Research efforts in our group revolve around using lasers to initiate and study chemistry. A variety of excimer, dye, Nd:YAG, and Ti:sapphire lasers are employed to investigate problems in areas ranging from gas-phase Doppler spectroscopy to laser-assisted thin-film growth. Mass spectrometers are used extensively for product detection, but materials characterization methods such as x-ray photoelectron spectroscopy (XPS) and scanning Auger microscopy (SAM) are enjoying increasingly popularity within our group. Below, snippets describing several individual project areas are presented.
Doppler Spectroscopy
Photolysis/probe experiments are conducted to measure correlated product state distributions. The experimental approach combines time-of-flight spectroscopy and velocityaligned Doppler spectroscopy in a complementary fashion. Examples include the photolysis of H2Se. Here, correlated product state distributions as a function of photolysis wavelength are being measured.
Laser-Assisted Clustering- and Film Growth
Laser-assisted reactivity in constrained gas expansions involving group III and group V compounds is being studied. For example, NH3 (or ND3) along with trimethylgalliun is introduced into a high vacuum chamber through a customized dual-source pulsed nozzle assembly. Excimer laser output (193 or 248 nm) is focused into the mixing and reaction region of the nozzle source, and laser irradiation of the gas mixture results in the formation of new adducts, TMGa:NH2 or TEGa:NH2, and diverse, higher mass GaN-containing species. Power and wavelength dependence studies are used to reveal the photolytic origins of individual mass spectral features, and the influence of reaction conditions on actual film growth is also being examined.
Site-Specific Chemistry
The focus of our work has been on how H and D atoms can be used as labels to investigate photolytic as well as bimolecular competition between chemically-distinct reactive sites. In the photolysis experiments, the question is not whether H or D is formed. Rather, H and D are used as labels to determine at which site bond cleavage occurs. With an abstracting agent such as Cl, a change in HCl versus DCl production serves as an indicator of the propensity that a particular site has for reactivity . Moreover, the internal energy distributions of the reactive product can be determined through the use of multiphoton ionization.
Laser Ablation and Film Re-Deposition
Here, we combine laser ablation with selective ionization in order to generate and subsequently separate neutral mixtures. For the example of laser ablation (λ = 308 nm) of K4In4Sb4 irradiation of the ablation plume with 248 nm laser light induces selective ionization of potassium. Subsequent electric field extraction leads to macroscopic depletion of potassium from the ablation plume. Quantification via SAM of thin films deposited from such ablation plumes reveals that even without optimization of the spatial overlap between the ablation plume and the ionization laser, the technique leads to ~60% removal of potassium. Extensions to other systems are underway.
