Photolysis Dynamics Laser Chemistry Lab

In one-photon dissociation of propionyl chloride at 248 nm, time-resolved Fourier-transform infrared emission spectroscopy is used to detect the fragments of HCl and CO in the presence of Ar. The inert gas Ar plays a role to enhance the internal conversion. The time-dependence of high-resolution HCl spectra yields a bimodal rotational distribution in the early stage. The total rotational and vibrational energy partitioned in HCl are evaluated to be 1.7 ± 0.3 and 8.8 ± 1.9 kcal/mol, respectively. The CO appearance indicates that HCl may be eliminated through a five-center mechanism accompanied with three-body dissociation of C2H2, HCl, and CO. A four-center mechanism forming HCl and CH3CHCO also contributes to the HCl fragment with a feature of rotational bimodality. However, the probability for the HCl contribution from the hot Cl reaction is negligible. The reaction with CH4 is carried out to evaluate the HCl and Cl elimination rate constants. © 2010 Elsevier B.V. All rights reserved.

Kinetics of photoinduced electron transfer (ET) from oxazine 1 dye to TiO2 nanoparticles (NPs) surface is studied at a single molecule level by using confocal fluorescence microscopy. Upon irradiation with a pulsed laser at 630 nm, the fluorescence lifetimes sampled among 100 different dye molecules are determined to yield an average lifetime of 2.9 ± 0.3 ns, which is close to the value of 3.0 ± 0.6 ns measured on the bare coverslip. The lifetime proximity suggests that most interfacial electron transfer (IFET) processes for the current system are inefficient, probably caused by physisorption between dye and the TiO2 film. However, there might exist some molecules which are quenched before fluorescing and fail to be detected. With the aid of autocorrelation analysis under a three-level energy system, the IFET kinetics of single dye molecules in the conduction band of TiO2 NPs is evaluated to be (1.0 ± 0.1)×104 s-1 averaged over 100 single molecules and the back ET rate constant is 4.7 ± 0.9 s-1. When a thicker TiO2 film is substituted, the resultant kinetic data do not make a significant difference. The trend of IFET efficacy agrees with the method of fluorescence lifetime measurements. The obtained forward ET rate constants are about ten times smaller than the photovoltage response measured in an assembled dye-sensitized solar cell. The discrepancy is discussed. The inhomogeneous and fluctuation characters for the IFET process are attributed to microenvironment variation for each single molecule. The obtained ET rates are much slower than the fluorescence relaxation. Such a small ET quantum yield is yet feasibly detectable at a single molecule level. © 2010 American Chemical Society.