[Editors] MIT laser method unveils ultra-fast photochemical reactions

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT laser method unveils ultra-fast photochemical reactions
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For Immediate Release
MONDAY, OCT. 2, 2006
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

PHOTO AVAILABLE

CAMBRIDGE, Mass.--MIT researchers have made a fundamental advance in 
understanding how different environments affect chemical reactions by 
devising a novel way to observe ultra-fast photochemical reactions - 
reactions induced by a pulse of laser light - in crystals.

The new MIT experiments show that the reaction dynamics, including 
whether the product molecules remain or recombine to reform the 
original compound, depend with exquisite sensitivity on the local 
"cage" environment formed by neighboring molecules in the crystal. 
Cage effects of this sort play crucial roles in many natural and 
industrial chemical processes.

The method they have developed allows them to observe other 
light-induced changes in solids, including those used to burn CDs and 
DVDs. For some materials, these transitions may be reversible, 
allowing information to be both written and erased.

"This is a very active area of research for both fundamental and 
practical reasons," said Keith Nelson, MIT professor of chemistry and 
leader of the team. "What we're able to see, in a simple and direct 
way, is how different local environments around the reacting species 
lead to extremely different dynamics and different outcomes."

The work was published in the Aug. 31 online issue of Science. 
Nelson's co-author on the paper is Peter Poulin, a former graduate 
student in his lab.

In their experiments, the researchers studied one simple reaction in 
different crystalline environments.  When I3-, a chain of three 
iodine molecules with a negative charge, is struck with a pulse of 
ultraviolet light, the chain splits into two fragments - one of one 
iodine atom and one of two iodine atoms.  However, what happens to 
the products after the initial splitting is wholly dependent on the 
environment in which the reaction occurs, Nelson and Poulin found.

The researchers staged the reaction in three different crystals - one 
with a round, open cavity in which the separated products could move 
freely; another where the products were constrained to move within a 
two-dimensional plane; and another where the products could move in 
only one dimension, through a linear channel.

In all three crystals, a pulse of light splits the I3- molecule into 
two fragments almost instantly.  But the researchers focused their 
attention on what happens in the picoseconds (one-millionth of 
one-millionth of a second) after the initial reaction.

In the crystal with a round, open cavity, the two fragments remain 
separate, exactly as they would if the reaction occurred in a liquid 
environment.

"The separate fragments aren't really interacting with each other on 
a fast time scale," Nelson said.

In contrast, in the more constrained environments of the other two 
crystals, the two fragments spent some time apart, then abruptly 
reformed. That suggests that the fragments flew apart but then 
bounced off the crystal walls and reattached to each other, Nelson 
said.

"They split up, move apart, crash into the neighboring molecules that 
form their 'cages,' bounce back, recombine and it's all over," he 
said. "The entire 'dance' is almost perfectly synchronized among 
millions of molecules throughout the irradiated region of the 
crystal."

Conducting such experiments in a crystalline environment proved much 
more technically challenging than studying reactions in liquids, as 
is normally done. In liquids, researchers can measure what is 
happening by firing an initial "excitation" pulse that sets off the 
reaction, then a "probe" pulse that monitors progress at a particular 
delay time. The measurement is repeated many times with different 
probe delays to get data for each point in time. Reaction products 
can be conveniently replaced with fresh material in between 
repetitions of the measurement by flowing a stream of reactants in 
the liquid.

However, the experiments in a crystal cannot be repeated over and 
over because the reaction products accumulate and cannot be flowed 
away. In fact, after just a single laser shot, the irradiated region 
of a crystal was visibly discolored due to the presence of the 
products.

Instead of repeating the measurement many times, the researchers used 
only one excitation pulse, then 400 different probe pulses, all 
arriving with different delays. The probe pulses were formed from one 
pulse which was passed through a glass echelon (stairstep structure) 
so that different parts of the beam went through different 
thicknesses of glass and therefore were delayed by different amounts. 
That way, all of the necessary data could be gathered from a single 
measurement.

This allows the effects of the surroundings on reaction dynamics to 
be studied incisively, unlike in liquids where the reactants have 
widely varying local environments that give rise to very different 
dynamics.

"The effects we observed in the different crystals surely occur all 
the time in liquids and in partially ordered systems like biological 
media, but directly observing them and comparing them to simple 
models is normally impossible," according to Nelson.

"What we did is develop a way to get all of the time-dependent data 
in one shot of the laser," Nelson said. "The method allows us to 
study ultrafast chemical and structural change even in materials that 
are permanently altered or destroyed in the measurement. Materials 
subjected to high-pressure shock waves or other extreme conditions 
are also in our sights."

The research was partly funded by the Office of Naval Research.

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Written by Anne Trafton, MIT News Office