Controlling the Quantum World The Science of Atoms, Molecules, and Photons (2007) / Chapter Skim
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5 Exploring and Controlling the Inner Workings of a Molecule
5 Exploring and Controlling the Inner Workings of a Molecule
Pages 98-119
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From page 98... ...
Our ability to control the positions, velocities, and relative spatial orien tations of individual atoms and molecules has led to a stunning array of precision measurement technologies and devices based on AMO science, leading to an enormous range of experiments that reveal qualitatively new phenomena. In this section, the committee focuses on the emerging ability to observe the inner work ings of atoms and molecules on their natural timescales, and to manipulate them to achieve desired effects.
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From page 99... ...
exPloring controlling inner workings molecule and the of a ing attosecond (10–18 s) timescale, the time it takes an electron to orbit an atom, to the timescale of ≈14 billion years, the age of our universe.
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From page 100... ...
the earliest direct observations of molecular vibrations were performed using a sequence of two pulses, in which the first pulse excited the molecule and the second pulse was used to probe the resulting molecular response as a function of the time interval between the pulses. this pump-probe approach has made it possible to
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From page 101... ...
exPloring controlling inner workings molecule 0 and the of a BOX 5-1 Stopping Time In the experiments that produced the data at the right of Figure 5-1-1, an x-ray pulse of a few at toseconds in duration is used to knock an electron out of a tightly bound inner orbital of krypton atoms to produce krypton ions. The "hole" in the inner orbital is not stable and is quickly filled by an electron from an outer orbital, with the concurrent ejection of a second electron.
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From page 102... ...
controlling Quantum world 0 the FIGURE 5-2 Time evolution of molecular excited states. Excited states of polyatomic molecules often have a mixed character that results in interesting effects when the molecule absorbs ultraviolet light.
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From page 103... ...
the fact is that the calculation of the chemical dynamics in any but the simplest systems remains an extremely difficult problem. the difficulty is in quantum mechanics itself, which is a far more challenging theory than classical physics.
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From page 104... ...
. According to quantum theory, if there are multiple paths between the starting point and the target for a process, then one cannot tell even in prin ciple which path was taken; the paths are said to be "indistinguishable." in such cases, the quantum paths can interfere, much like waves interfering on a beach.
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From page 105... ...
The crests and troughs are the result of constructive and destructive interferences, respectively, between the waves produces by the two pebbles. In contrast to pebbles in a pond, which are described extremely well by the laws of classical physics, atoms and molecules are quantum objects governed by the laws of quantum mechanics.
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From page 106... ...
controlling Quantum world 0 the How do we choose the appropriate pulse shape to achieve a particular chemical reaction? A sufficiently accurate a priori determination of this pulse shape from knowledge of the quantum structure -- that is, the orbits -- of the molecule is not possible with current theoretical and computational capabilities for most systems.
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From page 107... ...
there have even been attempts to control the pathways of energy flow in bacterial lightharvesting proteins. Quantitative electronic structure and dynamics calculations to predict the optimum laser pulse to control a given process are currently beyond the capabilities of AMo theorists for all but the simplest model systems.
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From page 108... ...
controlling Quantum world 08 the depend on how the molecules are aligned when they come together. Similarly, the interaction of light with molecules depends on the relative orientation of the mol ecules and the direction of the light's oscillating electric field.
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From page 109... ...
exPloring controlling inner workings molecule 0 and the of a information on the anisotropy of molecular interactions, yielding insight into how reactions actually occur. in addition, the ability to align single molecules will dramatically simplify the analysis of single-shot, and even single-molecule, structure determinations that may become feasible at new x-ray light sources like the Linear coherent Light Source, currently under construction.
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From page 110... ...
or in chemical physics, can we learn how to make organic solar cells for energy applica tions? How do x rays damage DNA, and can we learn how to use and control this process for applications in medicine?
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From page 111... ...
experiments can even be performed that determine the spin of the electrons. Synchrotron light sources such as the Advanced Light Source, the Advanced Photon Source, the National Synchrotron Light Source, and the Stanford Synchrotron Radiation Laboratory have enabled such experiments by providing intense sources of far-ultraviolet light and x rays, albeit with relatively long pulse durations.
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From page 112... ...
the separation of timescales for molecular vibration and electron motion re sults from the vastly different masses of the atomic nuclei and the electrons: the heavier nuclei generally move much more slowly than the lighter electrons. Shortly after the birth of quantum mechanics, this realization led to the development of the Born-oppenheimer approximation, in which it is assumed that the electron motion within a molecule can follow instantaneously any changes in the positions of the nuclei.
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From page 113... ...
exPloring controlling inner workings molecule and the of a FIGURE 5-3-1 Photoelectron angular distributions for fixed in-space carbon monoxide (CO) and nitrogen (N2)
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From page 114... ...
the ability to control the electronic wavefunction will allow scientists to drive molecules through such surface crossings along different trajectories and follow their outcome, providing the means to map out the detailed character of the potential energy surfaces and the electron motion near the crossing and to elucidate mechanisms for the nonadiabatic processes. in such experiments, the motion of the nuclei could be monitored using ultrafast, high-energy x rays, such as those that will be produced at XFeLs, discussed in chapter 4.
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From page 115... ...
target is bombarded by a fast ion, as well as heavyparticle diffraction images of molecules and thin solids using picosecond pulses of high-energy, heavy-particle beams. PROBING TIME-DEPENDENT MOLECULAR STRUCTURE WITH ELECTRONS imaging the nuclei in atoms and molecules with matter waves of subatomic particles has its roots in the earliest days of quantum mechanics.
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From page 116... ...
controlling Quantum world the FIGURE 5-5 Ultrafast electron diffraction images, after Fourier filtering high-frequency noise, showing the melting of aluminum, captured in several stages only a few picoseconds apart. Each line shows the diffraction data and the reconstructed image of the aluminum lattice at a particular time as the aluminum melts.
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From page 117... ...
exPloring controlling inner workings molecule and the of a recorded (see Figure 5-5)
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From page 118... ...
Because the recollision is nearly instantaneous -- the time interval between each ionization and recollision event is only a fraction of a cycle of visible light, or approximately 3 fs -- the electron diffracts from a relatively unchanged molecule, since the positions of the atomic nuclei cannot change appreciably on this timescale. can illuminating a molecule with its own electron be useful as a probe of the dynamics of the molecule itself?
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From page 119... ...
exPloring controlling inner workings molecule and the of a times approaching the timescales of electrons in individual molecules? or that designer molecules for health care will be created and reproduced using moleculemilling machines in which every atom is placed individually?
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