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Ultrafast Heating

February 14, 2014

As Lady Macbeth so famously said in scene VII of MacBeth by William Shakespeare, "If it were done when 'tis done, then 'twere well It were done quickly."[1] The idea that speed is beneficial to some processes is at the core of such important matters as pizza delivery and adhesive bandage removal.

Figure caption"Macbeth and Banquo encountering the witches," from Holinshed's Chronicles (1587), a source book used by Shakespeare in writing MacBeth.

(Via Wikimedia Commons.)

In materials science, rapid thermal processing, also called rapid thermal annealing, is a method of quickly heating an object to cause changes to its surface properties. It's effected by either passing the object, such as a semiconductor wafer, quickly through a heated zone; or, by application of an intense burst of radiation from high intensity lamps or lasers.

Rapid thermal annealing is commonly used to thermally diffuse material from a thin surface overlayer into wafers and other objects, to repair ion implantation damage, and for densification of deposited surface features, such as electrical conductors. It's used also for making ohmic contact to semiconductor wafers.

Application of considerable energy in a short interval is required for such processing, so lasers have proven ideal for rapid thermal processing. Even lasers have their limitations, since you can pack only so much power into the very short pulses from femtosecond lasers. Unless you're working with chirped-pulse amplifiers, or the building-sized lasers of the National Ignition Facility, you need to limit the size of your heated object if you want to generate large temperature changes in a short time.

A team of German physicists from Hamburg, Germany, at the Center for Free-Electron Laser Science at the Deutsches Elektronen-Synchrotron (DESY), and the University of Hamburg, have done simulations on how water will react to ultrafast heating.[2-3] As you realize by looking through a thick layer of water to the bottom of your cup, water doesn't absorb electromagnetic radiation at all wavelengths. The German team looked at terahertz radiation which is readily absorbed by water.

DESY logo on its entrance signDeutsches Elektronen-Synchrotron (DESY) logo on its entrance sign.

(From a photograph by Uvainio, via Wikimedia Commons.)

The computer simulations were performed at the Forschungszentrum Jülich, and the computations required 200,000 hours of processor time on a massively parallel computer.[DESY] The goal of the simulations was the heating of a small volume of water (a nanoliter) by 600 degrees Celsius in half a picosecond.[3] An intense terahertz pulse can accomplish this.[2] The short time is critical, since the rotational correlation time of water, the time it takes for a molecule to rotate a radian (about 57.3 degrees), is 1.7 picoseconds.[4]

Although these are just simulations, it would be possible to do such an ultrafast heating on a nanoliter volume through use of a free-electron laser.[3] Under such an excitation, the water molecules would remain in place for almost a millisecond, although the molecular vibration is equivalent to 600°C. The ensemble of molecules would be a structureless, gas-like system at the density of liquid water, but with the hydrogen-bonding removed.[2-3]

Water molecules heated by a terahertz flash.Artist's rendering of water molecules heated by a terahertz flash to 600 degrees centigrade.

The excited water molecules are in essentially the same place as they were in the liquid.

(Deutsches Elektronen-Synchrotron image by Oriol Vendrell.)

Since water is a solvent for so many chemical reactions, the capability for such an ultrafast heating leads to quite a few research possibilities. Says DESY scientist, Oriol Vendrell,
"Water is not just a passive solvent, but plays an important role in the dynamics of biological and chemical processes by stabilizing certain chemical compounds and enabling specific reactions."[3]
The trick, of course, is being able to extract data from your experiment in such a short time frame. This would be possible using ultrashort X-ray flashes from a 3.4-kilometer-long X-ray free-electron laser (XFEL) being built at DESY campus. This XFEL, synchronized with the terahertz burst, would generate 27,000 intense X-ray laser flashes per second, so it can record the chemical reactions as they happen.[3]

As for the adage that "a watched pot never boils," an eye blink is about 350 milliseconds. You can boil a lot of water with ultrashort terahertz radiation in that time.

References:

  1. The Tragedy of Macbeth by William Shakespeare, free versions at Project Gutenberg.
  2. Pankaj Kr. Mishra, Oriol Vendrell and Robin Santra, "Ultrafast Energy Transfer to Liquid Water by Sub-Picosecond High- Intensity Terahertz Pulses: An Ab Initio Molecular Dynamics Study," Angewandte Chemie - International Edition, vol. 52, no. 51 (December 16, 2013), pp. 13685-13687.
  3. Ultrafast heating of water - This pot boils faster than you can watch it, Deutsches Elektronen-Synchrotron/Helmholtz Association Press Release, December 16, 2013.
  4. D. Lankhorst, J. Schriever and J. C. Leyte, "Determination of the Rotational Correlation Time of Water by Proton NMR Relaxation in H217O and Some Related Results," Berichte der Bunsengesellschaft für physikalische Chemie, vol. 86, no. 3 (March, 1982), pp. 215-221.