Professor Hai-Lung Dai's Research Interests

Energy Transfer and Reaction of Highly Vibrationally Excited Molecules Intramolecular isomerization and collisional deactivation of molecules excited with 10,000-50,000 cm -1 vibrational energy are investigated. The excitation is prepared by electronic excitation followed by internal conversion, or by the stimulated emission pumping technique. IR emission spectra from the excited molecules are recorded by a newly developed nanosecond time-resolved FTIR technique to reveal the energy and structural evolution of the molecules following excitation.

Structure, Spectroscopy and Dynamics of Transient Radicals A new approach for detecting previously unknown vibrational modes of transient radicals has been demostrated. Photodissocaition of precursors produces the desired transient radical with high vibrational excitation. IR emission from these excited species is then detected by nanosecond time-resolved FTIR emission spectroscopy. The vibrational bands, with rotational resolution, allow the determination of the radical structure. Time-resolved IR emission spectra also enable the deduction of energy relaxation and reaction of the excited radicals.

Dynamics and Photochemistry of Molecules Adsorbed on Surfaces The presence of a surface provides several important factors affecting the chemical properties of a molecule. In addition to rapid quenching of molecular excitation, the surface may change reaction energetics and alter the reaction channels, provide new excitation channels through substrate electrons, and align molecular adsorbates to facilitate a particular reaction. All these effects have been identified in our study of laser-induced polymerization of formaldehyde on silver. These unique aspects of surface photochemistry and explored in several different molecular systems on metal and oxide surfaces.

Nonlinear Optical Probe of Surfaces/Interfaces and Ultrathin Films Nonlinear optical techniques based on second harmonic generation and transient grating scattering are developed for probing the structure, kinetics and dynamics of a variety of systems involving a surface or interface: ultrathin molecular films, the solid-liquid interface in colloids, and metal or semiconductor surfaces. For example the glass transition temperature of ice has been determined by second harmonic Raleigh scattering, the surfactant adsorption onto microparticles in colloids can be quantitatively characterized by second harmonic generation, ultrafast carrier dynamics at a silicon surface are revealed by transient grating scattering.