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Very Fast Atomic Scale Processes Using Ultra Short Pulses of X-Rays

What do melting chocolate and bubbles in a champagne glass have in common? Besides being treats one might sample at a sophisticated soiree, they are both handy examples of first-order phase transitions in which a material transforms from one phase to another—that is, atoms changing from an orderly arrangement into a more chaotic arrangement.

Now, in an experiment led by Aaron Lindenberg, an international collaboration of scientists has uncovered new clues about the first instants of that process. The results are published in the April 4 edition of Physical Review Letters.

"We did not at all expect to see what we saw," said Lindenberg, "although in the aftermath we can go back and realize perhaps we should have. What's amazing about the process is that it spans such a huge range of time scales."

The process of melting, or in the case of champagne, of bubbling, has long been of interest to scientists. Phase transitions take place in the tiniest fraction of a second. In the case of Indium Antimonide (InSb), a semiconductor used by scientists to study such processes, the first steps in melting take a few hundred femtoseconds, a quadrillionth of a second. But no one knew what happened after that.

In the current study, the group used a laser to excite the sample and then measured the structure of the disordered liquid using X-rays, a technique called "pump-probe." Critical to the experiment is timing the initial laser used to pump the sample with energy, and the X-ray beam used to probe the results, to within mere femtoseconds. The resulting diffuse pattern of scattered X-rays from the disordered sample is used to map out where the atoms are at a given instant. Subsequent repeats of the pumping and probing at different relative delays between the laser and X-ray beam enables the researchers to reconstruct how the material evolves over time.

Lindenberg and colleagues found that the structure of the disordered liquid was far different from what one would have expected. Tiny atomic-scale bubbles, called nucleation events, form first and seed the process, a unique transient state of matter in which large fluctuations dominate the response of the material.

The group captured the process on a timescale 100 times shorter than any other previous X-ray study. The results give scientists a deeper understanding of how disordered materials behave on short timescales, and could lead to improved materials processing techniques, such as electronics manufacturing.

The current study also represents the last scientific paper to come from SLAC's Sub-Picosecond Pulse Source (SPPS) collaboration, led by Jerry Hastings, which was undertaken to study very fast atomic scale processes using ultra short pulses of X-rays. The work at SPPS presages the science to come from SLAC's Linac Coherent Light Source (LCLS), now under construction, which will create coherent X-ray laser pulses that are even shorter.

"SPPS was a remarkable success," said SSRL Director Jo Stohr. "It was great to see prominent X-ray scientists from all over the world coming to SLAC to participate in this unique experiment. It is an indication of what is yet to come with LCLS."

Note for Indium Antimonide
Indium antimonide (InSb) is a narrow gap semiconductor material from the III-V group used in infrared detectors, including thermal imaging cameras, FLIR systems, infrared homing missile guidance systems, and in infrared astronomy. The indium antimonide detectors are sensitive between 1-5 µm wavelengths.

Indium antimonide was a very common detector in the old, single-detector mechanically scanned thermal imaging systems.

Indium antimonide is a crystalline compound made from the elements indium and antimony. It has the appearance of dark grey silvery metal pieces or powder with vitreous lustre. When subjected to temperatures over 500 °C, it melts and decomposes, liberating antimony and antimony oxide vapors.

Indium antimonide photodiode detectors are photovoltaic, generating electric current when subjected to infrared radiation. InSb has high quantum efficiency (80-90%). Its drawback is a high instability over time; the detector characteristics tend to drift over time, and between cooldowns, requiring periodical recalibrations, increasing the complexity of the imaging system. Due to their instability, InSb detectors are rarely used in metrology applications. This added complexity is worthwhile where extreme sensitivity is required, e.g. in long-range military thermal imaging systems. InSb detectors also require cooling, as they have to operate at cryogenic temperatures (typically 80 K). However, large arrays (up to 1024x1024 pixels) are available. HgCdTe and PtSi are materials with similar use.

A layer of indium antimonide sandwiched between layers of aluminium indium antimonide can act as a quantum well. This approach is studied in order to construct very fast transistors. Bipolar transistors operating at frequencies up to 85 GHz were constructed from indium antimonide in the late 1990s. Field effect transistors operating at over 200 GHz have been reported more recently (Intel/QinetiQ). Some models suggest terahertz frequencies are achievable with this material. Indium antimonide semiconductors are also capable of operating with voltages under 0.5 V, reducing their power requirements.

Note for Linac Coherent Light Source
LCLS, a free-electron x-ray laser, will use the last kilometer of the three kilometer SLAC linear accelerator; the world's longest & highest energy electron linac.

Short, intense bunches of electrons are injected into the linac. As the linac accelerates the bunches they pass through bunch compressors which pack them into even shorter bunches. The electrons are then excited (agitated) as they pass through an undulator magnet. Once accelerated & compressed these electron bunches pass through a long undulator magnet, where they emit radiation (x-rays) as they oscillate in the alternating magnetic field. This magnet trick is basically how undulators in the 50-odd synchrotron light sources around the world operate.

One key difference between current synchrotron sources and LCLS is that the x-rays will be emitted coherently and at the same wavelength—the essential properties of a laser. Coherent means that all the x-ray photons are in phase with each other and going in the same direction, like skiers making simultaneous, in-sync S-turns down a mountain slope. [Conventional lasers excite electrons that are bound to atoms within the lasing cavity. LCLS is a “free electron” laser because the electrons are independent from atoms: they fly down the linac unchaperoned.]

LCLS photons will be emitted between 1.5 and 15 Å corresponding to 8keV to 800eV, much shorter wavelength and much higher energy than visible light. In particular the short wavelength end, 1.5 Å, is ideal for studies on the atomic scale, where dimensions are of this order. There are 10 billion Ångströms in one meter.

To make the x-ray beam tremendously bright, LCLS will use a new technique called Self-Amplified Spontaneous Emission. SASE takes advantage of the interaction of an internal electron bunch with the spontaneously emitted x-rays travelling along with this bunch as it traverses the periodic magnetic field of the undulator. This interaction results in a microbunching of the electron beam which greatly amplifies the number of emitted x-rays. This means one bunch of 6 billion electrons can generate a pulse with one trillion coherent x-rays. More x-rays results in sharper pictures of smaller things.

This powerful combination—laser-like x-ray beams with extreme brightness (a trillion x-rays in a needle-thin beam), with short wavelength (on the scale of atoms) and short pulse duration (1 to 230 femtoseconds)—makes LCLS a revolutionary machine.

In figure, X-ray scattering images (above) and corresponding 3D depictions (below) of nucleation events, or "bubbles," forming in the semiconductor Indium Antimonide in the first instances after being hit with a laser pulse.