Shammai Speiser

Novel aspects of intermolecular and intramolecular electronic energy transfer in solution

Abstract Intermolecular and intramolecular singlet—singlet electronic energy transfer in solution is discussed. Special attention is given to novel developments such as temperature-dependent long-range dipole—dipole interactions, dual fluorescence resulting from short-range intramolecular transfer, structure determination by studies of energy transfer, the involvement of high-lying electronic states and dye laser operation using donor—acceptor pairs.

Intramolecular electronic energy transfer via exchange interaction in bichromophoric molecules

Abstract A theory is presented for intramolecular electronic energy transfer in bichromophoric molecules. Expressions are given for the donor moiety fluorescence (phosphorescence) decay and for its fluorescence (phosphorescence) quantum yield in terms of the average distance between the donor and acceptor moieties and the donor—acceptor bridge flexibility. Comparison with available experimental data supports the predictions of the analysis.

On the possibility of observing photochemical reactions induced by multiphoton absorption

Abstract Focusing of a laser beam (either through self-focusing or by lenses) may enhance multiphoton induced photochemical processes considerably (from 200 – 500 fold for a three-photon process). Above threshold power, the probability for multiphoton dissociation increases with the order of the process. This might be a pathway for the production of plasma in dense media.

On the origin of matrix elements for electronic excitation (energy) transfer

The origin of electronic energy transfer (EET) between two chromophores (D and A) is explored further for several molecular situations that may be encountered in experiment—namely, nonoverlapping active‐space orbitals of the D and A chromophores, forbidden electronic excitations for both chromophores, and an allowed and a forbidden electronic excitation for the D and A chromophores, respectively. The theory is illustrated via the results of calculations of the EET matrix elements for model systems with both four–eight active‐space electrons and all of the electrons included explicitly. In each case, it is found that any overlap contribution to these matrix elements is associated much more with charge‐transfer and penetration terms rather than it is with the Dexter exchange integral. The calculated magnitude of the latter integral is always smaller than that of the Coulomb integral.

Computer simulation of an energy transfer dye laser (ETDL)

Abstract Computer calculations of an energy transfer dye laser (ETDL) performance are presented. It is shown that the ETDL output can be manipulated by varying the donor or acceptor concentrations. Experiments done on the anthracene-perylene ETDL are in agreement with the predictions of the present calculation.

Laser‐Induced Photolysis of Iodoform

Iodoform dissolved in various solvents was irradiated with a Q‐switched ruby laser. The two‐photon photolysis was studied by measuring the concentration of the liberated iodine. Different two‐photon absorption cross sections were obtained for iodoform in different solvents. The major solvent effect was found to be due to the self‐focusing properties of the medium. After appropriate corrections for this effect, good agreement was obtained between measured and calculated cross sections. It was also found that the self‐focusing length of a liquid could be determined “chemically” from the degree of photolysis.

Photoquenching III. Analysis of the dependence of pulsed-laser-pumped dye laser performance on pumping conditions and on the dye molecular characteristics

Abstract Computer calculations are presented for the study of pulsed-laser-pumped dye laser (PLPDL) systems. A set of coupled differential equations for the population of the ground and excited siglet states of the dye medium and for the transient dye laser intensity are solved for various combinations of dye parameters and pumping conditions to yield the gain function and the dye laser intensity profile. It is shown that photoquenching effects due to the absorption of the exciting pulse by the lasing state of the dye and to the self absorption of dye Liser light by the dye medium determine the PLPDL performance. A delay time depending on the pumping intensity is predicted for dye systems exhibiting significant photoquenching. The efficiency of a PLPDL system will generally reach an optimum value for the range of pumping intensities employed.

Photoquenching II. Pulsed-laser-pumped dye laser systems

Abstract Kinetic analysis of a pulsed-laser-pumped dye laser system is presented. Photoquenching effects arising from absorption of the pump light by the excited singlet state are included in the analysis. Triplet state formation is neglected in view of the short pump pulse duration. The calculated gain function is compared with experiments in which dye laser gain was measured as a function of the pumping laser intensity. It is shown that proper description of pulsed-laser-pumped dye laser systems should include photoquenching effects. In these cases the gain is not saturated at high pump intensities and instead there exists an optinum pump power which depends on the dye molecular characteristics.

Calculation of the exchange integral for short range intramolecular electronic energy transfer in bichromophoric molecules

A novel calculation of the exchange integral used for describing short range electronic energy transfer (EET) is presented. The calculation is applied to describe intramolecular EET in specially designed bichromophoric molecules. The results indicate that the simple Dexter exponential distance dependence, which is usually assumed to account for short range exchange interaction is valid in some limited cases. However, a more elaborate approach, such as presented here, is required in order to fully describe the molecular geometry effects on short range intramolecular EET.

Spatial light modulation by nonlinear absorbers

A novel spatial light modulator (SLM) based on nonlinear absorption of light by excited molecules is suggested. A kinetic analysis is applied to calculate the relative populations of molecular levels in singlet and triplet manifolds. It is shown that for many actual cases a probe light can be modulated by propagating through a medium excited by another light source. The dependence of the molecular SLM on the photophysical parameters of the SLM medium is discussed.