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  • Bull's Eye! | JILA-PFC
    chemistry would render theoretical chemical physics much less interesting As it is theorists spend months determining the particular molecular shapes vibrations and energy states that make the simplest chemical reactions possible Recently Deskevich worked on a tricky theoretical problem with Fellow David Nesbitt Michael Hayes a graduate student in chemistry Kaito Takahashi a postdoc in Taiwan and Professor Rex Skodje in CU s Department of Chemistry and Biochemistry The researchers created the potential energy surface or PES right which shows in theory how the potential energy of a reaction changes with the position of the reactants and products A PES contains thousands of points created via a detailed quantum mechanical analysis of the energy states and atomic and molecular orientations that play a role in the reaction The process of creating PES takes hundreds of hours but it s time well spent How a molecule travels on a good PES reflects its real life behavior The researchers used the new PES to monitor the transfer of a hydrogen atom from a chlorine atom to a fluorine atom which is a very simple chemical reaction In chemistry shorthand the reaction looks like this HCl F HF Cl What they found was that it doesn t take a lot of energy to make the reaction happen if the reactants collide in a very specific orientation Energetically the reactants have to climb up a hill and go around a corner for a reaction to take place For that to happen the fluorine atom must collide with the hydrogen end of the HCl molecule such that the two reactants form a transitory F H Cl molecule which is bent at the H forming an angle of exactly 123 5 To form the perfect angle the F has to hit the HCl at a slightly

    Original URL path: http://jila-pfc.colorado.edu/highlights/bulls-eye (2016-04-29)
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  • Spectral Shapes | JILA-PFC
    reactive but short lived free radicals produced during the breakdown process The molecules which contain either fluorine or chlorine are an important source of atmospheric halogen atoms Elucidating their 3D structure and dynamical behavior will help scientists better understand atmospheric chemistry as well as their fundamental molecular properties Using slit jet supersonic expansion and high resolution infrared spectroscopy JILA researchers have determined key vibration modes of both the chloromethyl and fluoromethyl radicals The latter is shown below the fluorine atom is teal the carbon atom dark grey and two hydrogen atoms light grey Erin Whitney who earned her Ph D in January and is now at the National Renewable Energy Laboratory Research Associate Feng Dong and Fellow David Nesbitt were also able to resolve exquisite features in the molecular spectra that permitted them to deduce the structures of the two radicals Whitney and her colleagues were able to detect two stretching patterns for the carbon hydrogen bonds in fluoromethyl radical In one mode the two hydrogens move toward and away from the carbon at the same time symmetric in the other one hydrogen moves in as the other moves out antisymmetric This observation in conjunction with a theoretical analysis allowed the researchers to figure out that the four atoms of this molecule do not lie in the same plane rather the fluorine atom bends up at an angle of 29 They also deduced that CH stretch vibrations make the molecule s electrical charge slosh among the fluorine carbon and hydrogen atoms in a surprisingly counterintuitive way Usually the carbon hydrogen bond forms a dipole with the bond s negatively charged electron wave function hanging out nearer the carbon atom This is also true for CH 2 F radical but the presence of the electron hungry F atom makes the charges

    Original URL path: http://jila-pfc.colorado.edu/highlights/spectral-shapes (2016-04-29)
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  • Trapped! | JILA-PFC
    reaction the recombination of iodine bromide clusters IBr CO 2 n dissociated by laser light The IBr CO 2 n cluster shown at right consists of an iodine atom purple a negatively charged bromine atom green and four carbon dioxide solvent molecules In the latter the oxygen atoms are red and the carbon turquoise When the Lineberger group set out to study the dynamics of solvent induced recombination of IBr CO 2 n clusters they thought they knew what to expect After all they d done similar experiments on an ion of molecular iodine I 2 dispersed in carbon dioxide I 2 CO 2 n They d even figured out how to add solvent molecules to the clusters one at a time which allowed them to monitor changes occurring with increasing solvation In the I 2 CO 2 n system the iodine molecules become surrounded by a solvent shell that builds randomly from wherever the first CO 2 molecule aligns itself When laser light dissociates the iodine molecule into two iodine atoms the atoms stay inside the shell and rapidly recombine The group found that even when a small number of solvent molecules are present the iodine atoms recombine in about 10 ps The rate of recombination increases as more solvent molecules are added With IBr CO 2 n clusters the researchers found that the solvent shell always forms first around the bromide end probably because it is smaller than the iodine However when they photodissociate the IBr molecule things get very interesting In clusters with 6 or fewer solvent molecules the iodine and bromine atoms recombine in about 10 ps as in the I 2 CO 2 n experiments However the recombination rate shoots up to and stays at 1000 ps for clusters with 8 10 solvent molecules The

    Original URL path: http://jila-pfc.colorado.edu/highlights/trapped (2016-04-29)
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  • Gold Fever | JILA-PFC
    be easily attached to these nanoparticles Physicists have been investigating gold nanoparticles in optical trapping experiments because they enhance trapping efficiency and potentially increase detection sensitivity Medical scientists have targeted cancer cells in tumors with gold nanoparticles and then illuminated the tumors with fiber optic lasers The gold nanoparticles absorb the light heat up and destroy the tumor In the field of biophysics one has to consider both issues heating and enhanced optical trapping Research Associate Yeonee Seol Graduate Student Amanda Carpenter and Fellow Tom Perkins routinely perform precision optical trapping experiments on molecular motors and DNA binding proteins As physicists Seol and her colleagues were intrigued by the promise of gold nanoparticles being able to significantly boost the trapping efficiency and detection sensitivity of their optical trapping assays As biologists however they were worried whether laser induced heating of gold nanoparticles could deactivate the proteins under study compromising their results The researchers decided to perform their own experiments to compare gold nanoparticles with polystyrene beads What they discovered was that the trapping efficiency and detection sensitivity of experiments using large gold nanoparticles were six times higher than those using polystyrene beads of comparable size In experiments using low laser powers 100 μW the gold beads cause temperature increases of less than 1 C Thus small gold nanoparticles make an excellent choice for enhancing signal detection in biophysical assays where they are used to tether biological molecules of interest Additionally because they are small they make room for larger DNA or other biomolecules within what is often a limited detection range Unfortunately the most common trapping wavelength of infrared light 1064 nm induced dramatic heating of the gold beads equal to 266 C per watt of laser power The temperature gradient in C around one gold bead is shown at

    Original URL path: http://jila-pfc.colorado.edu/highlights/gold-fever (2016-04-29)
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  • Constant Vigilance | JILA-PFC
    for the basic structure of matter predict that α may have changed over vast spans of cosmic time with the largest variations occurring in the early universe However the Standard Model says a has always been the same Our basic understanding of physics depends on scientists ability to determine whether or not α is an inconstant constant Astronomers and precision measurement experts are teaming up to solve this mystery by taking advantage of the special properties of highly reactive OH molecules OH is abundant in outer space and naturally emits microwave radiation at four specific frequencies when it returns to its ground state after being excited by absorbing radiation from nearby stars or colliding with other interstellar molecules These natural masers can be detected on Earth some after being created billions of years ago in the early universe And because each OH transition frequency has a different dependence on the value of the fine structure constant it should be possible to compare the value of α 10 billion years ago with its value today provided astronomers and laboratory scientists can measure two of the four OH transition frequencies precisely enough to detect small differences in α Until recently the best laboratory and astronomical measurements of the two main OH transitions had uncertainties of 100 Hz much too large to detect small changes in α However Graduate Students Eric Hudson and Brian Sawyer and Fellows Heather Lewandowski and Jun Ye have now improved the precision of one OH transition frequency measurement by 25 fold to 4 Hz and of a second by tenfold to 12 Hz The researchers now including NRC Postdoctoral Fellow Benjamin Lev are currently working to achieve similar improvements in the measurement precision of the remaining two satellite OH transitions The researchers expect to have new results by

    Original URL path: http://jila-pfc.colorado.edu/highlights/constant-vigilance (2016-04-29)
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  • Flashdance! | JILA-PFC
    with a laser Then suppose you could generalize this understanding to a whole cloud of similar atoms and predict the temperatures your experimental physicist colleagues could achieve with laser cooling This way cool theoretical analysis comes compliments of Graduate Student Josh Dunn and Fellow Chris Greene The researchers have completed a detailed calculation of atom light interactions which they use to treat strontium 87 Sr and magnesium 25Mg atoms completely quantum mechanically in three dimensions And they don t use a single approximation from classical physics Although a direct quantum mechanical calculation is practically impossible they simulated the atom dynamics by rolling dice with a computer Actually they generated random numbers with a Monte Carlo simulation This theory uses random numbers to produce approximate solutions to various problems that are otherwise too difficult to solve Their project was inspired by a laser cooling experiment in Jun Ye s lab a couple of years ago in which more cooling of 87 Sr atoms was observed than was supposed to be possible with this kind of atom a fermionic isotope with no electronic spin Naturally the JILA theorists decided to analyze what was being observed in Ye s lab In the process they created theoretical renderings of laser cooled 87 Sr atoms as they become progressively colder right When they began their work the researchers knew that alkaline earth metals such as 87 Sr and 25Mg have multiple states that can be excited by laser photons Normally having lots of excited states means that the extra laser cooling known as sub Doppler cooling works fairly well However the excited states in these particular atoms are very close together in energy and until now physicists had predicted that such closeness would impair sub Doppler cooling When Josh Dunn decided to look into the

    Original URL path: http://jila-pfc.colorado.edu/highlights/flashdance (2016-04-29)
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  • Heme Motions | JILA-PFC
    What is this thing called heme And how does it do such amazing work inside our bodies Scientists know that heme is a large ringed molecule called a porphyrin that has an iron atom sitting in the middle of it In the heme molecule shown at right the iron atom is green and the atoms in the porphyrin ring are carbon teal nitrogen dark blue hydrogen not shown and oxygen red In the figure below the heme is embedded in and bonded to cytochrome c Understanding how heme functions at the biomolecular level is a hot research topic for biophysicists including Research Associate Byung Moon Cho Graduate Student Fredrik Carlsson and Associate Fellow Ralph Jimenez The JILA researchers use photon echo spectroscopy to study the motions of proteins associated with or attached to heme groups Understanding these motions will help scientists figure out how heme proteins accomplish their important work inside our cells Recently the JILA scientists began a series of studies to measure and quantify the motions of cytochrome c and two other zinc containing porphyrins To allow them to make the most precise measurements possible they substituted a zinc atom for the iron atom in the cytochrome c

    Original URL path: http://jila-pfc.colorado.edu/highlights/heme-motions (2016-04-29)
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  • Charting the Fermi Sea | JILA-PFC
    One of the key questions in ultracold physics is how atoms behave in the middle of this continuum called the BCS BEC crossover BCS stands for the Bardeen Cooper Schrieffer theory of superconductivity developed in the mid 1950s Graduate Student Cindy Regal former JILA Postdoc Markus Greiner now assistant professor of physics at Harvard former JILA Visiting Fellow Stefano Giorgini now at Italy s Universitá di Trento JILA visitor Marilù Chiofalo and Fellows Debbie Jin and Murray Holland are tackling this question head on The researchers believe that the condensation behavior of Fermi systems evolves smoothly from BEC behavior where most fermions have formed molecules through the BCS BEC crossover to Cooper pairs of atoms that form a superfluid as shown on the right In the crossover region pairs of fermions interact strongly with each other Depending on experimental conditions in particular changes in the magnetic field these pairs can behave more like molecules or more like Cooper pairs Murray Holland s theory team is currently working on developing a new theory to explain the quantum mechanical behavior of fermions in the BCS BEC crossover In the meantime Cindy Regal and Debbie Jin continue to take advantage of an earlier analysis they performed that predicted that Feshbach resonances and magnetic tuning could be used to investigate atoms in this region A Feshbach resonance is a special value of a magnetic field around which small changes in field strength have dramatic effects on the atomic interactions in an ultracold gas In 2003 Jin s group used these predictions to observe the condensation of atom pairs in the BCS BEC crossover Since then many experiments at JILA and elsewhere have used Feshbach resonances and magnetic tuning in experiments designed to improve science s understanding of crossover physics Researchers continually compare the results

    Original URL path: http://jila-pfc.colorado.edu/highlights/charting-fermi-sea (2016-04-29)
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