Page 24 - Visions of the Future Chemistry and Life Science
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14  G. ROBERTS



                                  The use of molecular beams to lock reactants together within femtos-
                               econd striking distance is not the only way to perform ultrafast spectros-
                               copy of bimolecular reactions. Another is to initiate the final approach
                               trajectory of collision between a metastable atom or molecule in a high-
                               pressure atmosphere of a second partner, thereby reducing the time
                               required for the collisional encounter to below a picosecond. This approach
                               is illustrated in Figure 1.5(c) for collisions between excited sodium atoms
                               Na* and molecular hydrogen, in which the outermost electron of the
                               sodium is first promoted to an energised state by an ultrafast laser pulse.
                               The Na* H system serves as a paradigm for transfer of matter and energy
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                               in atom–molecule scattering since, as shown in Figure 1.5(c), the atom and
                               molecule can either form NaH H by swapping a hydrogen atom or can
                               transfer the initial excitation energy of the sodium atom to the intact H
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                               molecule, resulting in the emergence of a deactivated sodium atom and
                               vibrationally excited hydrogen. The trajectory of the scattering event again
                               proceeds via one or more curve crossings between potential energy sur-
                               faces, representing the different forces between the atom and molecule at
                               different stages of the collisional evolution.
                                  Current research at the Max-Planck-Institut für Quantenoptik in
                               Garching, Germany, is concentrating on the mechanism of collisional
                               deactivation via electronic-to-vibrational energy transfer, in which the
                               temporal progress from initial to final states is monitored by the simulta-
                               neous absorption of three 20fs probe photons and re-emission of a fourth
                               by the [Na* . . . H ] intermediate configuration as it forms and breaks apart.
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                               This type of coherent scattering spectroscopy is extremely sensitive and
                               enables the appearance of deactivated sodium atoms to be probed as a func-
                               tion of time as they emerge from the curve crossing. Experimental meas-
                               urements are supported by theoretical calculations of the cross sections for
                               light scattering in real time, from which the wavepacket motion over the
                               intersecting potential energy curves can be deduced. These reveal that the
                               [Na* . . . H ] species formed during the initial approach stage persists for
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                               durations up to 120fs before it fragments, during which time the excitation
                               energy carried by the Na* atom is funnelled into the H coordinate by
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                               repeated multidimensional transfer of population between the colliding
                               partners. The collision is said to be ‘sticky’, as the Na* H collide, bounce
                                                                              2
                               off one another and exchange energy and population over a time scale that
                               is very long compared to the period of H vibrations (about 8fs).
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