apparatus is completely operative and first experiments on the photodissociation of CH₃SCH₃ have been performed. A joint work on the photodissociation of this molecule between the Madrid (P5) and Nijmegen (P3) groups has been already published.

At Oxford (Theory) (P6), early on in the first year of the network the first version of the hyperspherical coordinate reactive scattering program ABC was finalized and made available for general use (D. Skouteris, J. Castillo and D.E. Manolopoulos, Comput. Phys. Commun. 133, 128 (2000)). This is the program that is used by several of the other groups in the network to perform their QM reactive scattering calculations. D. Skouteris is currently doing post-doc work in Stuttgart (P7) (though not on EC funds), and he will soon (from 1 September 2001) take a post-doc position within the Network in Perugia (P1). J. Castillo has done this past year post-doc work (again not on EC funds) with the Madrid team (P5) and he is now taking an academic position in Spain. At around the same time, the ABC program was used to study a very interesting transition state resonance in the low energy threshold region of the F+HD->HF+D reaction in collaboration with groups in the USA and Taiwan. This study was particularly important because it provided the first conclusive evidence for the observation of a reactive scattering resonance in a molecular beam experiment (K. Liu, R.T. Skodje and D.E. Manolopoulos, Comm. Atomic Molec. Phys, in press). More recently, collaboration with M. H. Alexander (USA) and H-J. Werner (Stuttgart) (P7) resulted in a highly detailed ab initio study of spin-orbit effects in the F(²P)+H₂ reaction, using a modified version of the ABC program. This study of electronically non-adiabatic effects in chemical reactivity was one of the original objectives of the network and it has already led to a joint publication with the Stuttgart group; moreover, it is propaedeutic to the study of similar effects on the Cl(²P)+H₂ reaction.

At Stuttgart (P7), initially calculations using the ground state PES have been carried out for the reaction Cl(2P)+H₂ and D2 and compared with experimental results obtained in Perugia (P1) and QCT results obtained in Madrid (P5). This has resulted in three joint publications. Using new coupled potential energy surfaces for the Cl(²P_3/2,1/2)+H₂ reaction, non-adiabatic and spin-orbit effects in the entrance channel have been studied using exact QM reactive scattering calculations. This made possible for the first time to address theoretically the question about the reactivity of spin-orbit excited Cl(²P_1/2) atoms. It was found that the reaction of excited Cl has a lower energy threshold than of ground state Cl, and therefore at low collision energies the excited state is more reactive than the ground state. However, at energies above the threshold for the Cl(²P_3/2)+H₂ reaction the cross sections for ground-state chlorine atoms become much larger. This is to be expected due to the fact that only the ground state correlates adiabatically with the products, but it is in strong contrast to recent experimental results of K. Liu. Further experimental and theoretical work will be necessary to resolve this discrepancy. Publications are in preparation.

At Munich (P8), the accurate full-dimensional calculation of the H+CH₄->H₂+CH₃ reaction rate (F. Huarte-Larranaga and U. Manthe, J. Phys. Chem. A 105, 2522 (2001)) has been carried out and this can be considered as a major step forward in the accurate quantum treatment of reaction processes. Previously only reactions with up to four-atoms had been treated accurately in their full dimensionality. In the above mentioned calculations a six-atom reaction could be described in an accurate full-dimensional calculation for the first time. This progress was made possible by the combined use of direct rate constant computation based on flux correlation functions and multi-configurational time-dependent Hartree wave packet propagation. The results of these calculation provide new insight into the multi-dimensional character of polyatomic reaction systems and provide an important benchmark for the development of approximate dynamical methods and accurate potential energy surfaces. Collaboration with Stuttgart (P7) on rate constant calculations for the Cl+H₂ reaction including SO effects have been initiated.