Session W4: Breakthroughs in Molecular Physics

Molecular Programming with DNA

Information can be stored in molecules and processed by molecular reactions. Molecular information processing is at the heart of all biological systems; might it soon also be at the heart of non-biological synthetic chemical systems? Perhaps yes. One technological approach comes from DNA nanotechnology and DNA computing, where DNA is used as a non-biological informational polymer that can be rationally designed to create a rich class of molecular systems -- for example, DNA molecules that self-assemble precisely, that fold into complex nanoscale objects, that act as mechanical actuators and molecular motors, and that make decisions based on digital and analog logic. I will argue that to fully exploit their design potential, we will need to invent programming languages for specifying the behavior of information-based molecular systems, to create theoretical tools for understanding and analyzing the behavior of molecular programs, to develop compilers that automate the design of molecules with the desired behaviors, and to expand experimental techniques so that the implementation and debugging of complex molecular systems becomes as commonplace and practical as computer programming.   

High-Throughput Microwave Spectroscopy for Chemical Kinetics and Trace Detection

Recent developments in high-speed digital electronics have made it possible to develop a new generation of broadband microwave spectrometers for molecular spectroscopy. These spectrometers acquire a broadband microwave spectrum (up to 12 GHz bandwidth in the current designs) in a single data acquisition event. Chirped pulse excitation is employed to efficiently polarize a molecular gas sample over the 12 GHz bandwidth using a pulse duration of about 1 microsecond. Following sample polarization, the free induction decay signal from the molecular rotational spectrum is directly digitized using a high-speed digital oscilloscope. The frequency domain spectrum is obtained by Fourier transform following coherent, time-domain signal averaging. The spectrometer design provides new capabilities for high-throughput chemical analysis. Applications to chemical identification of molecules of astrochemical interest will be presented. The broadband technique is well-suited to laser experiments where isomerization kinetics of highly excited molecules can be measured on the picosecond time scale through line shape analysis. Microwave-laser experiments for chemical reaction dynamics in pulsed jet samples and room-temperature gases will be presented.   

Observing Energy Flow and Controlling Molecular Dynamics in Gases and Liquids

Preparing isolated molecules in selected vibrational eigenstates permits the control of a variety of dynamical processes, such as the passage of molecules through conical intersections between potential energy surfaces and the selective cleavage of chemical bonds. Experiments on isolated molecules set the stage for studying the same processes in liquids, where frequent interactions complicate the molecular dynamics. One challenge is understanding the factors that control of the flow of the initially deposited energy within the molecule and into the surrounding solvent. Experiments using 100-fs laser pulses to prepare and probe vibrationally excited molecules are able to follow this redistribution of vibrational energy. Other experiments use these pulses to probe electronically excited states and to observe the passage of molecules through conical intersections. The goal of these experiments is a comparison of the dynamics of isolated molecules with those of molecules in solution.   

Exotic Molecules in the Laboratory and Interstellar Space

Fourier transform microwave spectroscopy of supersonic molecular beams has developed into a remarkably sensitive technique for studying unstable molecules. It has proven particularly effective for the detection of the kind of large reactive carbon chains often found in space, such as polyynes, radicals, and carbenes, whose rotational spectra are greatly simplified at the very low rotational temperature that is readily achieved in a supersonic molecular beam source. Although laboratory detection remains challenging, the rotational spectra of many new carbon, silicon, and sulfur molecules in various states of ionization have been detected, including a number which possess ring, ring-chain, trapezoidal, or even bicyclic structures. Precise molecular geometries have been determined by means of isotopic substitution for nearly one-half of the newly found molecules. On the basis of the laboratory data, more than 10\% have been detected in space and, with large radio telescopes under construction or the discovery of better astronomical sources, it is possible that nearly all may eventually be found. This talk will provide a broad overview of our recent work, illustrating with a few specific examples the power of our laboratory techniques, and how these techniques can be applied to challenging problems in astronomical spectroscopy. Many of the results are of general interest to the chemical physics community, providing new information on molecular structure, chemical bonding, and isomeric distributions.