Laser photoexcitation

Triplet-triplet annihilation (TTA), i.e., deactivation of triplet molecules as a result of their interaction, is one of the main pathways of triplet state decay in solutions, crystals and the gas-phase. TTA may become the determining pathway of triplet state deactivation under conditions of high concentrations of triplet-state molecules resulting in particular from powerful laser photoexcitation. 

The initiation step is laser photoexcitation of the molecule to yield a caged geminate pair of atomic iodine radicals. The two competing steps open to the caged pair are 1) separative diffusion to yield atomic iodine radicals and 2) recombination to give molecular iodine. 

Laser photoexcitation of cesium atoms to the 7P state in the presence of hydrogen gas leads to the formation of cesium hydride by a process which is shown to be indirect. The concentration of the reactants is studied under various experimental conditions using a laser fluorescence technique. The rate coefficient of CsH formation is found to be proportional to the Cs(6S) density suggesting a two step process with the formation of an intermediate species which reacts with ground state cesium atoms. 

The Cs(7P) population is established by a balance between laser photoexcitation and quenching plus excitation transfer by colli-... 

A similar mechanism applies to covalently bound B[a]PDE-DNA adducts and it has been shown that the 355-nm laser photoexcitation of the pyrenyl residues can be used to inject holes into double-stranded DNA at the site of modified guanine residues [47]. 

Recently, stepwise laser photoexcitation and ionization has been used to identify Rydberg series in atomic uranium. 

Pulse Radiolysis Measurements of Intramolecular Electron Transfer with Comparisons to Laser Photoexcitation... 

This chapter will discuss the abilities of pulse radiolysis and laser photoexcitation to contribute to an understanding of the fundamental and important process of electron transfer (ET). We begin with a brief discussion of ET reactions. 

V. Like the classical Marcus theory, equation 1 predicts an inverted region, although the decrease of rates for highly exoergic reactions may be less pronounced than in the Marcus theory. The quantum mechanical theory also predicts modifications of the effects of temperature and polarity. Some principal features of these predictions have been verified by experiments using both pulse radiolysis and laser photoexcitation. 

This chapter will begin by delineating the capabilities of the two techniques, radiolysis and laser photoexcitation, for examination of factors that regulate ET rates. We will begin with a list summarizing the capabilities of the two techniques. Descriptions will be supplemented by reference to published work and examples from pulse radiolysis experiments in our laboratory, along with a brief description of the experimental setup. In this discussion we shall see... 

Despite the many superior features of laser photoexcitation, pulse radiolysis has played a central and decisive role in developing our understanding of ET reactions. Figures 4 and 5 illustrate two of the reasons for the importance of pulse radiolysis. 

Girvan HM, Heyes DJ, Scrutton NS, Munro AW (2007) Laser photoexcitation of NAD(P)H induces reduction of P450 BM3 heme domain on the microsecond time scale. J Am Chem Soc 129 6647-6653... 

Brownsword, R.A., Schmiechen, P., Volpp, H.-R., Upadhyaya, H.P., Jung, Y.J., Jung, K.-H. Chlorine atom formation dynamics in the dissociation of CH3CF2CI (HCFC-142b) after UV laser photoexcitation. J. Chem. Phys. 110, 11823-11829 (1999)... 

Fig. 11.5 Various rates of laser photoexcitation or deposition of light energy into a molecule can provide various types of laser-induced chemistry from mode-selective photochemistry for very fast, subpicosecond rates of excitation to ordinary thermal chemistry for low-rate, millisecond excitation. The real values of relaxation times depend on the density of the irradiated substance.

---