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H + H2 REACTION DYNAMICS
Nate Bartlett, Justinas Jankunas

Following an over thirty-year history of gas phase reaction dynamics in the Zarelab, we continue the study of H + H2 reaction. Indeed, it is a very exciting time for our section as significant recent advances in experimental results and theoretical knowledge allow us to start contemplating ever more complex experiments.

Using a home-built three-dimensional ion imaging experimental apparatus1 we were previously able to study reactive and inelastic scattering in an H + H2 reaction and its isotopic analogues. We did this by using a tunable photolysis of HBr in the HBr/D2 mixture. This generated fast H atoms that then went on to scatter either reactively or inelastically off D2, to give HD (v=1,3, j) or D2 (v=1-4, j), respectively. The products of interest were state-selectively ionized using [2+1] resonance-enhanced multiphoton ionization (REMPI). The resulting ions were accelerated toward a multichannel plate (MCP) coupled to a delay-line anode which measures the three-dimensional velocity distribution of the reaction products which can then be converted to a differential cross section using the PhotoLOC (photoinitiated reaction analyzed with the law of cosines) technique developed in the Zarelab.

The experimental results on reactive scattering in an H + D2 --> HD (v=1, j) + D reaction agreed well with the theoretical predictions.2 As expected, we found two different reaction channels if the spin-orbit coupling in Br is taken into account. Also, as the rotational excitation of HD increased the differential cross sections (DCSs) shifted from backward to sideward scattering, again in an agreement with the theoretical predictions. H + D2 --> HD (v=3, j) + D reaction exhibited an even richer dynamics.3 In particular, time-delayed forward scattering was attributed to a glory effect that resulted from a near- and far-side quantum interference. Inelastic scattering experiments also revealed a few interesting features.4 It was shown, for example, that the translational H atom energy was transformed into a vibrational D2 energy mainly via a bond elongation with forward scattered products rather than bond compression and backward scattered products.

We are actively working towards the goal of using aligned samples of D2 as scattering targets in the H + D2 reaction. It has been theoretically shown that reactant polarization can greatly alter the chemical reactivity.5 As a first step we have used stimulated Raman pumping to prepare highly aligned and oriented samples of H2, HD and D2 under collision-free conditions.6 We have prepared both, states which undergo time-dependent rotational depolarization due to coupling between the rotational and nuclear spin angular momenta, and those that do not. The time-dependent rotational polarization was interrogated using polarized [2+1] REMPI for pump-probe delay times of up to 13 s, about 10 times longer than has ever been measured with this method. We found that the calculated depolarization and our experimental data were in excellent agreement (see figure below). States for which no depolarization occurs we found that the prepared samples retained their initial degree of alignment for at least up to 8 s, about 1000 times longer than what will be required for the use in a chemical reaction. This gives us confidence to further pursue our goal to use an aligned D2 molecule in a scattering study. Finally, we are also very eager in testing the geometric phase effects predicted theoretically7 for an H + H2 reaction: an experiment that would truly push the limits of our understanding of the simplest bimolecular reaction as well as make us (re)think of how good our experimental apparatus really is!

1 K. Koszinowski, N. T. Goldberg, A. E. Pomerantz, and R. N. Zare, J. Chem. Phys. 125, 133503 (2006).
2 K. Koszinowski, N. T. Goldberg, J. Zhang, R. N. Zare, F. Bouakline, and S. Althorpe, J. Chem. Phys. 127, 124315 (2007).
3 N. T. Goldberg, J. Zhang, D. J. Miller, and R. N. Zare, J. Phys. Chem. A 112, 9366 (2008).
4 N. T. Goldberg, J. Zhang, K. Koszinowski, F. Bouakline, S. Althorpe, and R. N. Zare, PNAS 105, 18194 (2008).
5 J. Aldegunde, M. P. de Miranda, J. M. Haigh, B. K. Kendrick, V. Saez-Rabanos, and F. J. Aoiz, J. Phys. Chem A 109, 6200 (2005) and references therein.
6 N. C.-M. Bartlett, D. J. Miller, R. N. Zare, A. J. Alexander, D. Sofikitis, and T. P. Rakitzis, Phys. Chem. Chem. Phys. 11, 142 (2009).
7 J. C. Juanes-Marcos, S. C. Althorpe, and E. Wrede, Science 309, 1227 (2005).