Chemical Physics

David Nesbitt and J. Mathias Weber use the tools of physics to better understand the structure and dynamics of relatively "simple" ions and molecules. In one major program, Nesbitt and his group use high-resolution slit-jet infrared laser spectroscopy to investigate the dynamics of photon absorption by different entities, including HF and HCl dimers, H₃⁺ (abundant in the interstellar medium), OH^- (fundamental to acid/aqueous chemistry), OH₃⁺, CH₅⁺ (whose five protons swarm around the central carbon atom such that the very concept of molecular structure is problematic), and ethyl radical (a model for decomposition products of atmospheric chlorofluorocarbons). By probing high concentrations of these materials at very low temperatures, Nesbitt and his group have been able to understand their spectra well enough to determine precisely how they vibrate and rotate in response to laser light. They then compare their findings with the predictions of quantum mechanics. This research has yielded important insights into the hydrogen-bonding potentials and intramolecular vibrational dynamics of HF and HCl as well as the structure of gas-phase ethyl radical.

Investigations of the structure and behavior of atoms and molecules on the quantum level are particularly challenging when the molecule under investigation appears in small amounts or is rapidly transformed into something else during combustion or other rapid chemical reaction. Nesbitt's group has developed an innovative method for studying such elusive chemicals. The method combines infrared laser spectroscopy and slit-jet cooling, which cools the molecules close to 0 K, making them easier to observe. It also uses an electric discharge to create a desired molecule in sufficient quantities for study since short-lived chemicals typically exist in vanishingly small amounts. Using this method, Nesbitt's group recently obtained the first-ever high-resolution IR spectra from cyclopropyl radical, an important, but short-lived, combustion intermediate.

Nesbitt and his group probe the potential energy of more complex ions such as OH₃⁺. By substituting atoms of deuterium for one or more of the hydrogen atoms in this molecular ion, the researchers can use high-resolution spectroscopy to monitor changes in the vibrational spectrum that correspond to specific quantum mechanical behaviors such as "quantum mechanical tunneling" and "forbidden" states in which the molecule appears to be in two different configurations at the same time. The method is important because quantum mechanical calculations have not yet evolved to the point where they can predict the energy levels of molecules with five or more particles.

Nesbitt conducts a major research program to better understand the fundamental principles that control chemical reactions. The researchers direct molecular beams of highly reactive molecules into a vacuum chamber, so that they intersect. The probability that a chemical reaction will occur at the point where the beams cross is low enough to allow them to study the dynamics of single collisions when they do occur. Experiments using a beam of hydrogen or deuterium molecules and a beam of fluorine atoms have confirmed that their tools are accurate enough to understand single-collision reactions in terms of quantum mechanics. However, both the experiments and the theory to explain even simple chemical reactions are pushing the frontiers of knowledge. In fact, understanding the quantum mechanical behavior of each atom in relatively simple chemical reactions (such as F + H₂O → HF + OH^-) is going to be one of the major challenges of physics in the 21st century. The challenge comes from the dynamical complexity that exists at the quantum mechanical level. A chemical reaction involving only four atoms like this one requires an understanding of what is happening with six internal coordinates in six-dimensional space!

The Nesbitt group is meeting this challenge with experiments that stretch chemical bonds (making them easier to break), explore the use of induced vibrations (increasing the likelihood of bond breaking), use polarized light to align reactants (allowing the investigation of steric effects on chemical reactivity), and hold one reactant, such as water, in place to see how this affects collision dynamics. The ability to predict factors that increase or decrease the likelihood of a chemical reaction could also shed light on similar processes in biological systems.

Nesbitt and his group have begun efforts to probe chemical reactions in the liquid phase, which are far more complex and dynamical than reactions in solids and gases, which are beginning to be well understood. Their investigation of liquid-phase reactions have started with experiments that direct molecular beams onto the surface of a low-vapor-pressure liquid, so as to probe the single-collision dynamics between molecules that reflect off the surface of the liquid and reactants in the gas phase.

The group is investigating what happens when fact, cold carbon monoxide molecules collide with the surface of an oily liquid (perfluorether). The researchers measure the speed and temperature of a molecular beam of CO₂ created in a supersonic expansion as it hits the liquid surface. About half the CO₂ molecules splash off the surface as if it were a solid; a second fraction skips across the surface before being reflected. Only a tiny fraction of the original gas beam enters the liquid and stays there. The gas molecules that immediately reflect off the surface come off at temperatures about twice as hot as the temperature of the liquid because much of their energy of movement is transformed into heat. In contrast, the molecules that skip across the surface come off the liquid at room temperature, having already dissipated some of their energy of movement.

The Nesbitt group uses scanning laser microscopy to study artificial atoms, known as quantum dots, made from CdSe and silver nanocrystals. (Quantum dots are objects that confine electrons in all three dimensions.) By carefully separating the fluorescent light emitted by quantum dots from the incident laser light, the researchers can count and graph the number of photons emitted every millisecond in the field of view. This process allows them to "see" individual quantum dots and study their kinetics.

Nesbitt and his group have discovered that the quantum dots do not fluoresce uniformly; rather, they blink on and off in a pattern that most closely resembles fractal kinetics (chaotic motion). The researchers are now working to explain this phenomenon, which had not been observed prior to their experiments. They propose that a photon of light creates an electron-hole pair in the quantum dot. However, the quantum dot retains electrical neutrality. The wave functions of both the positively charged hole and the negatively charged electron are somehow squeezed into the dimensionless quantum dot because they can't go anywhere else. Even so, they don't exactly fit where they are. As Nesbitt says, "Clearly, there is new physics to be learned at the nanoscale."

J. Mathias Weber and his group combine mass spectrometry with laser spectroscopy to characterize positively and negatively charged ions and biomolecules. One project focuses on infrared spectroscopy of molecular and metal-containing cluster ions. The ions are produced in a pulsed supersonic expansion and mass selected in a time-of-flight mass spectrometer. Photon absorption leads to the evaporation of weakly bound ligands from the cluster. The absorption spectra of the clusters can be measured by monitoring the generation of photofragments as a function of the laser wavelength.

In a second project, the group is developing a three-stage photofragmentation spectrometer that uses an electro spray ion source to deliver ions into an ion trap. There, the ions can be cooled or reacted with small solvent molecules such as water. Next, the products are mass selected and illuminated with tunable pulsed laser light. Finally, the light-induced photofragments are analyzed with a time-of-flight spectrometer. This system allows Weber and his group to study the spectral and dynamical differences between unsolvated, sequentially solvated, and solution phase ion species. In the future, they plan to use it to investigate the photo physics and chemistry of complex molecules such as multiply charged anions, cationic and anionic salt cluster ions, and charged biomolecules.