Robert Bernheim

Laser Spectroscopy

The solution of fundamental questions concerning molecular structure and dynamics using laser spectroscopy is the research objective of Professor Bernheim and his students. The studies emphasize small molecules where comparisons between experimental findings and abinitio theory are possible or where an important controversy must be resolved. A recent example is the discovery and characterization of a shelf potential in the molecule Li2. Double-minimum and shelf potentials arise from state anticrossing in the electronic structure of the molecule. These situations present a unique opportunity to study non-adiabatic effects commonly referred to as Born-Oppenheimer breakdown in the electronic state structure of molecules.

Many techniques of laser spectroscopy are used in these investigations. These include high-resolution sub-Doppler studies with frequency-stabilized CW tunable dye lasers and multi-photon studies with high-power pulsed lasers together with nonlinear optical techniques.

In another investigation, Professor Bernheim and his co-workers are measuring the fundamental effects of laser-induced spin orientation on chemical kinetics and chemical equilibrium. The potential energy surface upon which reactants move and interact is selected by the optical orientation, opening up the possibility of "guiding" chemical reactions with lasers. The initial prediction that the optical orientation of atoms can result in a change in the equilibrium vapor composition of various elements was advanced by Dr. Bernheim some years ago. This prediction was subsequently verified experimentally in a number of laboratories around the world.

The effort at Penn State has concentrated on the reaction 2Li Li2 for a number of reasons. First, the molecular electronic spectra of the dimer exhibit a rotational structure which is easily resolved and makes possible a detailed, quantitative measurement of the shift in the equilibrium atom-dimer vapor composition and its dependence on other parameters in the experiment. Second, a comprehensive understanding of the excited electronic state structure of Li2 permits one to select, as probes of molecular density, those spectral transitions that are free from complicating factors such as perturbations that lead to "dark" processes, pre-dissociation being one example.

The atom-dimer vapor equilibrium 2Li Li2 has been investigated for pure 6Li, pure 7Li and a 50%-50% mixture of 6Li and 7Li. The point of equilibrium can be shifted by controlling the degree of lithium atomic spin orientation. Atomic orientation causes the dimer density to be reduced. Not only is the molecular component of the vapor composition decreased in the presence of atomic orientation, but it is also found that a nuclear spin polarization is generated in the remaining Li2 molecules. This is evidenced by a change in the ortho/para ratio of the homonuclear Li2 density to favor ortho (the totally symmetric nuclear spin combination). This work constitutes the first unambiguous demonstration of how lasers can be used to change the equilibrium composition of a chemical reaction.

The effects of nuclear spin can also determine the route of a photochemical reaction. Such a process has been discovered in Li2 where pre-dissociation of an electronically excited state results in the destruction of ortho molecules, thereby changing the ortho/para composition.

Another area of research conducted by Dr. Bernheim and his collaborators uses high-resolution infrared spectroscopy. Much of the current activity in state-to-state chemistry and reaction dynamics was stimulated by an early interest in the possible non-RRKM behavior in the isomerization of methyl isocyanide (CH3NC) to acetonitrile (CH3CN). Studies in other laboratories were carried out in which a C < H stretching overtone was excited and isomerization rates measured. However, no clear-cut deviation from a statistical (RRKM) rate behavior was found. Unfortunately, these early experiments were performed without a knowledge of exactly which specific sub-levels of the C < H stretch were being excited in the experiment. A completely resolved spectroscopic study of even the vibrational fundamentals of CH3NC had never been done. This situation is being corrected at Penn State with a high resolution spectroscopic analysis of a number of infrared transitions in CH3NC. In the work completed so far, numerous perturbations in the ro-vibrational state structure of CH3NC have been identified. The ultimate goal is to use this archetypical unimolecular isomerization reaction for a modern state-to-state dynamical study, but its resolved fundamental state structure must first be known and understood.

In all of these studies, there is a continual interaction with theoretical descriptions of the same phenomena. What emerges is a fundamental understanding of the electronic state structure of molecules and molecular interactions.