A two-photon absorption cross section measurement in nitric oxide

We report here the measurement of a two-photon absorption cross section for the R₂₂ + S₁₂ (J″ = 9.5) [A ²Σ⁺ (ν′ = 0) - X ²Π (ν′ = 0)] transition in the gamma band system of nitric oxide by measuring the third-order susceptibility using a four-wave mixing technique. A value of σ⁽²⁾ = (1.0 ± 0.6) X 10^?38 πg(2ω₁ ? ω_f) cm⁴ s was obtained.     

Dicyanostilbene-derived two-photon fluorescence probe for free zinc ions in live cells and tissues with a large two-photon action cross section

A novel two-photon fluorescence probe for Zn(2+) derived from dicyanostilbene as a two-photon fluorophore and 4-(pyridine-2-ylmethyl)piperazine as a novel Zn(2+) ligand was developed. The probe shows a 72.5-fold fluorescence enhancement in response to Zn(2+), a large two-photon action cross-section (580 GM), a noncytotoxic effect, and pH insensitivity in the biologically relevant range, and its dissociation constant (K(d)(TP)) is 0.52 ± 0.01 μM. The probe can selectively detect free Zn(2+) ions in live cells for 1500 s or so and in living tissues at a depth of 80-150 μm without interference from other metal ions and the membrane-bound probes.     

The two-photon excitation cross section of 6MAP, a fluorescent adenine analogue

6MAP is a fluorescent analogue of adenine that undergoes Watson-Crick base pairing and base stacking in double-stranded DNA. The one-photon absorption spectrum of 6MAP is characterized by a maximum around 330 nm with moderate quantum yield fluorescence centered at about 420 nm. To take advantage of this probe for confocal and single-molecule microscopy, it would be advantageous to be able to excite the analogue via two photons. We report the first determination of the two-photon excitation cross section and spectrum for 6MAP from 614 to 700 nm. The power dependence of the fluorescence indicates that emission results from the absorption of two photons. The one-photon and two-photon emission line shapes are identical within experimental error. A study of the concentration dependence of the fluorescence yield for one-photon excitation shows no measurable quenching up to about 5 microM. The maximum in the two-photon excitation spectrum gives a two-photon cross section, delta(TPE), of 3.4 +/- 0.1 Goeppert-Mayer (G.M.) at 659 nm, which correlates well with the one-photon absorption maximum. This compares quite favorably with cross sections of various naturally fluorescent biological molecules such as flavins and nicotiamide. In addition, we have also obtained the two-photon-induced fluorescence emission spectrum of quinine sulfate. It is approximately the same as that for one-photon excitation, suggesting that two-photon excitation of quinine sulfate may be used for calibration purposes.     

Fundamental limits of the dispersion of the two-photon absorption cross section

We rigorously apply the sum rules to the sum-over-states expression to calculate the fundamental limits of the dispersion of the two-photon absorption cross section. A comparison of the theory with the data suggests that the truncated sum rules in the three-level model give a reasonable fundamental limit. Furthermore, we posit that the two-photon absorption cross section near the limit must have only three dominant states, so by default, the three-level model is appropriate. This ansatz is supported by a rigorous analytical calculation that the resonant term gets smaller as more states are added. We also find that the contributions of the nonexplicitly resonant terms cannot be neglected when analyzing real molecules with many excited states, even near resonance. However, puzzling as it may be, extrapolating an off-resonant result to resonance using only the resonant term of the three-level model is shown to be consistent with the exact result. In addition, the off-resonant approximation is shown to scale logarithmically when compared with the full three-level model. This scaling can be used to simplify the analysis of measurements. We find that existing molecules are still far from the fundamental limit; so, there is room for improvement. But, reaching the fundamental limit would require precise control of the energy-level spacing, independently of the transition dipole moments-a task that does not appear possible using today's synthetic approaches. So, we present alternative methods that can still lead to substantial improvements which only require the control of the transition moment to the first excited state. While it is best to normalize measured two-photon absorption cross sections to the fundamental limits when comparing molecules, we show that simply dividing by the square of the number of electrons per molecule yields a good metric for comparison.     

Enhancement of two-photon absorption cross section and singlet-oxygen generation in porphyrin-cored star polymers

We report a newly synthesized polymer of a star-shaped porphyrin compound(TPA-FxP) with four oligofluorene arms at its meso positions with the pronounced enhancement of the two-photon properties and the generation of singlet oxygen by utilizing the two-photon excited fluorescence resonance energy transfer.The steady-state spectra and transient triplet-triplet absorption spectra give evidence that the enhanced two-photon absorption cross section results from not only the through-space energy transfer(Frster...     

Variationally stable calculations for molecular systems: polarizabilities and two-photon ionization cross section for the hydrogen molecule

The variationally stable method of Gao and Starace [B. Gao and A. F. Starace, Phys. Rev. Lett. 61, 404 (1988); Phys. Rev. A 39, 4550 (1989)] has been applied for the first time to the study of multiphoton processes in molecular systems. The generalization in theory is presented, as well as the calculation of properties such as the static and dynamic polarizabilities of the hydrogen molecule and the generalized two-photon ionization cross section. The Schwinger variational iterative method [R. R. Lucchese and V. McKoy, Phys. Rev. A 21, 112 (1980)] has been applied in the achievement of the photoelectron wave function, while a Hartree-Fock representation has been used for the target. This research has been motivated by the scarceness of ab initio calculations of molecular multiphoton ionization cross sections in the literature.     

Modified (22pi) smaragdyrins with large two-photon absorption cross section: a structure function correlation

[structure: see text] Two-photon absorption (TPA) cross-section values for a series of 22pi smaragdyrins bearing phenylacetylenylphenyl and [(phenylacetylenyl)phenylacetylenyl]phenyl meso links and their Rh(I) derivatives are reported.     

Enlarged pi-electronic network of a meso-meso, beta-beta, beta-beta triply linked dibenzoporphyrin dimer that exhibits a large two-photon absorption cross section

Enlargement of the pi-electronic network of meso-meso, beta-beta, beta-beta triply linked diporphyrin has been exploited by preparing a corresponding dibenzo-fused porphyrin dimer that exhibits a perturbed absorption spectrum and a large two-photon absorption cross section.     

New cyano-substituted organic dyes containing different electrophilic groups: aggregation-induced emission and large two-photon absorption cross section

Three aggregation-induced emission active dyes (3a–c) were synthesized and their one- and two-photon absorption properties have been investigated. They were all found to be weakly fluorescent in THF solution, while they exhibited dramatic fluorescence enhancement in water/THF mixtures. The solid fluorescence of 3a–c was recorded and their fluorescence quantum efficiency (Φ_F) values were determined to be 8.0%, 8.1%, and 16.4%, respectively. Moreover, the two-photon absorption (2PA) cross-sections (σ) of 3a–c were measured and 3a showed the highest value of 702 GM. The excellent aggregation-induced emission and 2PA properties provide a promising alternative for biophotonic materials.     

Enhancement of a two-photon absorption cross section (TPACS)--design and synthesis of a novel class of photochromic molecules with large TPACS

Two novel photochromic compounds were designed and synthesized; the two-photon absorption (TPA) cross section of both were measured with a nanosecond laser pulse; their TPACS (delta) values are around 25 x 10(-46) cm4 s photon-1 molecule-1 in acetonitrile; the molecular structure of the target compounds have a 3-methyl-1-benzothiophen-2-yl moiety, which can greatly enhance the two-photon absorption cross section.     

Dependence of the two-photon absorption cross section on the conjugation of the phenylacetylene linker in dipolar donor-bridge-acceptor chromophores

The nonlinear optical properties of four isomeric dipolar two-photon chromophores are compared. The chromophores consist of a carbazole electron donor coupled to a naphthalimide electron acceptor by a phenylacetylene bridge. By variation of the connectivity of the bridge at the phenyl groups, four compounds with 0, 1, and 2 meta linkages are synthesized. The linear and nonlinear optical properties of these compounds are measured. Despite similar linear absorption cross sections, the two-photon absorption cross section delta of the all-meta compound is almost a factor of 10 lower than the all-para compound. By taking the detailed molecular conformations into account in order to calculate accurate dipole moment changes, we find that the decrease in delta results largely from the decreased charge transfer ability with increasing number of meta linkages. We find that a two-state model can be used to predict semiquantitatively the observed trend in delta on the basis of the linear optical properties of the molecules. This work illustrates the dramatic effect the ground-state polarizability can have on the nonlinear optical response of organic compounds and also provides a way to quantify the ability of meta linkages to inhibit charge transfer in their ground-state configuration.     

Effective two-photon absorption cross section of heteroaromatic quadrupolar dyes: dependence on measurement technique and laser pulse characteristics

The linear and nonlinear optical properties of the heteroaromatic push-pull-push two-photon absorbing dye N-methyl-2,5-bis[1-(N-methylpyrid-4-yl)ethen-2-yl]-pyrrole ditriflate (PEPEP) are reported. The determination of the two-photon absorption (TPA) cross-section spectrum has been performed with different techniques: femtosecond TPA-white light continuum probe experiments, two-photon-induced fluorescence, and open aperture Z-scan measurements using both nanosecond and femtosecond laser pulses. The measured TPA cross sections and their wavelength dispersion show a marked dependence on the parameters of the laser pulses and on the measurement technique employed. These properties are discussed in terms of the different microscopic mechanisms that can contribute to the multiphoton absorption processes, with different weight depending on the measurement conditions and on the photophysical parameters of the dye.     

Actinide targets for neutron cross section measurements

The Advanced Fuel Cycle Initiative (AFCI) and the Generation IV Reactor Initiative have demonstrated a lack of detailed neutron cross sections for certain “minor” actinides. For some closed-fuel-cycle reactor designs more than 50% of reactivity will, at some point, be derived from “minor” actinides that currently have poorly known or in some cases not measured (n,γ) and (n,f) cross sections. Using a combination of resurrected techniques and new developments, we have made a series of targets including highly enriched ²³⁹Pu, ²⁴⁰Pu, and ²⁴²Pu. Thus far, we have electrodeposited these actinide targets. The chemical purification and electodeposition techniques will be described.     

The photoionization cross section for diatomic CH

Many body perturbation theory is used to calculate an approximate Hartree-Fock photoionization cross section for CH. the results show a large increase at threshold compared to that obtained from unperturbed atomic matrix elements. At higher energies the cross section approaches the atomic value.     

Very large infrared two-photon absorption cross section of asymmetric zinc porphyrin aggregates: Role of intermolecular interaction and donor-acceptor strengths

Very large two-photon absorption (TPA) cross sections at the infrared region have been revealed for J-aggregates of asymmetric zinc porphyrin using quantum-chemical calculation. The TPA properties are evaluated for monomer and aggregates of a series of push-pull porphyrins, whose syntheses are known in the literature. The two-photon absorption cross section can be greatly enhanced by increasing the strengths of the electron donor/acceptor. We also present a quantum-chemical analysis on porphyrin aggregates to understand the role of intermolecular interactions and the relationship between structural and collective nonlinear optical properties. It has been observed that the TPA properties change tremendously as monomers undergo J-aggregation and the magnitudes of TPA cross sections are highly dependent on the nature of aggregates. The importance of our results with respect to the design of photonic and photodynamic therapy materials has been discussed.