Guy Taieb

Kinetics of the Mg(3P,1P)+N2O reaction

The reaction of Mg atoms with N2O has been studied in steady‐state conditions, by the laser induced fluorescence technique and the chemiluminescence spectrum analysis. Mg atoms are produced in a Broida‐type apparatus. Experimental evidence is presented which shows that both Mg(3s3p3P) and Mg(3s3p1P) excited states reacts with N2O whereas the ground Mg(3s2 1S) state does not. Mg(3s3p3P) atoms produced in the oven are also responsible for the formation at Mg(3s3p1P) in the afterglow, through triplet–triplet annihilation. Chemiluminescence is partly due to the direct reaction of Mg 1P with N2O and partly due to secondary reactions. The kinetic behavior of reactants and products is studied and modeled. Absolute cross sections for quenching of Mg(3s3p3P) and Mg(3s3p1P) by N2O, and for the state‐to‐state reactions of Mg(3P)+Mg(3P)→Mg(1P)+Mg(1S) and Mg(1P)+N2O→MgO(B 1Σ+)+N2 have been measured. Rotational and vibrational temperatures of the B 1Σ+ state of MgO have been calculated via simulation of the chemilumine...

V-V energy transfer in highly excited CO by infrared-infrared double resonance

Abstract An infrared laser double resonance technique has been used to study the rate constants of the V-V exchange processes: CO(υ) + CO(υ′) → CO(υ−1) + CO(υ′+1) ± ΔE, with both υ and υ′ having large values (≈ 10). Experimental results are consistent with a theoretical model of Sharma and Brau. Transitions with |Δυ| ⪢ 1 are suggested.

The a 2Π-X2Σ+ system of MgCl. Evidence for predissociation in the a 2Π state of MgCl

Abstract The A 2Π-X2Σ+ system of MgCl has been reinvestigated. Laser excitation spectra of MgCl formed in a Broida oven, from the reaction Mg (3P) +R-Cl (R = Cl, CH2Cl, CHCl2, CCl3, C2H4Cl, C4H8Cl, C6H5, and CH2C6H5) have been recorded under medium resolution. The sudden cut-off of the vibrational structure (at VA⪰7), in all cases, is interpreted as arising from an avoided crossing between the … (3π)4 (4π)1 regular A 2II state and the … (3π)3 repulsive and inverted (D)2Π state. The strong interaction between the two 2Π states results in a predissociation of the A state and would explain the observed anomalous r-dependence of ΔGv +1 2 and Avso. An overall fit of all the presently available spectroscopic data provided a consistent set of vibrational and rotational constants for the A and X states of MgCl.

Temporal Characterization of a Plasma Produced by Interaction of Laser Pulses with Water Solutions

The temporal evolution of a plasma formed by the interaction of a Nd-YAG 10 ns laser pulse with the surface of aqueous solutions of CaCl 2 , has been observed by analyzing the variation of the emission spectra of ions and neutral atoms in the time interval after the laser pulse, 500 ns T e using a Saha-Boltzmann analysis. The broadening of the resonance line of CaI at 4227 A was used to deduce the electronic density N e , with the aid of the semiclassical theory of Stark broadening. A value of T e ≈ 28000 K decreasing slightly to 21000 K, and an exponential decay for N e were found, with N e ≈ 1:25 × 10 18 cm -3 at 500 ns and τ e = ( 1200 ± 50 ) ns.

Population distribution of MgCl formed from Mg+R-Cl reactions. Determination of the dissociation energy of MgCl

Abstract The reactions of Mg( 1 S, 3 P) and R-Cl, with R=Cl, CH 2 Cl, CHCl 2 ,CCl 3 , C 2 H 4 Cl, C 4 H 8 Cl, C 6 H 5 and CH 2 C 6 H 5 , have been studied under steady state conditions in a Broida oven. Formation of ground state MgCl(X 2 Σ + ) was interrogated by laser excitationof the MgCl(A–X) transition and that of excited state MgCl (A 2 Π) by detection of MgCl(A–X) chemiluminescence. Population analyses of recorded spectra were carried out on the basis of vibrational and rotational constants derived from a new overall fit of all available spectroscopic data. Our results are: (i) Only Cl 2 reacts with Mg ( 1 S) and only Cl 2 reacting with Mg ( 3 P) produces electronically excited MgCl(A). (ii) The laser excitation spectra observed from the Mg( 3 P) + R-Cl reaction are almost identical and the highest observed vibrational level is well below that permitted by the reaction exoergicity. A predissociation in the MgCl (A) state is responsible for this sudden cut-off of the vibrational structure at v ⪰ 7. (iii) An accurate value for the MgCl dissociation energy, D o =3.37 ± 0.01 eV, is derived from the MgCl chemiluminescence. In addition, a LIF spectrum previously recorded under single-collision conditions was reinvestigated using an inversion/simulation procedure and taking into account the A state predissociation. The derived population distribution is consistent with a direct interaction mechanism dominated by repulsive energy release in the Mg + ---Cl − ---Cl ionic complex.

Kinetics of Laser Induced Plasma on an Aqueous Surface

Emission spectra of MgI and MgII from a plasma induced by the interaction of a laser pulse with the surface of aqueous solutions of MgCl 2 , were recorded by a time resolved spectroscopy method to obtain information on the processes involved in the formation and the evolution of the two different species. A kinetic model based on ion-electron recombination produced during the relaxation of the plasma is constructed with the aim to explain the origin and the temporal shift of observed Mg + and Mg emissions. Comparison with the experimental results is presented and discussed.

Laser Spectroscopy of the C2Π‐X2Σ+ Transition of CaBr and CaI. Structure and Dynamics of the C2Π State

CRD and LIF spectra of the C2Π‐X2Σ+ transition of CaBr and CaI radicals formed by chemical reaction, have been recorded. A global treatment of the laser data together with previous conventional absorption and emission spectroscopy measurements provided accurate vibrational parameters and for the first time, rotational constants for the C2Π state of CaBr and CaI. Comparison of the CRD and LIF spectra pointed out the occurrence of a non‐radiative deexcitaion channel for the Ω = 3/2 component of the CaI C2Π state. In addition, the spectral analysis revealed an anomalous structure for this same substate, consistent with the Ω selective predissociation of the C2Π state of CaI recently recognized.

Resonance-enhanced two-photon excitation of CaI

Induced fluorescence following visible (620–655 nm) laser excitation of the CaI radical has been detected not only in the same region (B, A–X transitions), but also in the UV (315–330 nm). The UV two-photon excitation spectrum consists of narrow bands appearing at laser frequencies located within certain bands of the Δv = 1, 0 sequences of the B2Σ+−X2Σ+ and A 2Π12-X2Σ+ systems. The main peaks are tentatively assigned to resonance-enhanced excitation of a single vibrational level of the lowest Rydberg D2Σ+ state from successive vibrational levels of the ground state. The excitation process is a one-color two-photon optical—optical-double-resonance via B2Σ+ and A 2Π12 intermediate levels. This analysis is supported by the absorption spectrum observed long ago by Walters and Barratt. The absorption and laser excitation complementary data have been used to derive approximate molecular constants for the D state.

Calculation of the resonance escape factor of magnesium spectral line shapes in the case of a MgCl2—water plasma

The resonance escape factors for the lines emitted by a neutral magnesium atom MgI at 285.2127 nm (3 1S–3 1P) and of ionic magnesium MgII at 279.5528 nm (3 2S–3 2P) are calculated assuming a Voigt profile and in the case of MgCl2‐water plasma. The dependence of the escape factor on the optical thickness τ0 from the line center which itself depends on the two main spectral line shape broadening mechanisms (pressure and Doppler effects) are considered. The variation of the resonance escape factors with the temperature and the MgCl2 molar proportion are also investigated. This calculation is useful for the application of laser induced breakdown spectroscopy in the quantitative analysis of elemental composition.