T. J. Chuang

Theory of Fluorescence Depolarization by Anisotropic Rotational Diffusion

The general expressions for the time‐dependent fluorescence depolarization caused by anisotropic rotation diffusion have been obtained. It is shown that after an instantaneous exciting light pulse, the parallel and perpendicular components of fluorescence can have a maximum of six exponential decays and the difference of these two components a maximum of five decays. The present results differ from those of previous studies and the differences are discussed.

Studies of effects of hydrogen bonding on orientational relaxation using picosecond light pulses

Abstract Orientational relaxation of rhodamine 6G in a series of normal alcohols, ethylene glycol, chloroform and formamide has been studied using picosecond light pulses. The effects of hydrogen bonding and the structure of the liquids on the molecular rotational motion are examined.

Picosecond studies of the cage effect and collision induced predissociation of iodine in liquids

Abstract In this work we report the first time dependent study of the role of the cage effect in the geminate recombination of iodine atoms produced by photodissociation of I 2 . We have also directly observed the rate of the collision induced predissociation of the excited iodine molecule, I 2 (B 3 Π o+u ).

Multiple photon excited SF6 interaction with silicon surfaces

Infrared laser induced SF6–silicon interactions have been studied and the surface reaction yields have been determined as a function of the laser frequency, the laser intensity, and the gas pressure in both perpendicular and parallel beam incidences on the solid surfaces. The results clearly show that vibrationally excited SF6 molecules promoted by CO2 laser pulses are very reactive to silicon, particularly when the solid is simultaneously exposed to the intense ir radiation. The laser excitation of the Si substrate alone cannot cause the heterogeneous reaction to occur. The present gas–solid system thus provides an example which clearly establishes the direct correlation between surface reactivity and vibrational activation. Additional experimental measurements also demonstrate that the thermal fluorine atoms generated by SF6 multiple photon dissociation at high laser intensities can react with silicon to form volatile product. The study thus provides further insight into the silicon–fluorine reaction dy...

The role of cavity sites in surface-enhanced Raman scattering

Abstract Thermal desorption spectra (TDS) of pyridine from silver films deposited in ultra high vacuum are reported. Marked differences in the TDS are seen depending on the deposition conditions and the thermal history of the films, which have been correlated with surface-enhanced Raman scattering (SERS). These results as well as some of the observations in electrochemical systems are discussed in light of the recent Xe probe analysis carried out by Albano et al.

Infrared laser radiation effects on XeF2 interaction with silicon

The presence of the laser radiation field on a silicon surface has been shown to influence the XeF2–silicon reaction rate. It is observed that the radiation enhancement in the surface reaction yield depends on the laser power, but is independent of the CO2 laser wavelength or the partial pressure of the active gas in the pressure range of 10−4–10−5 Torr. A model based on the variation of the sticking probability of XeF2 on Si as a function of fluorine surface coverage modified by the laser radiation is proposed to account for the experimentally determined characteristics of the gas–solid system. The results show that the laser‐enhanced chemical process may not be due entirely to the thermal effect. Furthermore, the study provides a better understanding for the laser induced interaction between SF6 and silicon, and for the fluorine–silicon surface chemistry in general. The radiation effect by the infrared photons is also compared with those induced by energetic electrons and ions.

Studies of excited state charge‐transfer interactions with picosecond laser pulses

The methods of picosecond laser photolysis have been employed to study the inter− and intramolecular charge−transfer interaction between excited anthracene and N,N−diethylaniline in polar as well as nonpolar solvents. The formulation of reaction kinetics coupled with molecular reorientational motion is developed. The experimental results, at low diethylaniline concentrations, are analyzed in terms of diffusion model and Noyes molecular pair model. The transient behavior due to instantaneous flux in early time regions is clearly evident. The values of critical intermolecular distance and primary rate constant for electron transfer are determined and are consistent for all solutions studies. In the high diethylaniline concentration region, analysis of orientational relaxation times and the rate of charge−transfer reactions clearly reveals the dynamic nature of the interacting processes.

Quantitative study on the photoexcitation process of SiF4 at 49–120 nm

The photoabsorption and fluorescence cross sections of SiF4 were measured in the 49–120 nm region using synchrotron radiation as a light source. Absorption bands were tentatively assigned to Rydberg states and their oscillator strengths were determined. The fluorescence was dispersed to identify the emitting species. In the 49–58 nm region, the emitter is attributed to the excited SiF+4 (D) ion. In the 92–98 nm region, the fluorescence is likely produced by the excited SiF*3 radical. Photodissociation process of SiF4 in the extreme ultraviolet region is discussed.

Laser-induced CH2 and C2H4 formation and desorption from CH2I2 adsorbed on Al surfaces

Abstract The photochemical and thermal desorption behavior of CH2I2 adsorbed on an Al film has been studied by X-ray photoemission, thermal desorption and time-of-flight mass spectrometry with both UV and visible lasers. The products and the energetics of surface reactions are determined as functions of surface coverage and laser fluence. It is found that CH2 radicals and C2H4 molecules can be produced and desorbed from the surface when the adsorbate is electronically excited. In addition, laser-induced transient heating of Al substrate can lead to a decomposition reaction very different from the conventional thermal desorption.

Emission spectra of SiF3

Emission spectra of SiF3 radical were observed from photodissociative excitation of SiF4 at 99.1, 95.5, and 92.3 nm. The spectra show a broad visible band in the 350–800 nm region, a UV band in 290–340 nm, and a weak band in 240–280 nm. The visible band resembles the chemiluminescence spectra observed from etching of silicon by F or XeF2.