In this project funded by the Chemical Structure, Dynamics, and Mechanisms Program of the Chemistry Division, Professor Robert Levis of the Chemistry Department at Temple University seeks to understand the processes that occur when polyatomic molecules are subjected to laser fields on the order of the fields binding electrons to nuclei. In this laser intensity regime, the molecule can be reprogrammed to drive desired physical and chemical processes including coherent rotational and vibrational motion, chemical reaction and non-resonant vaporization. This research seeks to understand strong field control mechanisms through experimental and theoretical investigations. Students and postdoctoral associates are trained in experiment (lasers and ultrafast optics) and theory and as such, represent an important national resource for high technology science and innovation.
This research is leading to advances for biomedical diagnostics and therapeutics, macromolecule synthesis, nanomaterials synthesis, molecular electronics, and chemical/biological sensors. The findings of strong field chemical research have been leveraged to develop new chemical agent detection systems using adaptive strong field mass spectrometry, to develop new sensing paradigms, and to develop highly sensitive molecular monitors for stand off detection using laser filament-based methods.
Overview: The exploration of the interaction ultra-intense lasers with molecules has led to the discovery of useful phenomena including laser electrospray mass spectrometry, filament-based Raman spectroscopy, and the control of bond dissociation through shaped laser pulses. The experiments have also led to interesting new scientific phenomena including tunnel ionization as a means to prepare a launch state for coherent control, radical cation electronic spectroscopy and new means for impulsive rotational and vibrational spectroscopy. Five PhD students have been associated with this award over the last four years as have 4 undergraduate students. The work has resulted in some 20 papers and three patents. Intellectual Merit: The interaction of molecules with strong laser fields does not result in catastrophic dissociation primarily because of the ultra-fast time scale of the pulse, <100 fs. When such a pulse interacts with a van der Waals film of molecules, the major result is the transfer of the molecules into the gas phase. A major finding of the NSF-sponsored work was the discovery that laser vaporization results in the transfer of molecules into the gas phase without decomposition. When combined with atmospheric pressure mass spectrometry, a new, universal detection method results. This experiment, called laser electrospray mass spectrometry (LEMS) was performed on a wide variety of systems ranging from biological tissue samples like plant organs, to explosives, pharmaceuticals and protein structure determination. When an ultra-short infrared pulse interacts with a molecule in the gas phase, a new method to prepare molecules for coherent control experiments results. Laser wavelengths of wavelength 1150 nm and longer result in the preparation of relatively cold ions that can be subsequently excited with a second laser pulse to exert a degree of control over bond dissociation that is more than a factor of ten larger than previous coherent control experiments. This excitation scheme has also been used to probe the excited electronic state spectroscopy acetophenone. When an ultrafast laser pulse interacts with air over the course of a few meters a laser filament results. Such a filament has unique optical properties including the ability to self-shorten to a pulse that is only a few optical cycles, <8 fs. Such a pulse will impulsively excite all vibrational modes, meaning that the molecules move in perfect lock step. This make detection using Raman scattering one million times easier, and thus allows remote gas phase detection. This new spectroscopy has been used to detect signature molecules for improvised explosive devices. The mechanisms of laser filamentation have also been worked out as a result of NSF funding. Broader Outcomes: This research has translated into a method for mapping the distribution of chemicals in materials such as tissue. The laser electrospray mass spectrometry work has seeded a new method for systems biology and is of use for real time analysis of pharmaceutical formulations. We have leveraged the work as a new means to produce nanoparticles using shaped laser pulses. New spectroscopic methods for explosive device detection have also resulted from the research. Finally, five graduate students and four undergraduate students have been trained in the area of experimental and theoretical chemistry as part of this award. The graduates are all currently employed in industry, national laboratories or academia. These students represent an important national resource for high technology scientific endeavors and innovation. The laboratory actively recruits and trains underrepresented groups.
