Ricardo Rego Bordalo Correia

Resonance CARS overtones of NaH in a Na (3p) + H2 gas mixture

Abstract With the CARS technique (coherent anti-Stokes Raman scattering), we found the first, second and third overtone O, Q and S lines of NaH in a Na (3p) + H2 gas mixture, when the CARS pump-laser frequency was tuned close to electronic resonances between X 1Σ+ (ν″=O, J″) and the first excited state A 1Σ+ (ν′=1, J′=J″±1) in NaH. This opens new possibilities in the investigation of the nascent molecular products of the reaction between excited sodium atoms and hydrogen molecules, since resonance CARS can be used with good spatial and temporal resolution. We discuss the possible origin of the NaH CARS lines produced in the Na(3p) + H2 mixture.

P-R doublet enhanced CARS lines of NaH

Abstract We have observed an enhancement of the P and R lines from X1Σ+−A1Σ+ transitions in the NaH molecules through coherent anti-Stokes Raman scattering (CARS). The NaH molecules were photochemically produced in a sodium-hydrogen gas mixture. By changing the pump laser frequency ωpump the location of these P-R enhanced doublet CARS lines shift by the amount of detuning in the usual ωpump-ωstokes representation. However, in the 2ωp-ωs representation these P and R lines remain stationary. Double electronic resonances and upper state Raman overtone CARS lines have been observed. For P5(9,0) and R5(9,0) enhanced CARS lines we tuned the pump laser over the electronic resonance in order to obtain the resonance excitation curve for the double electronic resonance. This excitation curve exhibits a characteristics central dip.

CARS investigations of quenching and photochemical reactions in the Na+H2 collision system

SummaryWe have carried out parallel studies of the quenching process in Na(3p)+H2 collisions and the possible reactive process in Na(3p)+H2 (v″=1,2,3) collisions. Rich CARS spectra which were obtained at H2 pressure of 100 mbar and oven temperature of 600 K indicate the presence of vibrationally excited H2 and photochemically produced NaH molecules. Temporal resolution of NaH CARS lines was employed in order to rule out competing collisional processes. We make use of resonantly enhanced CARS methods which enabled us to achieve very high sensitivity for NaH detection.

The role of nuclear spin in inelastic Na (3p) + H2 collisions

Abstract Using the CARS (coherent anti-Stokes Raman spectroscopy) detection technique, we measured the rotational distribution in the v = 3 vibrational level of the para-H2 modification of the hydrogen molecule after inelastic collision with the Na (3p) atom. Para-H2 was used in a flowing-regime cell at a temperature of 523 K and at pressures of para-H2 ranging from 10 to 200 mbar. Our measurements show that the para-H2 modification is conserved in inelastic collisions. The conservation of the nuclear spin was also studied at very low hydrogen pressures, where we obtained the non-thermal rotational distribution at the highest attainable vibrational level v = 3.

Time resolved Hyper-Rayleigh Scattering

We extend the Hyper-Rayleigh Scattering (HRS) technique in time domain using two delayed polarized beams. We present the results in two model liquids, acetonitrile and carbon tetrachloride, analyzing the transient regime. (CNPq, DAAD)

Dye-induced narrowing in the spectral profiles of stimulated scattering in a CS2 liquid-core fiber system

The narrowing of the super-broadened wings of both stimulated Rayleigh and Raman scattering (SRS) spectral profiles in carbon disulfide (CS2) has been observed in a liquid-core hollow fiber system doped with 10 ppm of a dye. The observed red-shifted super-broadening effect of the SRS lines in transparent molecular liquids is caused by a nonlinear optical effect. It involves the light-induced reorientation of anisotropic molecules, a process in which the rotational work is done against the local viscosity. Pure and dye doped CS2 SRS spectra were obtained for resonant and nonresonant pumping in order to verify the dependence on the spatial anisotropy of the absorbing molecules. The remarkable change in the spectral profile for the resonant case is explained in terms of the change in the guest-host interaction induced by optical excitation into the dye. This effect is attributed to the light-induced molecular pre-alignment of the anisotropic molecules of CS2 caused by the reorienting mechanism of a majority of excited dye molecules along the optical pump field. Such phenomenon was reported recently in light-induced birefringence experiments by doping liquid crystals with an absorbing dye, leading to an enhancement factor by one to two orders in magnitude for the optical Kerr effect.

SRS line narrowing and optical induced birefringence in dye-doped CS2

It has been observed the effect of the addition of small concentrations of an absorbing dye (approximately 10 ppm of Coumarin 337) on the super-broadening process of CS2 stimulated scattering lines in a capillary fiber. This effect is attributed to the selective absorption of dye molecules, which creates an anisotropic distribution of these molecules in the excited- and ground states, which possess different static dipole moments. This asymmetry affects the orientation of the CS2 molecules through the modification of the local field, as measured by changing the spectral distribution of stimulated scattering and the optical induced birefringence of the liquid.