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Photonics Probes Hydrogen Exchange

Photonics Spectra
Jun 2002
Paul Mortensen

Theorists at Durham University in the UK and experimentalists at Stanford University in California have joined forces to investigate the dynamics of the simplest and most studied of bimolecular reactions, the hydrogen-exchange reaction. In this process, a hydrogen atom collides with molecular hydrogen, breaking the molecular bond and exchanging itself with one of the other atoms.

Zare1.jpg
A simulation of the H + D2 → HD + D reaction agrees well with experimental data. The blue contours represent the wave function of the reaction and display the evolution of the HD product as a function of R and θ, the center-of-mass scattering angles. At 40 fs after the collision, a fraction of the product is backscattered (to the right). At 48 fs, the forward-scattered product is beginning to emerge (to the left). Courtesy of Stuart C. Althorpe, Durham University.

The scientists have discovered independently via laser-based experiment and quantum simulation that, at a specific collisional energy, a tiny fraction of the collision product forms earlier and is backscattered and, unexpectedly, a second fraction is forward scattered approximately 25 fs later. They suggest that some form of trapping might cause this delay, and the answers go to the core of how chemical reactions occur.

Richard N. Zare and his colleagues at Stanford's Zarelab, together with Félix Fernández-Alonso of Consiglio Nazionale delle Ricerche in Rome, employed the "photoloc" technique (photo-initiated reaction analyzed with the law of cosines) to record the appearance, travel direction and speed of rotation or vibration of the molecules in the hydrogen-exchange process. To follow the reaction more easily, they created free hydrogen by laser photochemistry and used molecular heavy hydrogen (D2) to distinguish the reaction product from the initial hydrogen.

They expanded a mixture of HBr and D2 through a pulsed nozzle into the extraction region of a time-of-flight mass spectrometer and photolyzed the HBr at wavelengths between 203 and 225 nm with a tunable polarized laser to produce fast H atoms with well-defined speed and spatial distribution. The reaction continued for approximately 20 ns, and a probe laser ionized the HD products of interest. The researchers subsequently measured the distribution speeds of these products in the mass spectrometer.

"From an experimental viewpoint, this simple reaction system has been quite a challenging one," Fernández-Alonso said. "Only in the past decade has it been possible to measure, using photo-initiation and laser spectroscopic detection, reaction attributes at a sufficient level of detail of close comparison with theoretical predictions."

Quantum mechanics, however, prevents the imaging of the particles throughout the reaction. But through computer simulation, using no experimental data, Stuart C. Althorpe (now at Exeter University in the UK) and his former co-workers at Durham investigated the H + D2 → HD + D reaction, plotting where the atoms were moving during that time.

"We solved accurately the time-dependent Schrödinger equation governing the motion of the H and D atoms," Althorpe explained. The simulation is plotted as a movie that indicates when the HD forms and how it scatters. "This is the first time that this type of quantum simulation has been done for a bimolecular reaction."

The results of experiment and simulation agree very well, he said, except at a collisional energy of 1.54 eV. He attributes this disparity to the Berry phase effect, which currently is not addressed in the simulation. The theorists plan to model the H + D2 part of the reaction and to create movies that uncover the origin of the time delay.

The work highlights the benefit of performing both simulation and experiment, the researchers said, but it does not suggest that accurate models could replace the work in the lab. "A close match between theory and experiment can confirm the validity of ab initio approximations, but only a posteriori," Fernández-Alonso said. "Experiment and theory need each other."


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