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The synthesis, linear photophysical, two-photon absorption (2PA), femtosecond transient absorption, and superfluorescence properties of a new symmetrical squaraine derivative (1) are reported. Steady-state linear spectral and photochemical properties, fluorescence lifetimes, and excitation anisotropy of 1 were investigated in various organic solvents. High fluorescence quantum yields (≈0.7) and very high photostability (photodecomposition quantum yields ≈10(-6)-10(-8)) were observed. An open-aperture Z-scan method was used to obtain 2PA spectra of 1 over a broad spectral range (maximum 2PA cross section ≈1000 GM). Excited-state absorption (ESA) and gain was observed by femtosecond transient absorption spectroscopy, in which both reached a maximum at approximately 500 fs. Squaraine 1 exhibits efficient superfluorescence. The quantum chemical study of 1 revealed the simulated vibronic nature of the 1PA and 2PA spectra were in good agreement with experimental data; this may provide the ability to predict potential advanced photonic materials.

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We investigate the double K-shell ionization of heliumlike ions caused by the absorption of a single photon with energies being much smaller than the rest energy of an electron. In the near-threshold region, differential and total cross sections of the process are calculated for light ions, taking into account the leading orders of the 1/Z and ␣Z expansions. QED perturbation theory with respect to the parameter 1/Z exhibits a fast convergence in the entire nonrelativistic domain for moderate nuclear charge numbers Zу2. Going beyond the electric dipole approximation leads to a forward/backward asymmetry in the angular distributions for the ejected electrons with respect to the incident photon beam. A comparison of theoretical predictions for the ratio of double-to-single photoionization cross sections with available experimental data for a number of neutral atoms is also presented.

ABSTRACT:Certain organic compounds possess the ability to change color under the influence of light, called photochromism. This change is due to ultrafast chemical transition from open to closed ring isomers (photocyclization), which can be used for optical data storage and photoswitching applications. These applications require minimization of the irreversible photodegradation of the material, called photofatigue. This property is related to the chemical rate of byproduct formation. We use density functional theory methods to predict the mechanism and activation barriers to the byproduct formation for 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene in order to estimate its fatigue resistance. We also explain higher fatigue resistance for its methylated derivative. The methods used in this study may become a part of rational design strategy for the new photochromic materials.

Recombination of H + 3 with electrons was studied in a low temperature plasma in helium. The plasma recombination rate is driven by two body, H + 3 + e − , and three-body, H + 3 + e − + He, processes with the rate coefficients 7.5 × 10 −8 cm 3 s −1 and 2.8 × 10 −25 cm 6 s −1 correspondingly at 260 K. The two-body rate coefficient is in excellent agreement with results from storage ring experiments and theoretical calculations. We suggest that the three-body recombination involves formation of highly excited Rydberg neutral H3 followed by an l-or m-changing collision with He. Plasma electron spectroscopy indicates the presence of H3.

The degenerate two-photon absorption (2PA) spectra of several fluorene-based photosensitizers (PS) in solution were obtained over a broad spectral range (460-880 nm) by open aperture Z-scan and two-photon fluorescence methods under either picosecond or femtosecond excitation, respectively. A maximum 2PA cross section of ca. 300 GM was observed for the photosensitizers containing a benzothiazole substituent in the fluorenyl 7-position. The electronic structures and 2PA properties of these PS were analyzed using a time-dependent density functional theory method, resulting in reasonably good agreement between experimental and theoretical data.

Conjugated organic molecules with photochromic properties are being extensively studied as prospective optical switching and data storage materials. Among different photochromic compounds, diarylethenes demonstrate thermal stability, fatigue resistance, and high quantum yield. The mechanism of photoswitching in diarylethenes involves a symmetry-allowed conrotatory electrocyclic reaction, initiated by UV light. Replacement of one UV photon with two near-IR ones would offer a number of practical advantages, including drastic increase in storage capacity via three-dimensional multilayer design. For this purpose we designed a prototype molecule with a two-photon absorbing (2PA) pendant substituent, attached to the photochromic diarylethene moiety. However, this molecule was experimentally shown to have lost the photoswitching properties. We analyze reasons for this loss using quantum chemistry tools. Analysis of the nodal structure of the frontier Kohn-Sham orbitals, allowed us to trace the route of the problem to the lone pair orbital of the 2PA substituent falling within the HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of the photoreactive diarylethene moiety. We suggest a chemical modification of the 2PA substituent in order to restore the order of the orbitals. Potential energy plots along the reaction coordinate at the M05-2X/6-31G* theory level for the prototype 2PA photochromic molecule before and after the modification confirm the predictive capability of the proposed orbital approach. The Slater transition state method was used to obtain geometries along the reaction pathway by the constrained optimization of excited states, whereas potential energy curves were plotted using the recently proposed (Mikhailov, I. A.; Tafur, S.; Masunov, A. E. Phys. Rev. A 2008, 77 (1), 012510) a posteriori Tamm-Dancoff approximation to the time-dependent density functional theory in second order of the external field. We show that this combination is able to produce accurate potential surfaces for 1B and 2A excited states, as compared to available experimental data and results of high-level multireference wave function theory methods.