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What do Schroedinger's equation and Schoenberg's expressionism have in common? Not a lot you might think. However, researchers in Germany and the US have now modelled the hydrogen molecule, the archetypal subject of molecular modelling, using a theory of behaviour that emerges from music. The study demonstrates how a hydrogen molecule responds to laser pulses as if the molecule's vibrational motions, its quantum states, were the notes making up a changing musical chord and offers the opportunity of laser-controlled chemical reactions. Physicist Uwe Thumm and colleagues at Kansas State University working with Bernold Feuerstein and his team at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, are studying the properties of atoms and small molecules and hoping to find novel insights using laser light. In contrast with particular accelerators, laser pulses are more controllable allowing researchers to routinely observe the motion of nuclei within small molecules. Moreover, the peak intensity of a laser pulse can be very high and yet highly focused. The researchers modelled laser activation of dihydrogen and did some calculations to determine how laser pulses might influence the movements of the two protons in the hydrogen molecule. "The short answer is that the laser pulse either makes the molecules vibrate more violently or blows them apart, unsurprisingly" Thumm says. However, this simplistic image of a dihydrogen molecule behaving as two balls connected by a spring while easy to envisage, it is not at all accurate, because the particles involved follow the rules of quantum mechanics not classical mechanics. Thumm provides a metaphor for the behaviour of oscillating protons. Picture a marble dropped into a bathtub of dirty water, he says. The first circlular ripples would reveal where the marble had entered the water. However, once those ripples hit the sides of the bathtub, bounce back and interfere with each other, the pattern of ripples changes so that it would become almost impossible to pinpoint the initial entry point. The wave is delocalized, in other words. Similarly, the quantum ripples due to a pair of protons in dihydrogen dowsed with laser later for a few femtoseconds become so entangled that it is impossible to locate the protons with any kind of precision. "You quickly lose track of what the distance between the two protons is," Thumm explains, "All you can say is that they have a certain likelihood of being at a certain distance. This is in agreement with the marble experiment: Seconds after the marble was dropped, you cannot tell where exactly it plunged in." However, the marble and bathtub metaphor stretches only so far and behaviour at the quantum level is always ready to spring a surprise. While, the wave pattern is delocalized at 60 femtoseconds following the laser blast, after about 600 femtoseconds the distance between the protons again becomes well defined. "We call this a revival of the original motion of the protons," Thumm said, "It?s not going to happen in the bathtub, but it happens at the quantum level." The next step was to analyse the molecule's vibrational motion by breaking it down into its various frequencies. Each frequency being equivalent to a note in a chord. By scaling down the frequencies, the team were able to extend the metaphor and make the molecular oscillations audible. "This way you can listen to the vibrations and hear the revival. In the same way sound is analysed and decomposed, we decomposed the vibration with regard to the frequencies," Thumm explains. The researchers will next attempt to listen into to molecules a little more complex than hydrogen, such as water and methane. Each of these will produce their own unique "sound", the researchers suggest. Whether or not anything more complicated such as a polymer or protein would ever be accessible is a different matter, the same might be said in some cultural circles of both Schroedinger and Schoenberg. Related links:
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