Search results for "RADIATION"
showing 10 items of 5298 documents
Nonlinear optical spectra of conjugated polymers: Effect of long-range coulomb interactions
1993
Abstract Nonlinear optical spectra of conjugated polymers are theoretically studied in the Su-Schrieffer-Heeger model supplemented by long-range Coulomb interactions. Excitonic correlation for electron-hole excitations in explicitly taken into account by a standard method. Nonlinear susceptibilities χ (3) are calculated numerically with a standard sum-over-states method for finite chains (up to 1000 sites). Using moderate interaction strength, our calculations can reproduce many distinct features observed in the linear and nonlinear spectra (two-photon absorption, third-harmonic generation, and electroabsorption) of polydiacetylenes and some other polymers. Especially, a hump in the spectru…
Electric field induced alignment-orientation conversion in diatomic molecules: analysis and observation for NaK
1999
This article reports the observation of the molecular fluorescence circularity under irradiation with linearly polarised light. This alignment-orientation conversion phenomenon arises as a result of partial transformation from alignment of the ensemble of 1 P state molecular angular momenta into their orientation under the effect of non-linear dc Stark effect. Circularity rate up to 0.12 was observed in D 1 P! X 1 S fluorescence of 23 Na 39 K molecules in agreement with the theoretically predicted value.
Impact of the synergistic collaboration of oligothiophene bridges and ruthenium complexes on the optical properties of dumbbell-shaped compounds.
2012
The linear and non-linear optical properties of a family of dumbbell-shaped dinuclear complexes, in which an oligothiophene chain with various numbers of rings (1, 3, and 6) acts as a bridge between two homoleptic tris(2,2'-bipyridine)ruthenium(II) complexes, have been fully investigated by using a range of spectroscopic techniques (absorption and luminescence, transient absorption, Raman, and non-linear absorption), together with density functional theory calculations. Our results shed light on the impact of the synergistic collaboration between the electronic structures of the two chemical moieties on the optical properties of these materials. Experiments on the linear optical properties …
Photocatalytic and Catalytic Reactions in Gas–Solid and in Liquid–Solid Systems
2019
Abstract Heterogeneous photocatalytic and thermal-catalytic reactions are here presented focusing the differences and similarities between the two processes. When possible, the comparison was made for the same chemical reaction in the presence of an inorganic material playing the role of both catalyst or/and photocatalyst. Several examples are presented where a catalytic reaction can occur at milder experimental conditions and, particularly, at lower temperature when the system is also irradiated by UV and/or visible light. The differences in mechanistic aspects, conversions, and selectivities between catalytic and photocatalytic reactions will be also considered because these occurrences a…
Spatial separation of atomic states in a laser cooled ion crystal
1999
A laser cooled ion crystal containing several hundred Ca+ ions has been stored in a linear Paul trap. Cooling is provided by a red detund laser at the 4S1/2−4P1/2 resonance transition. A second laser serves for repumping of those ions which decay from the excited 4P1/2 level to the metastable 3D3/2 state. The ions can be additionally excited by a third laser to a long lived metastable 3D5/2 energy level which decouples them from the cooling laser radiation. The light pressure acting upon the laser cooled ions pushes them into the direction of the laser beam. The ions in the metastable 3D5/2 state, however, do not experience any light pressure force and diffuse to the crystal side which poin…
The use of X-ray-induced and thermostimulated visible and UV luminescence for understanding X-ray poling of silica glasses
2007
In this paper, we studied the X-ray-induced visible and UV luminescence (XRL) during X-ray irradiation, afterglow (after X-ray was switched off) and thermostimulated luminescence (TSL). Comparisons were performed between with and without applying an electric field of 1.5 kV/mm on the sample. The typical TSL curve for X-ray-irradiated samples present two peaks centered at 90 and 280 °C. The relative intensity of these two peaks is X-ray dose dependent. An external electric field enhances the XRL, but has no effect on the afterglow. The TSL and afterglow spectra are identical with one peak at 3.25 eV, whereas XRL spectrum displays an additional peak at 4.25 eV. A model of electron exchange be…
Time-resolved luminescence and induced absorption in PbWO4
1997
Abstract Luminescence and short-lived induced absorption are studied for two undoped PbWO4 crystals. Luminescence decay at LNT is delayed relative to irradiation pulse. The delay observed is either due to reabsorption of luminescence or due to creation of luminescence center excited states via energy or/and charge transfer after irradiation pulse. Short-lived absorption is observed within 1.1–3.6 eV spectral region at LNT as well as at RT. It is proposed that the short-lived absorption band peaking at ~ 3.5 eV is intrinsic.
Properties of methyl radical trapped in amorphous SiO2 and in natural SiO2-clathrate Melanophlogite
2013
Abstract We report an experimental investigation by electron paramagnetic resonance (EPR) on methyl radical (CH3 ) observed in γ-ray irradiated high-purity amorphous silicon dioxide (a-SiO2) and in a polycrystalline sample of Melanophlogite, a rare natural form of SiO2-clathrate. From the analysis of the EPR spectra we estimate the correlation time of the hindered rotational motion of CH3 molecules at T = 77 K in the two different materials. This physical quantity gives a quantitative measure of the freedom of motion of CH3 molecules trapped in the two solid systems, putting forward relevant information on the properties of the cavities/interstices in which the radicals are confined. In par…
Formation of HXeO in a xenon matrix: Indirect evidence of production, trapping, and mobility of XeO (1 1Σ+) in solid Xe
2004
IR spectroscopy, laser induced fluorescence (LIF), and thermoluminescence (TL) measurements have been combined to monitor trapping, thermal mobility, and reactions of oxygen atoms in solid xenon. HXeO and O(3) have been used as IR active species that probe the reactions of oxygen atoms. N(2)O and H(2)O have been used as precursors for oxygen atoms by photolysis at 193 nm. Upon annealing of matrices after photolysis, ozone forms at two different temperatures: at 18-24 K from close O ...O(2) pairs and at approximately 27 K due to global mobility of oxygen atoms. HXeO forms at approximately 30 K reliably at higher temperature than ozone. Both LIF and TL show activation of oxygen atoms around 3…
Spectral investigation of spin echo emission
1993
Abstract The spectral content of the echo radiation emitted after a two-pulse sequence is measured in a two-level spin system. The spectral profiles exhibit maxima and zeroes of spectral density depending on the exciting sequence parameters. A calculation based on a vectorial model relates the zeroes to those packets that happen to be transparent to the second (refocusing) pulse. Moreover we report on a new spectral narrowing effect which we tentatively ascribe to the instantaneous diffusion.