Time-resolved Laser Spectroscopy Research Articles

Photochemistry and Polymerization Efficiency of Bis-imidazole based Photoinitiator Systems

Photopolymerization efficiency of an acrylate formulation (Ebecryl 605) under light irradiation at 365 nm has been evaluated by using RTFTIR. The role of various multicomponent photoinitiating systems based on bis imidazole derivatives, in the presence or not of an aminobenzophenone, N-phenylglycine and several iodonium salt derivatives has been emphasized. Excited state processes in these two-, three- or four- component systems have been investigated through time resolved laser spectroscopy. The different photophysical processes and photochemical reactions involved are defined and the role of each partner is discussed : this helps to explain the high performance of the three- and four- component photoinitiating systems.

Lifetime Measurements in Neutral Erbium using Time-Resolved Laser Spectroscopy

Natural radiative lifetimes, measured by time-resolved laser-induced fluorescence spectroscopy, are reported for eighteen levels of neutral erbium belonging to the odd-parity 4f126s6p, 4f115d6s2, 4f115d26s and 4f115d6s7s configurations in the wavenumber range from 26 237 to 42 154 cm-1. Free erbium atoms were produced in a laser-induced plasma. Stimulated Brillouin scattering (SBS) techniques were employed to produce laser pulses as short as 1 ns for the excitation of short-lived levels, and special care was taken to avoid flight-out-of-view effects on the long lifetime measurements.

Dynamics of visible-to-ultraviolet upconversion in YAlO3: 1% Er3+

The dynamics of visible-to-ultraviolet upconversion in YAlO3: 1% Er3+ crystal has been investigated using the time-resolved laser spectroscopy when the sample was excited into the S-4(3/2) multiplets. It is found that the upconverted luminescence is produced by energy-transfer upconversion or by excited-state absorption depending on the excitation wavelength. The dependence of upconverted luminescence on pump power is presented, and the relative intensities of upconverted luminescence were accounted for by Judd-Ofelt theory. (C) 2002 Elsevier Science B.V. All rights reserved.

The cavity size effect on the fluorescence properties of 4′-dimethylaminoacetophenone complexed with cyclodextrins

Abstract Fluorescence properties of excited 4 ′ -dimethylaminoacetophenone (DMAAP) complexed with α-, β-, and γ-cyclodextrins (CDs) were studied by means of steady state and time-resolved laser spectroscopy. The 1:2 DMAAP–α-CD and 1:1 DMAAP–β-CD complexes exhibited dual fluorescence in neutral aqueous solutions while only the fluorescence from the locally excited state was observed in the case of DMAAP complexed with γ-CD. The CD cavity size effect on the excited state dynamics of DMAAP–CD complexes was further discussed. It revealed that polarity effect introduced by the hydrophobic cavity is more important in controlling of the photochemistry of DMAAP than the restriction of molecular motion inside the CD cavity.

Electron injection into DNA: synthesis and spectroscopic properties of pyrenyl-modified oligonucleotides.

The nucleoside 5-(1-pyrenyl)-2'-deoxyuridine (1) was prepared by a Suzuki-Miyaura cross-coupling reaction and subsequently used as a DNA building block in order to prepare a range of modified oligonucleotides using phosphoramidite chemistry. The DNA duplexes contain a pyrenyl group covalently attached to the nucleobase uracil. Upon excitation at 340 nm an intramolecular electron transfer from the pyrenyl group to the uracil moiety takes place which represents an injection of an excess electron into the DNA base stack. Based on the results obtained by steady-state fluorescence and time-resolved pump-probe laser spectroscopy it was possible to show that base-to-base electron transfer can occur from the Py-dU group only to adjacent thymines.

Concentration measurement of gas embedded in scattering media by employing absorption and time-resolved laser spectroscopy

Diode-laser-based absorption spectroscopy for the evaluation of embedded gas concentrations in porous materials is demonstrated in measurements of molecular oxygen dispersed throughout scattering polystyrene foam, used here as a generic test material. The mean path length of light scattered in the material is determined with the temporal characteristics of the radiation transmitted through the sample. This combined with sensitive gas-absorption measurements employing wavelength-modulation spectroscopy yields an oxygen concentration in polystyrene foam of 20.4% corresponding to a foam porosity of 98%, which is consistent with manufacturing specifications. This feasibility study opens many possibilities for quantitative measurements by using the method of gas-in-scattering-media absorption spectroscopy.

Inhomogeneous broadening of the dynamically split Kramers spectral line and up-conversion in the pair and quartet centers in CaF 2:Nd 3+

The site-selective and time-resolved fluorescence laser spectroscopy and kinetic measurements with high spectral and nanosecond temporal resolution was applied to analyze the high-energy wing of the M and N absorption bands of the 4I 9/2(1)→ 4G 5/2(1) crystal-field (CF) transition in a CaF 2:Nd 3+ (0.6 wt%) crystal at 4.2 K. It was found that at helium temperatures the dynamically split spectral line assigned as the 4I 9/2(1)→ 4G 5/2(1) (CF) transition of coherently coupled Nd 3+ ions in the pair M- and quartet N-centers of CaF 2:Nd 3+ (0.6 wt%) is inhomogeneously broadened. It consists of the pair M- and quartet N-centers with at least 0.1 A variation of the positions of the fluorescence-excitation spectral lines registered at the 4F 3/2(1)→ 4I 9/2(1) CF transition. Small fluorescence-lifetimes variation of the 4F 3/2 and 4D 3/2 levels from the small variation of the distances R between Nd 3+ ions in the pair is found. At least 2.7% variation of the value of the Nd–Nd distance R in the pair M-center was determined from the lifetime variation of the 4F 3/2 manifold with the assumption of a dipole–dipole interaction between the ions in the pair. The energy transfer up-conversion process responsible for the UV fluorescence observed when pumping the 4I 9/2(1)→ 4G 5/2(1) transition has been determined.

Tunable Diode Laser Study of the Reaction OH + ClO → HCl + O2

Time-resolved tunable diode laser spectroscopy has been used to measure the production of HCl in the reaction of OH and ClO radicals. The OH and ClO radicals were formed from the 308 nm laser photolysis of mixtures containing O3, Cl2, and H2O. Computer simulations of the chemistry were used to derive a rate coefficient of (1.25 ± 0.45) × 10-12 cm3 molecule-1 s-1 for the title reaction at 298 K. This reaction rate, which corresponds to a branching fraction of (6.5 ± 3.0)% relative to the currently recommended rate coefficient for the overall reaction, confirms that the reaction will have a substantial impact on the partitioning of chlorine compounds in the stratosphere.

Visible luminescence in KPb2Cl5:Pr3+ crystal

Abstract Steady-state and time-resolved laser spectroscopy of Pr3+-doped KPb2Cl5 crystal has been investigated for the first time. Visible luminescence from 3P1 and 3P0 states under excitation in the 3P2,1,0 states has been analyzed. The fluorescence decays of states 3P1 and 3P0 have been measured at 10, 77, and 295 K. Emission from level 1D2 is only observed when this level is directly excited. Anti-Stokes emission from levels 3P1 and 3P0 following excitation of the 1D2 state has been observed between 10 and 295 K. The temporal behavior of the upconverted emission indicates that an energy transfer upconversion is the dominant mechanism for the upconversion process in this crystal.

Coherent Excitation of Vibrational Modes in Gold Nanorods

The acoustic vibrational modes of Au nanorods with aspect ratios between 2 and 5 have been investigated by time-resolved laser spectroscopy. The results show that laser excitation launches a coherent vibrational motion that has a period that depends linearly on the length of the rod. Due to polydispersity in the samples, the measured period also depends on the probe wavelength (i.e., different probe wavelengths interrogate different length rods). Analysis of the data provides information about the size distribution of the sample and the homogeneous line width of the longitudinal plasmon band of the rods.

Time-resolved laser spectroscopy of multiply ionized atoms: natural radiative lifetimes in Ce IV.

Radiative lifetimes have been measured for two excited levels of Ce IV using the time-resolved laser-induced fluorescence technique. Ce3+ ions were produced in a laser-induced plasma. In the measurements, a suitable magnetic field was applied to reduce the recombination between electrons and the ions and thus the background light from the recombination, and special care was exercised to avoid flight-out-of-view effects on the lifetime measurements for the high-velocity ions. The experimental lifetime results, tau = 30(2) ns for the level 49 737 cm(-1) and tau = 30(3) ns for the level 52 226 cm(-1), were compared with relativistic Hartree-Fock calculations (tau = 30.5 and 30.0 ns) indicating a particularly excellent agreement.

Photopolymerization Reactions Initiated by a Visible Light Photoinitiating System: Dye/Amine/Bis(trichloromethyl)-Substituted-1,3,5-Triazine

Addition of a bis(trichloromethyl)-substituted-1,3,5-triazine (Tz) to a dye/amine photoinitiating system leads clearly to an increased efficiency of polymerization under visible light irradiation. The polymerization rates obtained with three triazine derivatives with either phenosafranine, eosin or Rose Bengal as light absorbing species, were measured. Whatever the investigated dye/amine mixture, the addition of Tz led to a strong synergistic effect: the inhibition time decreased and the polymerization rate increased significantly when the three components were used jointly. The spectroscopic and redox properties of the Tz were measured. The system dye/amine/Tz was investigated through time-resolved laser spectroscopy and steady state fluorescence experiments. A detailed reaction mechanism was elaborated which shows that Tz acts mainly as an inhibitor scavenger. The involved inhibitor is the reduced dye (DH∗) arising from the first photochemical reaction between the excited states of the dye and the amine.

Laser-assisted photochemistry of N, N-dimethyl-4-nitroaniline initiator of polymerization

Applying time-resolved laser (337 nm) spectroscopy, we have analysed the photochemical and photophysical behaviour of N, N-dimethyl-4-nitroaniline (DMNA) and its dependence on the irradiation time and the presence of N, N-dimethylaniline (DMA), as reducting agent, and lauryl acrylate (LA) monomer, under aerobic and anaerobic conditions. The laser-induced phosphorescence of the initiator is efficiently quenched by DMA inducing the appearance of new emissions as a result of radiationless processes. The spectral and temporal characteristics of these new bands together with their dependence on both the laser intensity and irradiation time allowed their assignment and provided evidence for the photosensitization process of DMA from the excited initiator to be the main photoinitiation mechanism of polymerization.

Infrared-to-visible upconversion in Nd3+-doped chalcohalide glasses

The infrared to visible upconversion processes have been investigated for ${\mathrm{Nd}}^{3+}$-doped chalcohalide glasses with different halide modifiers by using steady-state and time-resolved laser spectroscopy. Two different upconversion mechanisms have been identified depending on the infrared excitation wavelength. When the excitation wavelength is resonant with the ${}^{4}{F}_{3/2}$ state, three main bands at 538, 600, and 675 nm are observed and attributed to emissions from the ${}^{4}{G}_{7/2}$ level. These upconverted emissions occur via energy-transfer upconversion involving two neodymium ions in the ${}^{4}{F}_{3/2}$ state. However, nonresonant excitation at higher energies than that of ${}^{4}{F}_{3/2}$ state (between states ${}^{4}{F}_{3/2}$ and ${}^{4}{F}_{5/2})$ or in resonance with the ${}^{4}{F}_{5/2}$ state, causes an additional blue emission to originate from the ${}^{2}{P}_{1/2}$ state. This latter upconverted emission can be attributed to excited-state absorption of the pump radiation. The proposed upconversion mechanisms responsible for the different emissions from levels ${}^{2}{P}_{1/2}$ and ${}^{4}{G}_{7/2}$ are supported by both the time evolution of the upconversion luminescence after infrared pulsed excitation and the upconversion luminescence excitation spectra.

Landé factor andlifetime measurements in even-parity Rydberg series of Pb I using time-resolved laser spectroscopy

Landé gJ factors and lifetimes for the levels in theeven-parity Rydberg series 6pnp (n = 9-16) and 6pnf(n = 6-12) as well as for the levels from perturbingconfigurations of neutral lead have been measured usinglaser-induced fluorescence and Zeeman quantum-beat techniques.Two-step excitation was performed in a thermal atomic lead beam.Strong perturbations in the Rydberg series are manifested bothin the gJ factor and lifetime data.

Optical properties of Pr 3 +-doped lithium tetraborate glasses

The visible luminescence of Pr 3+-doped lithium tetraborate glass has been investigated for different Pr 3+-concentrations and temperatures by using steady-state and time-resolved laser spectroscopy. At low Pr 3+ concentration the luminescence is dominated by emission from the 1D 2 state which is populated by fast multiphonon relaxation from the 3P 0 level. As concentration increases the emission from the 1D 2 state shows a strong concentration quenching which indicates the existence of energy transfer processes. The time evolution of the decays from the 1D 2 level together with the concentration dependence of the energy migration rates suggest that migration of the excitation energy is competitive with cross relaxation in the concentration range studied.

Laser spectroscopy and upconversion studies of Pr3+-doped halide modified sulfide glasses

The visible luminescence of Pr 3+ -doped halide modified sulfide glasses has been investigated for different halide modifiers and Pr 3+ concentrations by using steady-state and time-resolved laser spectroscopy. The measured luminescence lifetime of level 3 P 0 is nearly independent on Pr 3+ concentration up to 0.7 mol% whereas the fluorescence of the 1 D 2 level shows concentration quenching for Pr 3+ concentrations higher than 0.5 mol% even at low temperature. The IR-to-visible upconversion obtained under continuous wave infrared laser excitation has been investigated for all samples. The excitation wavelength and pumping power dependencies of the upconverted emission allow to point out two different upconversion mechanisms to populate the 3 P 0 and 1 D 2 levels.

Lifetime measurements in Gd II and Gd III using time-resolved laser spectroscopy

Natural radiative lifetimes of 20 levels (energy range between 29 000-35 000 cm-1) in Gd II and 5 levels (energy range between 43 000-49 000 cm-1) in Gd III have been measured using time-resolved laser-induced fluorescence in a laser-produced plasma.

Spectroscopy andorange-blue frequency upconversion in Pr3+-dopedGeO2-PbO-Nb2O5 glass

The visible luminescence of Pr3+-doped lead-germanate glass ofcomposition (in mol%) 60GeO2-25PbO-15Nb2O5 hasbeen investigated for different Pr3+ concentrations andtemperatures by using steady-state and time-resolved laserspectroscopy. The fluorescence from the 1D2 level shows astrong concentration quenching for Pr3+ concentrations higherthan 0.1 mol%. The time evolution of the decays from the1D2 level and the concentration dependence ofthe effective decay rates are consistent with a dipole-dipolequenching process in the framework of a diffusion-limited regime.Anti-Stokes emission from the 3P0 level followingexcitation of the 1D2 state has been observed for thesamples doped with 0.1, 0.5, 1, and 2 mol% of Pr3+. Thetemporal behaviour of the upconverted emission from the3P0 level together with its quadratic dependence on theexcitation energy and its linear dependence on Pr3+concentration suggest that an excited-state absorption (ESA) is thedominant mechanism for the upconversion process in this glass.

Molecular Structure of p-Cyclohexylaniline. Comparison of Results Obtained by X-ray Diffraction with Gas Phase Laser Experiments and ab Initio Calculations

The results of a combined experimental and theoretical investigation of the molecular structure of p-cyclohexylaniline (pCHA) in the electronic ground and the first electronically excited state are reported. The experimental investigations are performed for the crystalline phase by X-ray diffraction for the first time and related to former gas phase results obtained by time-resolved rotational laser spectroscopy. The theoretical results, from new ab initio calculations at the MP2/6-31+G(d) and CIS/6-31+G(d) level of theory for the electronic ground and excited state, respectively, give an adequate description of the rotational constants as obtained by the gas phase experiments. Thus, a detailed comparison of the ab initio structure for the ground state with the X-ray structure is performed in order to ascertain differences in the molecular geometry between the gas and crystalline phase. In particular, the size of the aromatic and cyclohexyl ring, their mutual orientation, and the conformation of the NH2 g...