We study detailed mechanisms of photoinitiated chemical reactions in gas and condensed phases. Our goal is to understand reactive processes at a fundamental level, in particular those important in the atmosphere. Primary events, nonadiabatic transitions and the influence of the environment on reaction outcomes are examined. We exploit molecular beams for reactant kinetic energy control, state-specific laser excitations for varying reactants vibrational energy, and a variety of laser-spectroscopies and imaging for product diagnostics.
As an example, consider the technique of photoelectron and photofragment ion imaging. Here, "snapshots" of fragmentation or reactions are recorded with a position sensitive detector and a CCD camera. A single image interrogates state-selected products revealing their velocity and angular distributions, from which the reaction time scale and mechanisms are deduced. Photoelectron images pinpoint surface crossings that lead to dissociation and how ground and excited states of molecules are ionized. They are recorded using velocity mapping by projecting the photoelectrons on a position-sensitive detector. From the resulting image a complete 3D velocity distribution, including the speed and angular distributions, can be efficiently reconstructed using a method developed by our group and described in our group web site. The speed distribution represents the photoelectron or photofragment energy spectrum. The angular distributions convey information about the parent molecular orbitals and dissociation timescale. This technique allows us to rapidly obtain "road maps" for complex dissociation and ionization events that are germane to real-life environments. We also obtain correlated distributions, i.e. the state distribution of one product correlated with a specific state of the other, helping to sort among possible mechanisms.
The image on the left is of an HCl fragment in J=5 from the dissociation of the acetylene-HCl dimer (middle). The plot on the right displays the kinetic energy of the HCl(J=5) fragment from which the correlated rotational distribution of the acetylene co-fragment is obtained.
Photoinitiated reactions of free radicals and other transient species
Free radicals and diradicals are responsible for many reactions that affect our environment and participate in chemical synthesis, but their clean preparation remains a challenge. We have succeeded in preparing intense beams of hydroxyalkyl radicals, which are important in atmospheric chemistry and alcohol combustion. We identified dissociation channels, and in collaboration with theory showed that products are formed following nonadiabatic transitions. We use UV and visible lasers to promote molecules to electronically excited states, and infrared lasers to excite the radicals on the ground electronic state to specific vibrational levels. We then monitor energy flow in the radical leading to isomerization and/or dissociation. We have succeeded to excite the hydroxymethyl radical to high OH-stretch overtones and detected H fragments and their kinetic energy distribution. We also study energy rich molecules such as diazomethane and by looking at electrons ejected from excited electronic states we identify the nature of the states, their interactions, and vibrational frequencies.
Dynamics of hydrogen bonded and weakly covalently bound complexes
An important recent project involves studies of binary intermolecular interactions that involve weak covalent and hydrogen bonds. Hydrogen bonding is crucially important in biological systems and in the organization of molecular solids. Weakly covalent dimer bonds whose strengths is 10 - 30 kcal/mole (also referred to as incipient bonds) are encountered in dimers of free radicals, such as NO and NO2, and in Lewis acid-base pairs. The bonding and structure in these dimers are strongly influenced by the environment. For example, in the atmosphere, complexes of SO3 are important in aerosol and sulfuric acid formation. Despite their importance, the exact nature of the bonding and dissociation in these quasi-molecules is largely unexplored. We are studying them using photofragment and photoelectron imaging techniques. Recent examples include the hydrogen-bonded T-shaped complex of acetylene with HCl/DCl shown in the figures above, the linear complex of ammonia with acetylene and the covalently bound NO dimer. The experiments show state-specific effects in fragment vibrational and rotational state distributions, which are explained by theory as reflecting constraints on energy flow and linear to angular momentum transfer.
Transport and guest-host interactions of molecules in thin films
We examine the interactions of thin layers of ice and other molecules adsorbed on or embedded in the ice, as well as their bonding to insulating surfaces, such as MgO(100) single crystals. In these experiments, done collaboratively with Prof. Curt Wittig, we exploit FTIR spectroscopy, and laser and tempereature programmed desorption techniques. Specifically, probe molecules such as CO2 and N2O are coadsorbed and used to interrogate interactions in the thin ice layers, mobility and transport. These events are important in interstellar space as well as in heterogeneous chemistry in the atmosphere.