For the Sake of Science: Kicking Around a Nano-Sized Ball
John Morgan, Science Writer, UW Madison Synchrotron Radiation Center
January 18, 2006
There's a bit of good-natured ribbing going on at the UW Madison Synchrotron Radiation Center (http://www.src.wisc.edu), where researchers are hard at work to better understand Bucky Balls, the nickname for the famous molecule, Carbon-60 or C60. The ribbing comes from colleagues who ask, "why study a nano-sized ball?"
The answer is simple, say researchers Ralf Wehlitz and Pavle Juranic, who know that the knowledge needed to allow for important future uses of this famous molecule is hidden in basic science that not only hasn't been well defined, but has been blatantly overlooked. And their findings reported in the current issue of Physical Review Letters reveal the previously uncharted territory of the double-photoionization effect of C60.
"To put it simply, nobody has done it quite as accurately as we have," says Juranic, a graduate student researcher at SRC. "We just did this for five months (during the last year) and got very, very precise data down to such precision that the error bars are almost smaller than the points if you try to draw them in."
The photoionization effect is an offshoot of the photoelectric effect first described by Einstein, which explains what happens to the electrons of a material when it absorbs energy--like an atomic cakewalk, they are displaced or emitted. In the case of the current study, C60 was exposed to soft X-rays and the authors observed a particularly interesting and previously unreported modulation in the double-to-single photoionization ratio. This ratio essentially measures the strength of interaction between two electrons inside an atom or molecule.
"One idea why we have a modulation is that one electron is created on one side of the ball and is flying through the middle of the ball and kicks out the other electron on the other side," explains Wehlitz, a senior scientist at the SRC and an expert in atomic and molecular spectroscopy. "It's also possible that it is reflected on the other side--an electron bounces against it and goes turning around and then hits the second electron. So something can happen before both electrons are ejected."
Of note, conclude Wehlitz and Juranic, are the future possibilities for C60 and molecules like it. Yet it's this elementary knowledge that will enable future application of this molecule--perhaps enabling the super-small widgets of the future such as quantum computers or even as a nano-lubricant.
Using synchrotron radiation, "the photoionization has not been studied very carefully for C60, or much at all," concludes Juranic. "Which really could be important, because you want to know how much you can do with it."
For the figure above the lower panel shows the double-to-single photoionization ratio of C60 (black circles) and a smooth curve (blue) through the data. The abscissa is the inverse of the square root of the excess energy; the excess energy is the photon energy (axis on top of the figure) minus the double-ionization threshold (=19eV). The upper panel shows the log of the data-curve ratio of the lower panel. The "humps" are at certain energies, namely whenever the corresponding de Broglie wavelength lambda matches a geometrical distance of the bucky ball such as the diameter (D) of the ball, the diameter of a hexagon (H), and the distance between two neighboring atoms (C).
P. N. Juranic, D. Lukic, K. Barger, and R. Wehlitz:
Experimental Evidence for Modulations in the Relative Double-Photoionization
Cross-Section of C60 from Threshold up to 280 eV
Phys. Rev. Lett. 96, 023001 (2006)
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