Nonlinear spectroscopic studies using sum- and difference-frequency generation

A detailed study is presented of resonant sum- and difference-frequency generation for the mixed-crystal system azulene in naphthalene. The S0 -> S1, S0 -> S2 and S1 -> S2 transitions of azulene provide the 3 needed dipoles for the existence of c(2), and a relatively small static elec. field was used to break the interference between the waves generated in the 2 crystal sublattices. Second-order nonlinear processes can occur sep. in each sublattice. All the theor. predicted resonances, except the DICE effect, were obsd. The generated field intensity satd. at higher fields, and in certain circumstances exhibited hysteresis as a function of the d.c. field strength. The line-narrowing capabilities of these new forms of spectroscopy were explored

Brain fog in neuropathic postural tachycardia syndrome may be associated with autonomic hyperarousal and improves after water drinking.

Brain fog is a common and highly disturbing symptom for patients with neuropathic postural tachycardia syndrome (POTS). Cognitive deficits have been measured exclusively in the upright body position and mainly comprised impairments of higher cognitive functions. The cause of brain fog is still unclear today. This study aimed to investigate whether increased autonomic activation might be an underlying mechanism for the occurrence of brain fog in neuropathic POTS. We therefore investigated cognitive function in patients with neuropathic POTS and a healthy control group depending on body position and in relation to catecholamine release as a sensitive indicator of acute stress. The second aim was to test the effect of water intake on cardiovascular regulation, orthostatic symptoms, cognitive function and catecholamine release. Thirteen patients with neuropathic POTS and 15 healthy control subjects were included. All participants completed a total of four rounds of cognitive testing: two before and two after the intake of 500 ml still water, each first in the supine position and then during head-up tilt. At the end of each cognitive test, a blood sample was collected for determination of plasma catecholamines. After each head-up tilt phase participants were asked to rate their current symptoms on a visual analogue scale. Working memory performance in the upright body position was impaired in patients, which was associated with self-reported symptom severity. Patients had elevated plasma norepinephrine independent of body position and water intake that increased excessively in the upright body position. The excessive increase of plasma norepinephrine was related to heart rate and symptom severity. Water intake in patients decreased norepinephrine concentrations and heart rate, and improved symptoms as well as cognitive performance. Brain fog and symptom severity in neuropathic POTS are paralleled by an excessive norepinephrine secretion. Bolus water drinking down-regulates norepinephrine secretion and improves general symptom severity including brain fog

Resonant nonlinear spectroscopy in strong fields

A method is presented to describe multiple resonant nonlinear spectra in the presence of strong laser fields. The Liouville equation for the d. operator of the mol. system is transformed to a time-independent linear equation system. This can be easily solved rigorously by numerical methods or, after partitioning into a strong-field part and a perturbation, the soln. can be obtained anal. by a novel perturbative approach. The results account for power broadening. Rabi splitting of signals, and power-induced extra resonances, the latter being related to the pure dephasing-induced resonances in the weak-field limit. The method can be applied to a large no. of multiple resonant nonlinear spectroscopies, esp. CARS, CSRS, coherent Rayleigh scattering and sum- or difference-frequency generation

THE INFLUENCE OF INITIAL CONDITIONS ON DYNAMICAL PROCESSES IN MOLECULAR SOLIDS

Préparant par voie optique des excitations vibrationnelles dans les solides, nous proposons quelques idées nouvelles pour étudier les effets dynamiques qui en résultent. Nous avons prédit un effet significatif [voir référence 4]. La compréhension des différentes contributions à la largeur de raie Raman permet d'obtenir la valeur des temps de relaxation vibrationnelle pour les modes actifs en Raman.Recent ideas on the effects of optical preparation on the dynamics of vibrational excitations in solids are presented. A significant effect is predicted [see reference 4]. An understanding of the various contributions to the Raman linewidth in molecular crystals yields values for the vibrational relaxation times of the Raman active modes

Principles of time resolved cars studies of molecular solids

Recent studies of vibrational phase and population relaxation in molecular crystals at liquid helium temperatures are discussed. The technique of coherent antiStokes Raman scattering is becoming the method of choice for very high resolution Raman spectroscopy in those systems, so the important aspects of both time and frequency resolved CARS measurements are discussed in this article. In addition, the contributions to the residual low temperature vibron linewidth are systematically explored, drawing on experimental observations in crystalline N2, H2, benzene and naphthalene as examples. These observations are then brought into relation with current theories of exciton dynamics in condensed molecular systems. An important conclusion from these studies is the vibrational population relaxation rates show a high degree of mode selectivity, which has an impact on theories of chemical reaction dynamics. A qualitative, but detailed discussion of the mechanisms of trapping vibrational excitations by isotopic impurities is also presented

Picosecond phenomena II

xii+382hlm.;24c

Excited state dynamics of poly-di-n-hexyl-silane

International audienceA model of the optical excitations of the sigma-conjugated linear-chain polysilanes is presented. Support for this model comes from measurements of holeburning, fluorescence and transient absorption spectra