Abstract

It has been suggested that the process of laser induced filamentation can be used in a wide range of applications such as: material processing, controlled electric discharge, and innovative spectroscopic measurements including the remote sensing of pollutants and hazardous materials. During filamentation, emissions from small excited molecular species and atomic states, both neutral and ionic, are readily observed. Among the important excited state species observed in air is the excited triplet state of molecular nitrogen, N₂(C³Π_u)${\mathrm{N}}_2({\mathrm{C}}^3{\mathrm{\Pi }}_{\mathrm{u}})$. A method used to measure the emission properties of the excited states of nitrogen during filamentation is described. These measurements indicate that intersystem crossing from an excited singlet state is the dominate path to produce the triplet state. Alternative pathways, including neutralization of cationic species, were shown to be minor contributors under the laser-induced filamentation conditions of this study.

An empirical extension of the continuum model was used to reproduce the absorption spectrum of the optical probe merocyanine 540 in numerous solvents based on 27 probe-specific parameters in conjunction with the dielectric constant and refractive index of the solvent. The calibrated absorption spectrum of this dye allowed the accurate determination of the dielectric constant and refractive index of the bulk solvent. This study incorporates several binary solvent mixtures in addition to several pure solvents of differing functionality, including protic and aprotic solvents. A single, generally applicable, set of probe-specific parameters is presented. The accuracy of the determined solvent properties using this general set of probe parameters suggests that the influence of specific solvent–solute interactions on the absorption spectrum of this dye must be constant if not insignificant in the range of solvents studied, with the notable exception of water. Copyright © 2004 John Wiley & Sons, Ltd.

The association constants for charge-transfer (CT) complex formation of a series of methylated benzene donors with 1,2,4,5-tetracyanobenzene and tetracyanoethylene as acceptors were measured. In several cases the values determined previously using standard analysis techniques, such as Benesi–Hildebrand or related methods, were shown to be incorrect and a new method for determining the association constants for weak complexes is presented. A systematic error occured in the determination of these constants when standard analysis was carried out on weakly bound complexes. In general, the thermodynamic stabilities have been underestimated and the extinction coefficients for the CT absorptions overestimated. Furthermore, it was demonstrated that the ground-state stabilization of the complexes studied here is due primarily to non-bonded interactions and that the ion-pair contributions are minor in the ground state. A notable exception may be the tetracyanobenzene–hexamethylbenzene complex where preliminary evidence points to a significant contribution of the ion-pair state to the ground-state stability. This study raises significant questions about what is currently known concerning the thermodynamics of CT complexes because much of what is believed may be based on incorrectly determined constants. Copyright © 2000 John Wiley & Sons, Ltd.

Abstract— The evaluation of the equilibrium constants for charge-transfer complex formation has been of interest for five decades. During this time, absorption spectroscopy using Benesi-Hildebrand, or related methods, has been used to obtain the equilibrium constants. These methods require relatively high concentrations of donor or acceptor to be present in solution when weakly bound complexes are studied, conditions that lead to the formation of higher order complexes and inconsistent determinations of these constants. A new method is presented that allows weakly bound charge-transfer complexes to be studied under low concentration conditions and the equilibrium constants to be determined accurately for the first time. Using this method, the equilibrium constant for the formation of 1, 2, 4, 5-tetracyanobenzene/pentamethylbenzene charge-transfer complex was found to be K_CT= 6.8 ± 0.3 M⁻¹ with an extinction coefficient at 400 nm of ε_CT= 150 ± 30cm⁻¹M⁻¹.