In this award, funded by the Experimental Physical Chemistry Program of the Division of Chemistry, Professors Dudley Herschbach and Igor Lyuksyutov of Texas A & M University, together with their graduate student colleagues, will study the reactions of hydrogen atoms with molecules in crossed-beam experiments at ultra-low collision energies -- energies where the deBroglie wavelengths exceed 250 Ã…. Initial molecules to be studied will include halogens and nitrogen dioxide. In addition to these studies of reactions of ultra-slow chemical species, Herschbach and Lyuksyutov propose to conduct crossed-beams studies of a number of reactions involving atmospherically important HOx species.
By carefully controlling the collision energies of reactions, experiments like those being conducted here reveal important details for how chemical reactions occur -- details that are lost when studying reactions under normal laboratory conditions. In addition, by slowing normally speedy reactant molecules (ca. 1000 mph) to "school-zone" speeds, extreme quantum effects will become dominant -- resulting in unique behavior. Profs. Herschbach and Lyuksyutov will continue to lead a number of high profile outreach projects, including television work and a summer school for high school students entitled the "Coolest Place in Texas."
Towards Matterwave Chemistry At ordinary temperatures, gaseous molecules typically careen about with the speed of rifle bullets. Contending with this wildness has been a major challenge in efforts to probe intimate features of molecular interactions, and a chief theme in the historical development of experimental chemical physics over the past century. Our project was focused on pursuing a simple and versatile means to cool molecules to low temperatures. Thereby they can be calmed down to the speed of bugs rather than bullets. In accord with quantum mechanics, at such low speeds molecules will act like "matterwaves" rather than particles. Chemists think of chemical reactions, at ordinary thermal collision velocities, as involving transfer of ball-like atoms, or groups of atoms, between localized regions of the reactant molecules. However, slow molecules will react differently. In the matterwave realm, the collision wavelength can considerably exceed molecular diameters. Then the reactants will engulf each other, somewhat like the coupling of amoebas rather than the impact of balls. The process might be visualized as reactant waves mixing, sloshing or oozing about, then regurgitating forth the product molecules as waves--or as particles, if enough energy is released to markedly shorten the outgoing wavelengths. Anticipatory theory has predicted novel applications of matterwave beams, ranging widely from information processing via quantum computers to gravity wave detectors. Intrigued by such prospects, several laboratories have over the past decade explored means to generate matterwave molecular beams. But attaining very slow beams generally imposes drastic reduction in intensity, a severe handicap for most anticipated applications of very slow beams, especially studies of chemically reactive collisions. Two key outcomes of our project have significantly enhanced prospects for overcoming this intensity limitation, particularly for slow collisions. One key outcome is instrumental, with several components, assembled in a new molecular beam laboratory. The most unorthodox component launches a molecular beam from a supersonic nozzle near the tip of a rotor. By spinning the rotor at high-speed (comparable to bullets) the velocity distribution can be shifted downward or upward over a wide range. Beams of any molecule available as a gas can be used. We have used this source to produce both slow and fast beams of rare gases, O2, Cl2, NO2, NH3, and SF6. Our lab also employs more conventional stationary supersonic beam sources suitable for chemical species formed by discharge or photolysis, such as H, D, O or OH. Product species are detected by laser induced fluorescence and/or mass spectroscopy. The other key outcome is a change in perspective. For collision experiments, the matterwave character pertains to the relative velocity of the reactants. In molecular beam studies of reactions at ordinary thermal conditions, the reactant beams are crossed, usually at 90o. But to get very low relative velocity with crossed beams requires that both reactants be moving very slowly, so imposes a drastic cost in intensity. The redeeming strategy is to use merged, codirectional beams. Then neither beam needs to be slow, provided the beam speeds can be closely matched. That is readily accomplished by merging a reactant beam with fixed speed from a stationary source, with the other reactant beam of adjustable speed from our rotor source. The gain in intensity is huge, typically a million-fold or more. The merged-beam technique, previously used only with very high energy beams, has now been recognized as a "game-changing" breakthrough for experimental study of matterwave chemistry. Among other outcomes are theoretical treatments pertaining to the matterwave realm. These include treating "tunneling" of incident waves during slow approach of reactants and modification of intermolecular forces due to averaging over rapid molecular vibrations during that approach. Also treated are aspects of proposed quantum computers using "entanglement" interactions of polar molecules subject to external electric fields. Broader impacts associated with this adventurous research include improving tools useful in other domains of chemical physics, particularly spectroscopy, and fostering interest both among students and the wider public. In addition to numerous scientific and popular talks by both PIs, Herschbach served as narrator for a series of narrator for a series of 17 five-minute videos, directed by Dr. Jeff Seeman, comprised of interviews of high school students taking part in science fairs. These are available on a website: ArchimedesInitiative.org. Herschbach also served as advisor to the Harvard-Radcliffe Undergraduate Research Association, which produced a book titled Success with Science: The Winner's Guide to High School Research. He wrote the Forward, recruited Prof. Lisa Randall to write an Epilogue, and arranged for publication by the Research Corporation. Thousands of copies have been distributed to U.S. high schools and to many countries. Recently the book has been translated into Chinese and Arabic. At the invitation of Yo-Yo Ma, founder of the Silk Road Project, Herschbach joined in introducing some science into the Silk Road curriculum, now in use in many middle and high schools.
