UsingLasers to See the Shape of Molecules

A Korean research team has created a new technique forresolving the orbits of multiple molecular orbitals, a previously impossiblefeat

Ascientist in a crisp, white lab coat and protective eye goggles sits behind asafety shield, controller in hand.  Infront of him is a powerful titanium-sapphire laser, aimed at a crystal lens. Histhumb gently squeezes the trigger on the controller.  There is an imperceivable wisp of gas that isescaping from a nozzle and crossing the laser’s path.  Before he can even blink his eye the laser iscapable of firing more than a trillion times. On the screen a line of alternating pairs ofglowing, amorphous spots appear.  For thefirst time ever, someone has been able to peer down into the molecular level toobserve simultaneously in two dimensions.

Professor Hyeok Yunand his team from the Institute for Basic Science (IBS) and GwangjuInstitute of Science and Technology (GIST) in Korea have gotten a step closer to fullyunderstanding the complicated relationship of form and motion of molecules.

The structure and movement of molecules isnot feasible to observe via conventional microscopic methods.  In order to get information about molecularshape and the orientation of their orbits,researchers use a process called high harmonic generation (HHG).  To do this, a laser pulse tuned to a specifichigh frequency is directed into a jet of gas of the molecule being studied.  When the pulse meets the jet of gas, plasmais generated which emits specific color light.This interaction with the molecule and light generation reveals what iscalled the highest-occupied molecular orbital (HOMO).  The HOMO can be envisioned as the “shape” ofthe outside molecular orbits.  The pulsedlaser beam is converted to a high harmonic frequency which reaches the sensorwhere data from the interaction can be collected.  What the researchers see allow them to gatherinformation about the characteristics of molecule’s structure anddynamics.  As useful as this technologyis, researchers have been limited in what information they can obtain becausethey have been confined to observing the high harmonic frequency from a singlelaser pulse on a one-dimensional plane each time. 

To gather more information from themolecules during each test, Professor Yu’s team, have devised a method forresolving multiple molecular orbitals by using two-dimensional high-harmonicspectroscopy (HHS).  This HHS processinvolves pulsing a laser at an ultra-fast interval through a polarizing lenswhich splits the beam in two. 

The team focused the laser through a thincrystal which split the beam into two polarized waves traveling in the samedirection but now perpendicular to each other.When one beam traveling up and down while the other is moving side toside, the beams are moving orthogonally. When the two beams interacted with the gassample, they revealed not only the HOMO, but simultaneously the HOMO-1, a lowerlying molecular orbit.  In the past thesetwo orbits have been difficult to distinguish from one another, because HOMO-1has been overshadowed by the more energetic HOMO. According to Yun, “In thiswork, we approached molecules in two dimensions. HOMO-1 can be revealed withrelative ease in the orthogonal direction to the molecular axis, while HOMOdoes it in a parallel direction. Orthogonally polarized two waves enable us to probeboth orbitals in two dimensions and to separate signals to different harmonicfrequencies. Thus, we could resolve the signals from the two orbitals and couldsimultaneously obtained information on both orbitals.”

After combining the data collected fromeach laser pulse the researchers were able to use a clever technique calledtomography to piece the two-dimensional images together into athree-dimensional approximation.  Withthe three-dimensional approximation, they were able to discern the shape andrelative alignment of the HOMO and HOMO-1 orbitals, something that had neverbeen done before.

Thereis no loud applause, nobody waiting to congratulate Professor Yun on thisachievement.

“The ultimate goal”he says, “is to follow a chemical reaction in its own time scale. It leads us tohave direct insight and to understand fundamental mechanism abouttransformations in molecular scale. We expect this method can be a route or beof help to achieve the goal.”This new method willadvance future molecular research by allowing for independent and simultaneousobservation of the structures and dynamics of multiple molecular orbitals. Itwill enable the observation of multi-orbital dynamics during chemical reactionsof more complicated molecules.

by Daniel Kopperud

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Notes for editors

References  

Title of Paper: ResolvingMultiple Molecular Orbitals Using Two-Dimensional High-Harmonic Spectroscopy, Physical Review Letters,DOI: dx.doi.org/10.1103/PhysRevLett.114.153901

Authors: HyeokYun, ¹ Kyung-Min Lee,¹ Jae Hee Sung,^1,2 KyungTaec Kim,^1,3 Hyung Taek Kim,^1,2,*and Chang Hee Nam^1,3,†

¹Center for Relativistic Laser Science,Institute for Basic Science (IBS), Gwangju 500-712, Republic of Korea

²Advanced Photonics ResearchInstitute, Gwangju Institute of Science and Technology (GIST),

Gwangju 500-712, Republic ofKorea

³Department of Physics and PhotonScience, Gwangju Institute of Science and Technology (GIST),

Gwangju500-712, Republic of Korea

For further information or to request mediaassistance, please contact: Mr. Shi Bo Shim, Head of Department ofCommunications, Institute for Basic Science (+82-42-878-8189; sibo@ibs.re.kr)or M. Sunny Kim, Department ofCommunications, Institute for Basic Science (+82-42-878-8135; sunnykim@ibs.re.kr) 

About Institute for Basic Science (IBS) TheIBS was founded in 2011 by the government of the Republic of Korea. With thesole purpose of driving forward the development of basic science in Korea, IBSwill be comprised of a total of 50 research centers in all fields of basicscience, including mathematics, physics, chemistry, life science, earth scienceand interdisciplinary science. IBS has launched 24 research centers as ofJanuary 2015. There are one mathematics, eight physics, six chemistry, sevenlife science, and two interdisciplinary research centers.