SECOND 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 most of the commitments set out in the Joint Programme of work for the second year. In particular, strong, synergistic collaborations between the various teams, which had led already to several joint publications during the first year, have continued fruitfully during the second year and have led to several other (10) joint publications (4 printed, 1 submitted, and 5 in preparation). In addition, the various teams have published several other papers reporting work related to the network and containing acknowledgment to it.
We describe now the scientific highlights obtained during the second year period of the Network.

The principal targets of the Perugia group (P1) during the second year of the Network were to measure 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, in order to compare them with the results of theoretical calculations carried out in other laboratories of the Network. A characterization, by spectroscopic techniques, of OH radical beams was also foreseen. These objectives have been largely achieved. DCSs for Cl(²P)+H₂, measured during the first year at low collision energies (Ec), together with the results obtained previously at higher Ec, have been compared with the results of both quasiclassical trajectory (QCT) and quantum mechanical (QM) dynamic calculations carried out in Madrid (P5) and in Stuttgart (P7) on a new, ground state potential energy surface (PES) developed by ab initio means in Stuttgart (P7). This has led to a joint paper, published in the J. Chem. Phys (2001). New measurements of angular and velocity distributions, with higher sensitivity, have been carried out at even lower Ec to explore further the role of the rotational and spin-orbit excitation of the reagents. The results have been compared with the new, accurate quantum scattering calculations (including non-adiabatic effects) carried out for the first time in Stuttgart (P7) and Oxford (P6), in collaboration with Prof. M. H. Alexander (University of Maryland, USA) on the new multiple PES developed in Stuttgart (P7) for Cl(²P_3/2,²P_1/2)+H₂. A joint publication with (P7) is in preparation. In addition, the YR fellow Dimitris Skouteris, enrolled in Perugia from 1 September  2001 to work on the theoretical side of the project, has developed work along the line of designing and implementing time dependent programs for the calculation of reactive cross sections. The program is now working. The calculations have been carried out for the Cl + H₂ reaction. To this end the potential energy surface developed by the Stuttgart group (P7) has been used and the total reactive cross section has been calculated in the range of collision energy extending up to 0.3 eV. A comparison with values obtained from other approaches has been performed and the agreement was found to be satisfactory. A publication is in preparation.
 Time-dependent quantum calculations of reactive probabilities have also been performed for the Li+HF system for a sufficiently extended interval of energy. From these values estimates of the reactive cross section were obtained using an energy shifting model. The theoretical results were compared with experimental data obtained in the recent past by the group of Loesch at Bielefeld (P4). A publication is about to be submitted to Journal of Computational Methods.
After having studied the reaction N(²D)+D₂ and compared the results with QCT calculations, since accurate quantum scattering calculations were only available for N(²D)+H₂, DCS have been measured in Perugia (P1) also for the isotopic variant N(²D)+H₂: this has permitted to compare, for the first time for an insertion reaction, experimental DCS with accurate quantum scattering calculations (carried by the group of Launay in France) and with QCT calculations performed by the Madrid group (P5) on an ab initio PES: from these comparisons, clear quantum effects in the dynamics of this prototypical insertion reaction have been characterized and the PES probed unambiguously. A joint paper has just been submitted to Phys. Rev. Letters.
The DCS measured in Perugia (P1) for the other prototype insertion reaction C(¹D)+H₂ are being compared with the results of accurate quantum scattering calculations (carried out by Launay's group in France) as well as with QCT calculations performed in Madrid (P5) on a very recent ab initio PES. A joint publication is currently in preparation for J. Chem. Phys. DCSs for O(¹D)+D₂ at high Ec obtained in Perugia (P1) have been analyzed together with complementary results from Oxford (Expt) (P2) and compared with the results of QCT calculations on new ab initio PESs (both ground and excited state) in collaboration with Madrid (P5): the role of the first excited H₂O PES is analyzed.  A joint publication is in preparation. New measurements of DCS at low Ec have been carried out for O(¹D)+H₂ stimulated by the fact that accurate quantum scattering calculations are available for this isotopic variant (carried out by Launay's group in France); detailed comparisons experiment/theory (quantum and QCT) have been nearly completed and a joint publication with the Madrid team (P5) is in preparation
Finally, the set-up of a new experiment aimed at the spectroscopic characterization, by LIF (laser-induced-fluorescence), of supersonic beams of OH radicals is near completed. Collaboration with the highly experienced Nijmegen team (P3) is envisaged on this topic. The OH beams will be used to study, during the third year, the dynamics of the prototypical four-atom reactions OH + CO and OH + H₂, of interest also to other teams in the Network.

In Oxford (Expt) (P2), many of the recent activities of the group have been highlighted in a Feature Article in the Journal of Physical Chemistry A (which has recently been published on the WEB). This year significant progress has been made with studies of the following reactions

In the second year of the network the research at Nijmegen (P3) was aimed at the study of orientation effects in non-reactive scattering of OH radicals with HCl. Simultaneously, preparations were made for (1+1') REMPI detection in reactive scattering experiments (see Sect. B.3.2). The experiments on non-reactive scattering resulted in the determination of inelastic cross sections for rotationally inelastic scattering of OH by HCl and the measurement of the effects of OH orientation on these inelastic collisions.

At Bielefeld (P4), the original research objectives of the project have been modified due to the change of the Bielefeld group leader in the Network, as already mentioned in the First Year Report. The plans for creating an intense pulse of OH molecules by photodissociation and the study of the reaction OH + H₂ have been abandoned. Instead two new projects are being pursued: (i) photodissociation  and subsequent velocity map imaging and  (ii)  the reactive scattering of Li+HF->LiF+H. In the first project results are now available for HI and I₂ (data analysis is in progress) while for the latter the experimental equipment is being assembled and the required crossed molecular beam apparatus modified to reach very low collision energies (see Sects B.1 and B.2).
The original vacuum chamber which was used for the hydrogen exchange reaction was modified. In order to do velocity map imaging as developed in Nijmegen by Dr. André Eppink and Prof. Dave Parker (P2), it was fitted with an ion lens system with an imaging detector which consists of a pair of MCPs and  a phosphor screen. The image from the phosphor screen is taken with a CCD-camera and analysed with standard software. For the first test measurements photodissociation of HI at 266nm was done with subsequent 1+1’-REMPI detection of the H-atoms or D-atoms with 121.6nm and 364nm wavelength. This process is well known and the measurement showed that after some small adjustment the ion lens worked fairly well. During the measurements of HI-dissociation some peculiarities were noted. Among them a structured ring system at a higher mass, which was attributed to the dissociation of molecular iodine. Since the  comparison with older measurements of other groups yielded no immediate assignment of the processes involved, it was decided to investigate further. Therefore the measurements were repeated with pure I2 and different seed gases (Argon and Helium) to exclude the possibility of cluster processes of HI or of I2 with the seed gas. At the moment work on a full analysis of the data is underway.

The Madrid group (P5) has fulfilled most of the commitments within the Network for the second year. The quasi-classical trajectory (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, D₂ reactions. The Cl(²P)+H₂,D₂ reaction has been studied by using the QCT and quantum mechanical (QM) methods on a new potential energy surface (PES) developed by the Stuttgart group (P7), and the results have been compared with the experiments of the Perugia group (P1). In collaboration with J. M. Launay (Rennes) the groups of Perugia and Madrid have undertaken a thorough study of the N(²D)+H₂ reaction (included in the Projects Objectives 1b.), in which new experiments carried out in Perugia are compared with QM and QCT calculations on the most recent PES existing for this reactive system. In addition, a new QCT code has been developed to treat four-atom reactions and results concerning the H+H₂O and H+N₂O reactions have been obtained. All these developments have permitted the publication of three papers in which four groups within the Network have participated (Madrid (P5), Oxford-Expt (P2), Stuttgart (P7) and Perugia (P1)), and another paper has been submitted for publication. From the experimental side, the REMPI apparatus is completely operative and extensive experiments on the dynamics and stereodynamics of the photodissociation of CH3SCH3 have been performed. The velocity-map ion-imaging technique is being implemented and this will permit the performance of photon-initiated experiments with imaging detection of products.

At Oxford (Theory) (P6), early on in the second year of the network a coupled-channel statistical theory of atom-diatom insertion reactions was developed by combining the early statistical ideas of Pechukas and Light with the coupled-channel capture theory of Clary and Henshaw. The resulting theory was applied to the N(²D)+H₂ and O(¹D)+H₂ insertion reactions, which have been studied experimentally by the Oxford (P2) and Perugia (P1) groups, and was found to give results in excellent agreement with the exact quantum mechanical calculations of Honvault and Launay. This work was done in collaboration with the YR fellow Fermin Huarte-Larranaga from the Munich group (P8) during his research training visit (secondment) to Oxford, and it has led to a Network joint publication. Since then, the YR fellow Thomas Gonzales-Lezana and PhD student Edward Rackham have been applying the same statistical theory to the H+O₂ combustion reaction, with rather interesting results. For total angular momentum J=0, their statistical calculations are in excellent agreement with the exact quantum mechanical calculations of Meijer and Goldfield, and Dai and Zhang up to a total energy of 1.3 eV. However, for J>0, there is a marked discrepancy between the statistical results and the results of Meijer and Goldfield, which are the only "exact" results for this system that are available for comparison. In order to get to the bottom of this discrepancy the Oxford team are now continuing with their adaptation of the ABC program to treat insertion reactions, so that they can calculate their own "exact" results for comparison with the statistical theory (see Section 3 below - Research Method and Work Plan).
Finally, in a collaboration with Professor Bowman from Emory University, they have applied the ABC program to a detailed study of the O(³P)+HCl reaction and shown that the low-energy resonances that are seen in this reaction on the latest ab initio potential energy surface are due to quasi-bound quantum states in the reactant and product van der Waals wells. This study is relevant to the network because the same methodology can be applied to the O(3P)+HBr reaction which has been studied experimentally by the Oxford group (P2).

At Stuttgart (P7), using coupled potential energy surfaces for the Cl(²P_3/2,²P_1/2) + H₂ reaction, non-adiabatic and spin-orbit effects in the entrance channel have been studied. In the current period the analytic fits of the potentials were improved and finished. The fits have been used in exact quantum scattering calculations of integral and differential cross sections for Cl(²P_3/2,²P_1/2)+H₂. 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_3/2,²P_1/2)+H₂ reaction the cross sections for ground-state chlorine atoms become much larger.  The reactivity of the ground-state potential (correlating with Cl(²P_3/2)+H₂) is reduced as compared to single state calculations, which is due to additional inelastic scattering caused by non-adiabatic transitions to the non-reactive spin-orbit states. These theoretical results are in strong contrast to recent experimental results of K. Liu et al., who predict that the spin-orbit excited Cl(²P_1/2) state is more reactive than the ground state. Further experimental and theoretical work will be necessary to resolve this discrepancy.
A first report on this important studies have been accepted for publication in the prestigious journal SCIENCE. Several other publications, some joint with other teams (P1 and P8) of the Network, are in preparation.

At Munich (P8), accurate reaction rate calculation for the H₂+Cl->H+HCl reaction have been carried out. Employing the new potential energy surface developed by the Stuttgart team (P7), spin-orbit effects have been included in the dynamical calculation. The results confirm the accuracy of an approximate treatment of the spin-orbit coupling used in previous collaborative work (U. Manthe, W. Bian and H.-J. Werner, Chem. Phys. Lett. 313, 647 (1999)). The calculations are now complete and an article is in preparation. Extended reaction rate calculations investigated the H+CH₄->H₂+CH₃ reaction. Accurate full-dimensional (12D) quantum calculations investigated the
reaction dynamics in an increased energy and temperature range. These calculations required additional computational techniques: a rigorous statistical sampling scheme was employed to study thermal rate constants at temperatures above 500 Kelvin and to compute the reactant partition function. This has led to the publication of several papers involving the YR Fermin Huarte-Larranaga.