Page 24 - Visions of the Future Chemistry and Life Science
P. 24
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
2
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
2
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.
2
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
2
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
2
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).
2