First Pump-Probe Experiment at Linac Coherent Light Source Completed

(PhysOrg.com) -- The first experiment using the Linac Coherent Light Source to illuminate molecules via a "pump-probe" technique has been completed by an international team of more than 30 scientists from institutions including Lawrence Berkeley National Laboratory, LCLS and the joint SLAC/Stanford PULSE Institute. Ryan Coffee, physicist with the LCLS Laser Group, presented initial results in a seminar at SLAC on Wednesday, November 18.

Pump-probe experiments use one laser pulse, in this case an infrared pulse, to pump energy into a sample and then probe it with another laser pulse, in this case an LCLS X-ray pulse. Such experiments are ideal for looking at atomic and molecular interactions, which take place in tiny fractions of a second. The LCLS probe pulses were as short as a few quadrillionths of a second and a billion times brighter than any X-ray source produced in a laboratory.
Coffee and his colleagues looked at the quantum behavior of electrons in nitrogen molecules, N2. The results represent a step toward a fundamental understanding of how nature converts light into chemical energy and might one day help revolutionize solar power, Coffee said.
Nitrogen atoms distribute their electrons between a lower and a higher energy shell. Using X-rays, the team picked off two electrons from the lower level, allowing a higher shell electron to descend and fill the vacancy. The energy released during this downward plunge ejected another electron from the atom, a phenomenon known as the Auger effect.
The team wanted to study how the nitrogen molecules' orientation affected this reaction. To do this, they used the infrared laser to line up the nitrogen molecules so that they were all facing the same direction.
"In a sense, we tried to make the gas act a little bit like a crystal," Coffee said.
After hitting the nitrogen with X-rays, the researchers detected electrons flying off and measured how the molecules' alignment with respect to the X-rays influenced the Auger effect. They observed numerous features that had strong dependence on the molecules’ direction. The results are currently being prepared for publication.
Future work will focus on how atomic bonds change as molecules either break apart or rearrange. Coffee thinks such work will lead to a deeper understanding of how nature converts light into energy. Ultimately, he hopes the results will lead to technology that will help humans generate power from the sun.
"I'm going for the solar power revolution, though I don't know where it will come from," he said. His gut feeling is that the important atoms to look at are carbon, nitrogen and oxygen.
"That's where energy in nature comes from," he said.
Coffee added that the team owes a debt of gratitude to the LCLS Controls, Accelerator, and Laser Groups, who made the experiment's success possible.

Attosecond angular streaking

Petrissa Eckle, Mathias Smolarski, Philip Schlup, Jens Biegert, André Staudte, Markus Schöffler, Harm G. Muller, Reinhard Dörner & Ursula Keller(Department Physik, ETH Zurich, Wolfgang-Pauli-Str. 16, 8093 Zurich, Switzerland)

Ultrashort measurement-time resolution is traditionally obtained in pump–probe experiments, for which two ultrashort light pulses are required; the time resolution is then determined by the pulse duration. But although pulses of subfemtosecond duration are available, so far the energy of these pulses is too low to fully implement the traditional pump–probe technique. Here, we demonstrate 'attosecond angular streaking', an alternative approach to achieving attosecond time resolution. The method uses the rotating electric-field vector of an intense circularly polarized pulse to deflect photo-ionized electrons in the radial spatial direction; the instant of ionization is then mapped to the final angle of the momentum vector in the polarization plane. We resolved subcycle dynamics in tunnelling ionization by the streaking field alone and demonstrate a temporal localization accuracy of 24 as r.m.s. and an estimated resolution of approximately 200 as. The demonstrated accuracy should enable the study of one of the fundamental aspects of quantum physics: the process of tunnelling of an electron through an energetically forbidden region.

Nature Physics
Published online: 30 May 2008 | doi:10.1038/nphys982

Faster than a Speeding Bubble

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.

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."

Source: by Brad Plummer, SLAC Today