What the %$#@ is a Femtosecond Spectrometer…

If you’ve read the Femtochemistry post on this blog, you may be curious as to how a femtosecond laser works and what femtosecond spectroscopy has taught us.

Femtosecond Laser:

Let’s start with the laser, the proverbial hammer in any practicing femtochemist’s tool belt. The first question you might ask is why use a laser in the first place? We already know that whatever is ultimately used needs to pulse at the rate of femtoseconds. So why not use electronic circuits (in which a pulse is easy to induce with the help of a few switches)? The reality is that even by using the shortest wires and the best semiconducting materials, circuits can only produce pulses at a rate of billionths of a second, not the necessary quadrillionths. In order to reach the femtosecond time-scale, scientists had to exploit something even faster: light, or more specifically, the laser.

However, one problem with this medium is that light is being emitted constantly, meaning the laser is either on or off. To capture a “movie” of an ultrafast chemical reaction, scientists needed a way to rapidly stutter the light, similar to a strobe or a series of frames in a film. This was acheived by a revolutionary new technique pioneered at Bell Laboratories called mode-locking.

In order to emit “chunks” or “pulses” of light, the femtosecond laser uses a slightly counter-intuitive method. It produces light waves with up to 10,000 different frequencies that are evenly spread apart. The arrangement of the waves is such that 99.999… percent of the time there is destructive interference, meaning that the peaks and valleys of the various waves cancel each other out. However, for the infinitesimally small amount of time the waves are in phase, a small packet of coherent* light is generated.  Since the waves are evenly spaced, the laser releases these small “packets” at a constant rate.

*One of the defining characteristics of a traditional laser is that it is coherent, meaning that all of the light waves have the same wavelength and oscillate in unison. This property allows for the formation of a precise and concentrated beam (like the ones in the movies).

Synthesis of a periodic pulse train (red curve) by adding seven oscillations with slightly different frequencies (blue curves). The vertical lines indicate points in time where all the oscillations add up in phase (Courtesy of Dr. Rudiger Paschotta and the Encyclopedia of Laser Physics and Technology)

In femtochemistry experiments, the laser actually emits two different types of pulses. The first is called the “pump pulse” because it initiates the reaction (as noted in the earlier post, light has this ability by exciting the molecules to a higher energy state). The second, weaker type of pulse is called the “probe pulse”, which illuminates the reaction at different time intervals so that it can be observed. If need be, the probe pulses can be offset by bouncing them in between mirrors and through prisms, effectively lengthening their path to the reaction. This allows scientists to observe a chemical reaction at various points along its lifetime. For example, if they wanted to see the moment right before two atoms split apart, the probe pulse would have to be delayed for several hundred femtoseconds to allow the reaction to get to that stage.

Because it is impossible to take a traditional picture of a reaction at a certain point in time, scientists rely on colors in order to see what is going on. In nature, different atoms and molecules absorb and emit different wavelengths (colors) of light. Because these unique wavelengths are dependent on the exact characteristics of the different molecules, when a molecule is changing or is disturbed in any way throughout the reaction, it will have a slightly different wavelength signature. Thus, the molecules can effectively communicate to scientists what they are and what state they are in by emitting very particular colors of light. Of course, this is also dependent on the scientists attuning the pump and probe pulses to the particular wavelengths that will elicit a colorful response.

One of the most accessible examples of a commercial application for these lasers is their implementation in performing delicate surgeries in ophthalmology. “No-blade” Lasik procedures are faster, better, and do less damage than their manual counterparts. Cataract surgery is also more precise and simple thanks to this technology (here’s an informative video).

Chemicals in the different minerals emit visible light when subjected to UV in a phenomenon called fluorescence (Courtesy of Hannes Grobe)

Discoveries in Femtosecond Spectroscopy:

The first pioneer of femtosecond spectroscopy was Egyptian-born physical chemist Ahmed Zewail, who won a Nobel Prize in 1999. One of Zewail’s most important discoveries centered on the dissociation of sodium iodide into its composite elements of sodium and iodine (on their own, dangerous elements). It turns out that this reaction (and others like it) behaves a lot like your typical failed relationship. Not surprising when we use words like “bonds” between atoms.

When the reaction is captured with a femtosecond laser, we first see that the molecules are splitting up as they should be. However, 10 femtoseconds later, they’re reconciling back together. Another ten and they’re drifting back apart. This happens several more times (12 total according to Zewail) like two balls on the end of a spring, until the two molecules finally part ways for good.

When Zewail and his team tested a sample of sodium iodide in their femtosecond spectrometer, they found that the distance between the two nuclei in the excited molecules is about 2.8 Angstroms (ten-billionth of a meter) and that the atoms exhibit covalent bonding. However, as the molecules vibrate, the nuclei can get as far as 15 Angstroms apart, causing an electron to be transferred from the sodium atom to the iodine atom and the molecules become ionic!

"Spring" model of Sodium Iodide Dissociation reaction

Zewail noticed that during this periodic cycle, there is a critical point when the distance between the nuclei is 6.9 Angstroms. At this configuration, there is a (relatively) high probability that the excited molecules will complete the reaction by dissociating. This probability is 0.1, meaning that approximately one tenth of the time Zewail saw the release of free sodium and iodine atoms (still better than break-up rates in the US).

This early experiment also exhibits a crucial aspect of femtochemistry, namely, the importance of coherency. Because the starting pump pulse that initiated the reaction was composed of coherent laser light, it was able to synchronize all of the molecules in the experiment. This allowed Zewail and his team to accurately track the different stages of the reaction and helped them to notice the periodic behavior the sodium iodide molecules exhibited. Think of it like trying to track the path of a single strand of confetti amongst millions as they fall from the ceiling to the floor. It’s virtually impossible unless all of the confetti strands are falling in exactly the same way at exactly the same time.

Dr. Zewail is the Linus Pauling Chair Professor of Chemistry at CalTech and serves on President Obama's Council of Advisors on Science and Technology

Throughout the 1990s, Zewail and his colleagues studied more than 50 molecular reactions across the five laboratories of “Femtoland”, his Southern California lab. Among other things, they examined the mechanics of electron transfer and photoisomerisation. These landmark experiments led to an explosion of research in the field that has been facilitated in part by the introduction of relatively affordable lasers (such as the Titanium-sapphire femtosecond laser we have here at our Lab (100-300K)). Another breakthrough came in the form of cheap (less than 100K) fiber lasers that use optical fibers as their gain medium (the power amplifier of the light beam). These fibers can be further doped with rare earth ions to increase the laser’s gain bandwith (frequency range). Thus, fiber lasers have the capability of being mode-locked to produce femtosecond pulses.

To read more about Ahmed Zewail and his experiments, you can visit the Nobel Prize Committee’s page here. You can also visit Dr. Zewail’s very informative personal website here.

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One Comment

  1. Posted January 11, 2014 at 7:33 am | Permalink

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