Allen J. Twarowski

Multiphoton absorption spectra using thermal blooming: I. Theory

Abstract A theoretical treatment is presented which describes the formation of a thermal lens produced by pulsed laser excitation through multiphoton absorption processes. The effect of the thermal lens on a monitoring laser beam is discussed. Estimates are given showing that it is possible to obtain two-photon absorption spectra by this technique using commercially available pulsed dye lasers.

Multiphoton absorption spectra using thermal blooming: II. Two-photon spectrum of benzene

Abstract The two-photon absorption spectrum of benzene obtained from a thermal blooming experiment is presented. The feasibility of using thermal blooming to obtain two-photon spectra, discussed in the preceding paper, is thus verified. The dependence of the thermal blooming signal on laser energy and the time behavior of the thermal lens are also shown to agree with theoretical predictions. The two-photon spectrum of benzene enables us to locate the 1B2u. 1B1u, and 1E2g states. We thus definitively establish the state ordering in benzene as 1B2u

A search for a low-lying excited1 a state in 1,3,5-hexatriene

Abstract The two-photon absorption spectrum of 1,3,5-hexatriene has been investigated using thermal blooming techniques. The spectrum probes excited states lying 4.1 eV to 6.5 eV above the ground state. The investigation does not reveal the excited 1 A − g state predicted, by PPP calculations with single plus double excitation configuration interaction, to lie in this energy range. Calculations of two-photon cross sections indicate that the two-photon transition to this low-lying 1 A − g state would be weak in spite of the fact that the transition is symmetry allowed.

Excited state absorption spectroscopy and state ordering in polyenes. II. α,ω‐diphenylpolyenes

Calculated and observed excited singlet state absorption (Sn←S1) spectra of a series of diphenylpolyenes are presented. In diphenyloctatetraene and diphenylhexatriene, the S1 state is assigned as an 1Ag state, in agreement with results from two‐photon spectroscopic studies. In diphenylbutadiene, we assign the S1 state as a 1Bu, although two‐photon studies have indicated that 1Ag state lies slightly below the 1Bu. It appears that a 1Ag state is the lowest excited state of diphenylbutadiene in its ground state geometry, but when the excited states relax to their equilibrium configurations, the 1Bu becomes the S1. Good agreement between the PPP–CI calculations and experimental Sn←S1 spectra demonstrates the potential usefulness of this technique in assigning ππ* excited states of large molecules.

The photophysics of gas phase europium chelates. I. Temperature dependence of luminescence

Luminescence lifetimes for two gas europium chelate complexes have been measured as a function of gas temperature. Following ligand excitation the rate constant for Eu(+3) 5D0→7F2 in both molecules shows an Arrhenius temperature dependence. An activation energy of 5100 cm−1 is obtained for europium (3+) tris‐1, 1, 1, 2, 2, 3, 3‐heptafluoro‐7, 7‐dimethyl‐4, 6,‐octanedione, Eu(fod)3, and 4100 cm−1 is obtained for the activation energy of europium (3+) tris‐2, 2, 6, 6,‐tetramethyl‐3, 5‐heptanedione, Eu(thd)3. The quantum yield for production of molecules in the luminescing Eu(+3) 5D0 state is found to be independent of temperature when either Eu(thd)3 or Eu(fod)3 is excited in the ultraviolet absorption bands of the ligands.

Thiocyanate quenching of Eu(hfa)3 luminescence

Abstract Thiocyanate quenching of europium (III) hexaflouroacetylacetonate (Eu(hfa)3) has been studied. Quantum yield and kinetic measurements give evidence that a charge transfer mechanism previously proposed for the quenching process is not the major quenching process. While some quenching takes place at the hfa ligand the majority of quenching takes place at the europium (III) 5D1 and 5D0 states.

The excited singlet state absorption spectrum of 1,4-diphenylnaphthalene

Abstract The excited singlet state spectrum of 1,4-diphenylnaphthalene (DPN) has been measured and analyzed in terms of theoretical calculations. The results show that there is significant geometrical rearrangement in DPN upon excitation and that consideration of this change in geometry is important in calculating the excited state spectrum. A new method is also suggested for selecting the configurations to be used in the excited state spectral calculation. Calculations using an expanded set of configurations (S+S*) provide a more consistent analysis of the experimental spectrum than those using only singly excited configurations (S).

Photokinetics of gas phase europium(III) chelated with hexafluoroacetylacetonate

Abstract The europium chelate salt, triethylammonium tetrakis-hexafluoroacetylacetone europium(III), was evaporated into the gas phase and the luminescence properties of the gas phase material were examined. The rate of decay of luminescence of the gas phase europium chelate was observed to increase with temperature. An activation energy of 2650 cm −1 was measured for the barrier to non-radiative decay from the 5 D 0 state. The quantum yield for formation of the 5 D 0 state showed a slight temperature dependence at temperatures above 220°C.