Vibrationally Mediated Photodissociation
(Gas Phase)

The focus of the corner lab is the study of unimolecular reactions in the gas phase. In particular, we are interested in how vibrational excitation prior to photodissociation affects the molecule's transition to and evolution on the excited state. We have demonstrated this vibrational mediation for such systems as water, ammonia, and phenol.

Most of our studies are performed on molecular beams via action spectroscopy or velocity map ion imaging, although we do occasionally look at spectra for room temperature gases via photoacoustic spectroscopy in cells. In addition to maintaining a vacuum chamber, we also keep three Nd:YAG pumped dye lasers in operation. These nanosecond-pulse lasers allow excitation of molecular quantum states and state resolved detection.

In photodissociation, photofragments fly out from the interaction region with a given translational energy. This kinetic energy is determined by the difference in energy of the photon which caused the dissociation and the energy required to form the given fragments, causing a series of nested spheres - each containing information on the internal energy of the fragment states. These neutral fragments are ionized by a laser and projected by ion optics onto a spatially sensitive detector. The image produced from the crushing of the ion sphere is captured by a CCD camera, and established techniques allow for reconstruction of the full 3D sphere. Integration around each ring from a center slice yields an energy distribution; angular information is also available.

Our results for dissociation from excited state N-H stretches show remarkable differences. These states are only accessible from selectively excited vibrations in the ground state. We can see from the data that the symmetric stretch leads to nonadiabatic dissociation through the conical intersection to form ground state NH₂. For the antisymmetric stretch however, adiabatic dissociation is the dominant pathway. This can be inferred from the narrower energy distribution, but to identify the features from the background, images were obtained at many different energies.

People in this lab:

Amanda Cornelia Mike