Femtochemistry

Even if you don’t know the story or significance behind this simple animation, you have probably seen it before. The legend goes that during the 1870′s, English photographer Eadward Muybridge was commissioned by Leland Stanford, a former governor of California and horse enthusiast, to prove that in the course of a horse’s gallop there is a moment at which the animal is completely airborne. Little did Muybridge know that his ”proof” would become the most famous 3-second movie in history.

Of course, Muybridge’s movie isn’t the quality of your typical Hollywood fare. His primitive motion picture device (the Zoopraxiscope) ran at 8 frames per second (FPS) whereas most movies nowadays are screened at 24 to 30 FPS. Even better are top of the line high definition (HD) televisions that can show about 60 FPS and emerging “ultra” HDTV’s that are capable of up to 120 FPS.

However, typical audiences don’t need or really even want a life-like viewing experience when they tune into their weekly sitcoms. Nevertheless, the question arises, how “life-like” are we physically capable of achieving and what are the ramifications of ultra-fast motion capture? The astonishing answer is that scientists are now able to record movement down to the scale at which molecules vibrate and even faster than the speed of light.

First, let’s get a handle on how fast a camera would actually have to be in order to register molecular vibrations.

It can be argued that our physical world is defined by chemical reactions. After all, the act of molecules smashing into each other is what determines everything that goes on inside our bodies as well as all of our interactions with the outside world. Nevertheless, for years, actually observing molecules in the process of reacting was at best a chemist’s pipe dream. To explain why, we must go back to one of the first precepts of molecular motion. Namely, as you increase temperature, the movement of a molecule becomes more frenetic. In fact, reactions cannot occur until a molecule is imbued with just the right amount of energy (high temperature) to ”kick” it over its activation barrier. This is similar to a rocket having to overcome the force of gravity. The experimental problem is that a chemical reaction has so much built up energy that it occurs in a matter of femtoseconds*, or a quadrillionth (0.000000000000001) of a second.

[*To truly wrap your brain around how short a femtosecond lasts is impossible. According to the Committee for the Nobel Prize, in one second light travels from the earth to the moon while in one femtosecond it travels a fraction of the width of a human hair. Alternatively, the ratio between a femtosecond and a second is equivalent to the ratio between a second and about 32 million years.]

The first person to formalize this idea of the rate of a chemical reaction was Swedish physical chemist Svante Arrhenius who won the Nobel Prize in Chemistry in 1903. He derived mathematically that a high activation barrier can be overcome by high temperatures, resulting in a very fast chemical reaction. From then on, it was a race to see who could get the most information out of the smallest slices of time.

In 1923, a process for observing reactions in intervals of a thousandth of a second was developed by mixing two solutions from separate tubes and forcing the resultant mixture through an outlet tube of glass at high velocity. Then, in 1967, three scientists were awarded the Nobel Prize after achieving a resolution of microseconds using flash lamps as well as electrical, pressure, and heat shocks. This was based on the emergent field of photochemistry in which light was found to be able to initiate chemical reactions. The Swinging Sixties also saw the birth of the laser to replace the flash lamp, finally pushing the veil past the nano and picosecond time-scales.

One of the most significant discoveries throughout this process was that of “intermediate” substances between the original reactants and the final products of the reaction. With each successive refinement in resolution, these intermediates would become increasingly unstable and ephemeral. It is important to note that this “transition-state theory” was hypothesized as early as the 1930’s, but nobody would ever have thought that actually seeing the intermediate molecules could be possible. You can look at transition states like a slide on the playground. Molecules are activated, causing them to occupy a higher “transition” state (the top of the slide). Once over this crest, the molecule proceeds to fall to the lowest possible energy state (the bottom of the slide).

It wasn’t until Ahmed Zewail in the late 20th century that femtochemistry was born, culminating a century long voyeuristic desire to catch molecules “in the act”. Since then, we have been able to penetrate the strange world of molecular “slow motion”, from observing how electrons are transferred between ions to how plants process photons into usable energy.Femtochemistry promises to lead us into the future with improved materials to manufacture tomorrow’s electronics, new medical procedures to diagnose cancer and other diseases, and even an artificial photosynthesis. It may be that humans are close to unlocking nature’s deepest secrets.

For more, I recommend watching this TED talk by MIT’s Ramesh Raskar talking about commercial uses for femtosecond “cameras”. You can also read a very comprehensive review of the field of femtochemistry by Dr. Zewail himself.

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One Comment

  1. Posted February 11, 2013 at 6:42 am | Permalink

    Excellent post. I was checking continuously this blog and I am impressed!

    Very useful information specifically the last part :) I care for such information much.

    I was seeking this particular info for a long time. Thank you and good
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