Chih-Kai Lin

Dissociation rate of hot benzene

The dissociation rate of benzene and d6-benzene were measured under collision-free condition by multimass ion imaging techniques. The value of 1±0.2×105 s−1 and 5±1×104 s−1 were obtained for benzene and d6-benzene, respectively, with internal energy of 618 kJ/mol. The dissociation rate of benzene with internal energy of 483 kJ/mol was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 3×103 s−1.

Multimass ion imaging detection: Application to photodissociation

A constant momentum mass spectrometer with a two-dimensional ion detector in conjunction with a pulsed vacuum ultraviolet laser is used for the simultaneous measurement of the translational energy distributions of many different fragments. A description of the apparatus and its performance are presented. Preliminary experimental results on the photodissociation of toluene and benzene are given.

Vibrational predissociation spectra and hydrogen-bond topologies of H+(H2O)9–11

Vibrational predissociation spectra of protonated water clusters H+(H2O)n, n = 9-11, are presented. Examination of the spectra in the free-OH stretching region revealed predominance of a single absorption band at approximately 3690 cm(-1) for three-coordinate H2O acting as a double-proton-acceptor/single-proton-donor in the n = 11 cluster. In contrast, the intensity of the absorption band of two-coordinate H2O acting as a single-proton-acceptor/single-proton-donor at approximately 3715 cm(-1) decreases with cluster size, and that of one-coordinate H2O acting as a single-proton-acceptor at approximately 3740 and approximately 3650 cm(-1) diminishes nearly entirely at n > 10 in the spectrum. To deduce the information about cluster temperature, we measured the spontaneous dissociation rates of the cluster ions inside an octopole ion trap and fitted the measured rate constants to an empirical Arrhenius equation. Temperatures in the range of 150 K were estimated for all H+ (H2O)9-11, suggesting that the thermal effect may populate the structures other than the true ground state. The results, combined with previously acquired spectra for H+ (H2O)5-8 (J.-C. Jiang, Y.-S. Wang, H.-C. Chang, S. H. Lin, Y. T. Lee, G. Niedner-Schatteburg and H.-C. Chang, J. Am. Chem. Soc., 2000, 122, 1398) and Monte Carlo simulations with the OSS2 model potential (L. Ojamlie, I. Shavitt and S. J. Singer, J. Chem. Phys., 1998, 109, 5547), show a systematic change in hydrogen-bond topology from tree-like, single-ring, multiple-ring to cage-like isomers (and their mixtures) as the cluster size increases from n = 5 to n = 11.

Theoretical treatments of ultrafast electron transfer from adsorbed dye molecule to semiconductor nanocrystalline surface

In studying ultrafast electron transfer from a dye molecule to a nanosized semiconductor particle, pump-probe experiments are commonly used. In this system the electron transfer (ET) rate is faster than vibrational relaxation so that the ET rate should be described by a single-level rate constant and the probing signal (often in the form of time-resolved spectra) contains the contribution from the dynamics of both population and coherence (i.e., wave packet). In this paper, we shall present the theoretical treatments for femtosecond time-resolved pump-probe experiment and the dynamics of population and coherence by the density matrix method, and the calculation of single-level ET rate constant involved in a pump-probe experiment. As an application, we show the theoretical results using parameters extracted from experiments on a specific dye/semiconductor system.

Calculation of the vibrationally non-relaxed photo-induced electron transfer rate constant in dye-sensitized solar cells

In this paper we shall show how to calculate the single vibronic-level electron-transfer rate constant, which will be compared with the thermal averaged one. To apply the theoretical results to the dye-sensitized solar cells, we use a simple model to describe how we model the final state of the electron-transfer process. Numerical calculations will be performed to demonstrate the theoretical results.

Density matrix method and ultrafast processes

When femto-second (fs) time-resolved experiments are used to study ultrafast processes, quantum beat phenomena are often observed. In this paper, to analyze the fs time-resolved spectra, we will present the density matrix method, a powerful theoretical technique, which describes the dynamics of population and coherence of the system. How to employ it to study the pump-probe experiments and fs ultrafast processes is described. The ππ*→nπ* transition of pyrazine is used as an example to demonstrate the application of the density matrix method. Recently, Suzuki’s group have employed the 22 fs time resolution laser to study the dynamics of the ππ* state of pyrazine. In this case, conical intersection is commonly believed to play an important role in this non-adiabatic process. How to treat the effect of conical intersection on non-adiabatic processes and fs time-resolved spectra is presented. Another important ultrafast process, vibrational relaxation, which takes place in sub-ps and ps range and has never been carefully studied, is treated in this paper. The vibrational relaxation in water dimer is chosen to demonstrate the calculation. It should be noted that the vibrational relaxation of (H2O)2 has not been experimentally studied but it can be accomplished by the pump-probe experiments.