R. Naskrecki

An ultrafast excited state intramolecular proton transfer (ESPIT) and photochromism of salicylideneaniline (SA) and its “double” analogue salicylaldehyde azine (SAA). A controversial case

Two simple, structurally related photochromic Schiff bases, salicylideneaniline (SA) and salicylaldehyde azine (SAA) were studied in femto– and picosecond time domains. In both systems an ultrafast excited state intramolecular proton transfer (ESIPT) reaction was stated with the characteristic time below 50 fs. For SA this result is in contrast to the recent data published by Mitra and Tamai (S. Mitra and N. Tamai, Chem. Phys. Lett., 1998, 282, 391; S. Mitra and N. Tamai, Chem. Phys., 1999, 246, 463; S. Mitra and N. Tamai, Phys. Chem. Chem. Phys., 2003, 5, 4647), reporting on the corresponding time as long as 200–300 fs. The kinetics of decay of keto-tautomers in S1 states was followed by the transient absorption (410 nm and 470 nm for SA and SAA, respectively) and stimulated emission bands. About 10–30% of excited molecules give birth to the long-lived ground states of photochromic forms.

Mechanism and deactivation kinetics of S2-xanthione in acetonitrile, a quenching solvent, and of S2-exciplex measured by pico- and femtosecond laser spectroscopy

Abstract The Letter discusses the mechanism and dynamics of intra- and intermolecular electronic relaxation of the S 2 state of a xanthione molecule in acetonitrile, a quenching solvent, as well as the most important properties of the S 2 -exciplex formed upon the quenching process. Despite very efficient quenching of S 2 -XT and other thiones it has not been possible so far to observe the S 2 -exciplex. Therefore transient absorption system with 120 fs resolution was successfully applied for this purpose: the S 2 -exciplex was identified for the first time, its lifetime (∼30 ps) and transient absorption spectrum were determined as well as the mechanism of its formation and decay. Moreover, the authors observed the presence of a very fast intramolecular electronic relaxation process (S 2 v >0 →S 0 ) during S 2 -state deactivation which competes with intramolecular vibrational redistribution in the S 2 -state.

Femtosecond absorption and emission spectroscopy of the DCM laser dye

Abstract The results of a femtosecond laser investigation of the photophysical and photochemical properties of the DCM styryl laser dye molecule are reported. Femtosecond pump-probe experiments (absorption/gain spectroscopy using a white light continuum and up-conversion fluorescence spectroscopy) show a red shift of the gain and emission spectra and a blue shift of the S 1 → S N absorption band only due to a fast reorganization of the solvent cage around the highly polar fluorescent first singlet excited state of DCM. This first complete study (spontaneous emission, gain and absorption) of DCM with a pump-probe intercorrelation function around 300 fs (FWHM) demonstrates that the hypothesis of a transition from a locally excited (LE) to a twisted intramolecular charge transfer (TICT) state is not valid for DCM.

Implicit motor learning is impaired in strabismic adults.

Binocular vision disorders (BVD) are quite common in subjects with cerebellar dysfunctions. Also individuals with strabismus often suffer from many motor deficits, such as impaired body balance and walking. It is known that the cerebellum is necessary to maintain proper body posture but also to learn motor skills. It is conceivable that subjects with BVD would also have deficits in procedural (implicit) motor learning, one of the primary cerebellar functions. The primary aim of this study was to explore motor learning abilities in subjects with BVD (strabismic group, SG). Modified versions of a single reaction time task were used in the scheme proposed by Molinari et al. in 1997. A set of three different tasks (Experiment 1) were performed under dominant eye viewing to investigate (a) procedural (implicit) motor learning, (b) declarative (explicit) learning, and (c) simple stimulus-response associative learning. Because each task examined different aspects of motor learning abilities, it could be revealed which motor learning pathway is impaired in SG. Results showed that the SG had slower reaction times in all three tasks and demonstrated poor implicit motor learning ability compared to controls. To verify if these results were caused by reduced binocular vision or cerebellar deficits, per se, a nonstrabismic binocular anomalies group (NSG) was introduced, and all the same tests were performed (Experiment 2). These results revealed that there were no differences between the NSG and the control group with good binocularity. To conclude, the poor procedural learning ability and slower reaction times in strabismic subjects should not be explained as an effect of incomplete binocular vision that influences the maturity of the visual cortex and transformation of visual information into a motor program because binocular anomaly individuals without strabismus have motor learning abilities close to the controls. Some cerebellar deficits appear to be the origin of observed anomalies.

The influence of the excitation geometry on the temporal resolution in femtosecond pump-probe experiments

Abstract The influence of the intersection angle between pump and probe beams on the temporal resolution in time-resolved pump–probe experiments was investigated. The calculations were performed for the Gaussian temporal shape of the pulses and for both, rectangular and Gaussian spatial energy distribution of the beams. The results were experimentally verified in the transient absorption measurements with femtosecond resolution.

Femtosecond laser studies of the DCM push-pull molecule

We report the results of our femtosecond laser study of the photophysical and photochemical properties of the DCM styrenic dye. Our femtosecond pump-probe experiments using a white light continuum show a red shift of the gain spectrum and a blue shift of the S1 yields Sn absorption band due to a fast reorganization of the solvent cage around the highly polar fluorescent first singlet excited state. There is no evidence of any locally excited (LE) to twisted intramolecular charge transfer (TICT) state transition.

Rayleigh light scattering in molecular crystals of pivalic acid and nitro-t-butane

Abstract Two representatives of molecular crystals exhibiting plastic phase: pivalic acid (CH3)3CCOOH and nitro-t-butane (CH3)3CNO2 were studied. For these crystals the polarized and depolarized components of the scattered light were measured in the temperature range from the plastic to liquid phase and the depolarization degrees were determined. In both phases the square of effective anisotropy of optical polarizability Γ2 and the parameter of angular correlations JA were found from the measurements of the depolarized component Rvh.

Unstable Binocular Fixation Affects Reaction Times But Not Implicit Motor Learning in Dyslexia

Purpose Individuals with developmental dyslexia suffer not only from reading problems as more general motor deficits can also be observed in this patient group. Both psychometric clinical tests and objective eyetracking methods suggest that unstable binocular fixation may contribute to reading problems. Because binocular instability may cause poor eye-hand coordination and impair motor control, the primary aim of this study was to explore in dyslexic subjects the influence of unstable binocular fixation on reaction times (RTs) and implicit motor learning (IML), which is one of the fundamental cerebellar functions. Methods Fixation disparity (FD) and instability of FD were assessed subjectively using the Wesson card and a modified Mallett test. A modified version of the Serial Reaction Time Task (SRTT) was used to measure the RTs and IML skills. The results for the dyslexic group (DG), which included 29 adult subjects (15 were tested binocularly, DGbin; 14 were tested monocularly, DGmono), were compared with data from the control group (CG), which consisted of 30 age-matched nondyslexic subjects (15 tested binocularly, CGbin; and the other 15 tested monocularly, CGmono). Results The results indicated that the DG showed poorer binocular stability and longer RTs in the groups tested binocularly (RTs: 534 vs. 411 ms for DGbin and CGbin, respectively; P < 0.001) as compared with the groups examined monocularly (RTs: 431 vs. 424 ms for DGmono and CGmono, respectively; P = 0.996). The DG also exhibited impaired IML when compared with the CG (EFIML: 25 vs. 50 ms for DG and CG, respectively; P = 0.012). Conclusions Unstable binocularity in dyslexia may affect RTs but was not related to poor IML skills. Impaired IML in dyslexia was independent of the viewing conditions (monocular versus binocular) and may be related to cerebellar deficits.

Ultrashort Laser Pulses in Physics and Chemistry

Study of physical and chemical events accompanying light‐matter interaction in pico‐ and femtosecond time scale have become possible with the use of ultrashort laser pulses. With the progress in generation of ultrashort laser pulses, the ultrafast optical spectroscopy, as a tool for dynamic study, is still evolving rapidly.

Transient absorption studies of the reaction center of the photosynthetic bacterium Rb. sphaeroides R-26 in the blue spectral range

Transient absorption spectroscopy (with 120 fs time resolution) was applied to measure at room temperature the electron transfer in the reaction center of the purple photosynthetic bacterium Rb. sphaeroides R-26. Absorption changes were measured in the range from 400 nm up to 680 nm, after excitation with a laser pulse of 80 fs duration at 800 nm. The absorbance changes characteristic of the excited state of the primary donor and extending over the whole spectral range investigated, appeared within 120 fs. They gave rise to the bleaching of the Qx absorption band of bacteriochlorophyll at 600 nm, increased further by electron transfer to bacteriopheophytin in ~ 3 ps. Photoreduction of the bacteriopheophytin acceptor detected at 546 nm and 670 nm proceeded with the same time constant. Relatively the largest were multiphase absorbance changes in the blue spectral range between 415 nm and 450 nm. Apart from the immediate absorbance increase due to excitation of the primary donor, another fast increasing phase was detected, characterized by a wavelength dependent time constant ? from ~ 5.5 ps at 415 nm to ~ 1.9 ps at 450 nm. Both the photooxidized primary donor and photoreduced bacteriopheophytin contributed to the amplitude of this phase. The electron transfer from the reduced bacteriopheophytin to a quinone acceptor was observed as a decrease in the intensity of the transient absorption bands at about 422 nm and at 670 nm, with disappearance of the bleaching at 546 nm and increase of the bleaching at 600 nm, all in ~ 200 ps.