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March 24, 2004
Better Chemistry Through Femtosecond Lasers

A faster, cheaper, and more accurate way to analyze solids

In the quest to determine the chemical composition of solids with greater and greater accuracy, Berkeley Lab scientists are using extremely short laser bursts that span one-quadrillionth of a second.

These femtosecond-length laser bursts are used to zap a substance's surface and dislodge an aerosolized plume of particles that can be spectroscopically analyzed. The technique is much more sensitive than similar systems that rely on longer, nanosecond-length laser bursts, and may revolutionize scientists' ability to quickly and accurately analyze the chemical makeup of any solid—from nuclear material and hazardous waste to Martian rocks.

From left, Jhanis Gonzalez, Rick Russo, and Xianglei Mao with the mass spectrometer they use to analyze laser-ablated particles.

"It opens up the field of solid-sample chemical analysis," says Rick Russo of the Environmental Energy Technologies Division, whose research team became one of the first in the world to use femtosecond lasers to ablate samples for spectroscopic analysis three years ago.

Since then, Russo's team has proved the merits of the technique through research funded by two separate entities within the Department of Energy: the Office of Nonproliferation and National Security and the Office of Science, the latter through the Chemical Sciences Division of its Office of Basic Energy Sciences. Russo and his colleagues have shown that femtosecond lasers are better than nanosecond lasers when it comes to analyzing the isotopic ratios of the elements that compose glass, monazite, and zircon. And they've reported similar results with metal alloys.

Their success lies in the enormous benefits gained when switching from laser pulses that last several nanoseconds, or one billionth of a second, to several femtoseconds, which at one millionth of a nanosecond is one of the fastest manmade events. Both of these laser pulses can ablate tiny regions of a solid's surface into an aerosolized explosion, ready for analysis. But only a femtosecond laser can ablate a surface while barely heating it, thanks to the fact that nothing in nature occurs as quickly as a femtosecond, not even the movement of atoms.

In other words, a femtosecond laser pulse is there and gone before material has a chance to thermally react. This advantage sidesteps a phenomenon that has stymied laser-ablation-based chemical analysis for years: elements vaporize at different rates based on their unique thermal properties. Ablate brass with a nanosecond laser, which lasts long enough to heat the compound, and zinc may vaporize while the less volatile copper remains in the sample.

"This fractionation means you're not getting everything into the aerosol that's in the material," says Russo. "You're getting things based on thermal properties, which could leave out many elements and result in a misleading analysis."

Zapping a surface with a laser creates a plume of particles that can be analyzed spectroscopically.

But use a laser that's quicker than nature can react, and every element in a substance is aerosolized regardless of its thermal properties. This ability to only nominally heat a surface has helped femtosecond lasers make inroads into such delicate applications as the manufacture of nanomaterials, thin films, and micro-electromechanical systems. And now, thanks to Russo's team, the lasers are poised to change the way solids are analyzed.

"At the nanosecond scale, which is the current state of laser-ablation chemical analysis, there is a tremendous amount of fractionation," Russo says. "But a femtosecond laser explodes a sample into the vapor phase without any fractionation."

Based on its potential, Russo believes the technique could give geologists a better way to analyze the isotopic ratios of sedimentary layers to determine their age. It can also analyze soil samples to map how far a toxic plume has spread from a contamination site. To improve national security, it can be used to test suspicious substances for the presence of fissile material such as plutonium. And a variation of the technique could be used in the field to search for chemical leaks in industrial plants or to analyze Martian rocks.

Femtosecond lasers boast other advantages. In addition to barely heating a surface, they produce an aerosol composed of a much finer particle-size distribution than a longer duration laser. A nanosecond laser produces particles that range from five nanometers to five microns in diameter, which is too diverse for most spectroscopic devices. A femtosecond laser, on the other hand, yields particles that average approximately 200 nanometers in diameter, which makes spectroscopic analysis much more precise.

If it's in the substance, it's in the plume, thanks to a femtosecond laser's ability to ablate a surface without heating it.

These advantages underscore how far lasers have advanced since the early 1980s, when researchers had largely given up on their use in chemical analysis. Back then—and until a few years ago—longer duration laser pulses thermally damaged samples, or yielded a hodgepodge of particle sizes unsuitable for analysis. Because of these drawbacks, scientists have been forced to use more time-consuming chemical analysis processes that involve sample preparation, acid-fusion microwave dissolution, and chemical separations, among other steps.

"Now, with a femtosecond laser, we can take any solid sample, hit it with a beam, and there's the answer," Russo says. "People thought lasers weren't going to work for chemical analysis, but they have progressed tremendously in the past several years."

Other members of Russo's research team include Jhanis Gonzalez, Chunyi Liu, Samuel Mao, Xianglei Mao, Sy-Bor Wen, Jong Yoo, and Xianzhong Zeng.

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