Time-resolved Atomic Fluorescence Research Articles

Kinetic investigation of the molecular chemiluminescence SrBr(A2II, B2Σ+ → X2Σ+) and the time-resolved atomic fluorescence Sr(53P1 → 51S0) following the pulsed dye-laser generation of Sr(53PJ in the presence of CH3Br and CF3Br

A kinetic study is presented of the vibronic energy distribution in SrBr following the reactions of the optically metastable, electronically excited strontium atom, Sr(5s5p(3PJ)), 1.807 eV above its 5s2(1S0 electronic ground state, in the presence of CH3Br and CF3Br. Sr(53P1) was generated by pulsed dye-laser excitation of ground-state strontium vapour at λ=689.3 nm (Sr(53P1←51S0)) at elevated temperature (850 K) in the presence of excess helium buffer gas in a slow flow system, kinetically equivalent to a static system. The decay of Sr(53PJ) is then monitored by time-resolved atomic fluorescence from Sr(53P1) at the resonance wavelength following rapid Boltzmann equilibration within the 3PJ spin-orbit manifold using boxcar integration. Time-resolved molecular chemiluminescence to the ground state was also observed from electronically excited strontium bromide and investigated in detail via the SrBr(A2Πsol:32 → X2Σ+ (Δν=0, λ≈667 nm) and SrBr(B2Σ+ → X2Σ+) (Δν=0, λ≈651 nm) systems. The A2Π (177.6 kJ mol−1 and B2Σ+ (183.6 kJ mol−1) states of SrBr are both energetically accessible on collision between Sr(3P) and CH3Br and CF3Br. Both the atomic and molecular (A,B,–X) chemiluminescence emissions are shown to be exponential in form with both reactants and characterized by first-order decay coefficients which are found to be equal under identical conditions within experimental error. SrBr(A2Π) and SrBr(B2Σ+ are thus shown to arise from direct reaction with both CH3Br and CF3Br. The results obtained here in the time domain are compared with analogous chemiluminescence studies on Sr(3P) with halides, including bromides, using molecular beams and with previous time-resolved measurements on SrCl(A3II, B2Σ+ X2Σ+) that we have reported. The results are also compared with those from a series of investigations we have presented from time-domain investigations of molecular emissions from CaF, Cl, Br, I (A,B–X) arising from the collisions of Ca(43PJ) with appropriate halides.

Kinetic investigation of the time-resolved atomic fluorescence Sr(53P1→ 51S0) and molecular chemiluminescence SrCl(A2Π, B2Σ+→ X3Σ+following the pulsed dye-laser generation of Sr(53PJ) in the presence of CH3Cl and CF3Cl

A kinetic study of the vibronic energy distribution in SrCl following the collision of the optically metastable, electronically excited strontium atom, Sr[(5s)1(5p)1, 3PJ], 1.807 eV above its (5s)2, 1S0 electronic ground state, in the presence of CH3Cl and CF3Cl is presented. This is investigated in the time-resolved mode following the generation of the excited atom by pulsed dye-laser excitation at elevated temperature from ground-state strontium vapour in the presence of reactants and an excess of helium buffer gas in a slow-flow system, kinetically equivalent to a static system. The decay of Sr(5 3PJ) is monitored by time-resolved atomic fluorescence at the resonance wavelength [λ= 689.3 nm, Sr(5 3P1)→ Sr(5 1S0)+hν], following rapid Boltzmann equilibrium within the 5 3PJ spin–orbit manifold using boxcar integration. Electronically excited SrCl in the A 2Π1/2, 3/2 and B 2Σ+ states, respectively 1.855 eV (179.0 kJ mol–1) and 1.948 eV (188.0 kJ mol–1) above the X 2Σ+ ground state, was also monitored by time-resolved molecular chemiluminescence of the A, B–X systems under identical conditions to those employed for characterising the decay profiles of Sr(5 3PJ). Both atomic and molecular chemiluminescence emissions showed exponential decay profiles, characterised by first-order decay coefficients which were found to be equal under the same experimental conditions within experimental error. SrCl(A 2Π) and SrCl(B 2Σ+) are thus shown to arise from direct production on the collision of Sr(5 3PJ) with CH3Cl and CF3Cl where these processes are energetically favourable. These results from measurements in the time-domain are considered with analogous data for Ca(4 3PJ) collisions involving F, Cl, Br and I atom abstraction reactions and with chemiluminescence from SrCl(A, B–X) systems in the single-collision condition following the reaction of Sr(5 3PJ) with various Cl-containing molecules.

Kinetic investigation of the collisional quenching of Ca(43Pj) by C4H10at elevated temperatures (750–923 K). Time-resolved atomic emission and electronic energy transfer between Ca(43PJ) and CaH(X2Σ+) observed by Molecular Chemiluminescence

A detailed investigation is presented of the collisional removal of electronically excited calcium atoms, Ca[4s4p(3PJ)], 1.888 eV above its 4s2(1S0) ground state, with n-butane at elevated temperatures in a slow-flow system, kinetically equivalent to a static system. Ca(4 3PJ) was generated by pulsed dye-laser excitation of calcium vapour at λ= 657.3 nm [Ca(4 3P1)â†� Ca(4 1S0)] and monitored by time-resolved atomic fluorescence at the resonance wavelength over timescales during which Boltzmann equilibration had taken place with the 3PJ spin–orbit manifold. Absolute second-order rate constants for the collisional removal of Ca(4 3PJ) by butane were measured over the temperature range 750–923 K, yielding the following Arrhenius form: kR=(3.5+3.6–1.8)× 10–9 exp(– 58.7 ± 5.0 kJ mol–1/RT) cm3 molecule–1 s–1, The results are compared with analogous data derived from our previous investigations on the collisional removal of Mg(3 3PJ) by various hydrocarbons. Molecular chemiluminescence for the system CaH(A 2Π–X 2Σ+) was also monitored in the time domain following excitation of Ca(4 3PJ) and shown to arise from electronic energy transfer from Ca(4 3PJ) to ground-state CaH(X 2Σ+) both from its bi-exponential time dependence and the observation of CaH(X 2Σ+), generated in the reaction, by laser-induced fluorescence (LIF). CaH(X 2Σ+) was also detected by LIF from the reaction of Ca(4 1S0) in the flow, and the source of this is discussed. These CaH(A–X) chemiluminescence profiles which provide a spectroscopic marker for Ca(4 3PJ) in this type of system, coupled with the LIF observations on CaH(X) and constituting, to the best of our knowledge, the first observation of its type, are compared with analogous observations reported previously where molecular chemiluminescence from CaO is employed as a spectroscopic marker for Ca(4 3PJ) with oxidising reactants.

Kinetic investigation of the time-resolved atomic fluorescence Ca(43P1→ 41S0) and molecular chemiluminescence CaCl(A2Π, B2Σ+→ X2Σ+) following the pulsed dye-laser generation of Ca(43PJ) in the presence of CH3Cl

We present a kinetic study of the collisional behaviour of the optically metastable, electronically excited Ca[4s4p(3PJ)], 1.888 eV above its 4s2(1S0) electronic ground state, with molecular CH3Cl. Ca[4s4p(3P1)] was generated by pulsed dye-laser excitation of calcium vapour at elevated temperatures (900 K) and in the presence of CH3Cl and an excess of helium buffer gas at λ= 657.3 nm [Ca(4s4p(3P1)]â†� Ca[4s2(1S0)] in a slow-flow system, kinetically equivalent to a static system. Atomic fluorescence at the resonance wavelength following rapid Boltzmann equilibration within the Ca(4 3PJ) spin–orbit manifold was monitored using boxcar integration and exhibited exponential decay profiles which were characterised. Time-resolved chemiluminescence was also monitored under identical conditions for both the CaCl(A 2Π1/2, 3/2→ X 2Σ+) and CaCl(B 2Σ+→ X 2Σ+) systems, principally via the (0,0) transitions, on account of Franck–Condon limitations for which the X 2Σ+, A 2Π1/2, 3/2(2.000 eV) and B 2Σ+(2.089 eV) states are characterised by internuclear separations which are close in magnitude. These emissions also exhibit single-exponential decay profiles which are equal in magnitude to those from Ca(4 3PJ). In the case of the A–X chemiluminescence it was possible under some conditions to resolve the spin–orbit components of the (0, 0) transitions for A 2Π1/2 and A 2Π3/2→ X 2Σ+, whose exponential decay coefficients were also found to be equal in magnitude and thus indicating that these also are in equilibrium during the kinetic decays. The exponential form and equality in the first-order decay coefficients for the atomic and both A–X and B–X chemiluminescence profiles are in accord with a mechanism involving the production of these electronically excited states by direct reaction: Ca(4 3PJ)+ CH3Cl → CaCl(A 2Π1/2, 3/2v′⩽ 9)+ CH3(k2)(2), Ca(4 3PJ)+ CH3Cl → CaCl(B 2Σ+v′⩽ 7)+ CH3(k3)(3) Generation of the A 2Π and B 2Σ+ states from collision between Ca(4 3PJ) and CaCl(X 2Σ+), which is detected by laser-induced fluorescence, could only involve higher vibrational levels of the ground state, namely, v 3 and v 5, respectively. It is concluded that these (E, V–E) processes, involving the steady concentrations of CaCl(X 2Σ+) from the reaction of Ca(4 1S0)+ CH3Cl in the flow, would make only a small contribution to the generation of the A and B states, characterised by the same time dependences as observed in chemiluminescence, but whose contribution cannot be totally ruled out; a similar mechanism from CaCl(X 2Σ+) generated in the time domain from the reaction between Ca(4 3PJ) and CH3Cl would yield double-exponential molecular chemiluminescence decay profiles, which are not observed. The results for reactive channels into specific electronic states involving halogen abstraction are compared with analogous processes for Ca(4 3PJ) with O-atom and H-atom abstraction in the time domain, and halogen atom abstraction studied in molecular beams.

Kinetic study of the collisional behaviour of Mg(3 3P J ) with alkanes at elevated temperatures following pulsed dye-laser excitation

The collisional behaviour of electronically excited magnesium atoms in the Mg(3s3p(3PJ)) state, 2.712 eV above the 3s2(1S0) ground state, is investigated by time-resolved atomic fluorescence at λ= 457.1 nm [Mg(3 3P1)→ Mg(3 1S0)+hν] following pulsed dye-laser excitation of magnesium vapour at the resonance wavelength. Decay profiles for Mg(3 3P1), which demonstrate clear exponential behaviour, are recorded on time-scales within which a Boltzmann equilibrium has been established in the 3PJ spin–orbit manifold using signal averaging and computerised analysis. The kinetic behaviour of Mg(3 3PJ) is investigated as a function of temperature in the range 680–905 K in He and Xe alone and in the presence of CH4, C2H6, C3H8 and C4H10 using He as the excess buffer gas both in a static system and in a flow system, kinetically equivalent to a static system. The mean radiative lifetime, τe[Mg(3 3P1)] has been re-investigated with particular care to eliminate impurity quenching and employing low densities of Mg(3 1S0) and low laser pulse energies to minimise effects arising from both collisional quenching by the ground state and by energy pooling. This yields τe= 3.9 ± 0.2 ms, in accord with more recent theoretical calculations. The diffusion of Mg(3 3PJ) is also investigated in some detail as a function of temperature and considered quantitatively in terms of the first-approximation Chapman–Enskog equation. Absolute second-order rate constants for the collisional removal of Mg(3 3PJ) by the alkanes (RH) are measured as a function of temperature, yielding the following Arrhenius forms [kR=A exp(–E/RT)]: [graphic omitted]. These data are considered in terms of the bond-energy, bond-order (BEBO) approach, yielding estimates of activation energies consistent with the experimental observations and with H-atom abstraction to yield MgH. The results are also compared with analogous kinetic data for the removal of Mg(3 3PJ) by H2 and D2 investigated hitherto by a similar method.

Time-resolved molecular chemiluminescence investigations of SrO following pulsed dye-laser generation of Sr(5s5p( 3P J)) in the presence of N 2O

Molecular chemiluminescence from SrO is studied in the time domain following pulsed dye-laser generation of Sr(5 3P J ) in the presence of N 2O at elevated temperatures (675–1100 K). Time-resolved atomic fluorescence at λ=689.3 nm and SrO chemiluminescence were optically isolated with a small monochromator and recorded photoelectrically using boxcar integration. The atomic decay profiles were employed to characterize the Arrhenius parameters for overall quenching between Sr(5 3P J ) + N 2O described by the overall rate constant k= (4.2 ± 1.9) × 10 −10 exp (−25.8 ± 5.7 kJ mol −1/ RT) cm 3 molecule −1 s −1. Time profiles for SrO chemiluminescence were characterized under conditions identical in each case to those employed to record the atomic profiles. Quantitative comparison of the time dependences of the atomic and molecular emission profiles showed that the SrO “orange” and “red” arc band emitting states result from direct reaction between Sr (5 3P J ) + N 2O giving rise to simple exponenti decay of chemiluminescence. By contrast SrO(A′ 1Π) and SrO(A 1Σ +) result from energy transfer between Sr(5 3P J ) and SrO (X 1Σ + and display a bi-exponential time dependence. Estimates of vibrational temperatures within SrO (A′ 1Π) show some measure of vibrational excitation above the Boltzmann temperature for ambient conditions, and the temperature variation of the SrO chemiluminescence intensity overall is shown to be consistent with thermodynamic data for atomic strontium.

Delayed-choice experiments in quantum interference.

Following a suggestion by Wheeler, we have performed delayed-choice experiments in both the spatial and time domains. For the first experiment we use a low-intensity Mach-Zehnder interferometer, and for the second the technique of quantum beats in time-resolved atomic fluorescence. The results obtained show no observable difference between normal and delayed-choice modes of operation, in agreement with the predictions of quantum mechanics.

Kinetic studies of the ground state lead atoms Pb(6 3P 0) generated by pulsed irradiation and monitored by time-resolved fluorescence at λ = 405.78 nm (Pb(7 3P 1) → Pb(6 3P 2)) following optical excitation at λ = 283.31 nm (Pb(7 3P 1) ← Pb(6 3P 0))

We describe the first kinetic study of ground state lead atoms by time-resolved atomic fluorescence spectroscopy. Pb(6 3P 0) was generated by the pulsed irradiation of Pb(CH 3) 4 in the presence of helium using a high intensity magnetically pinched Garton-Wheaton light source. The transient atom was monitored in the time-resolved mode by off-resonance (direct-line) fluorescence at λ = 405.78 nm (Pb(7 3P 1) → Pb(6 3P 2)) following resonance excitation at λ = 283.31 nm (Pb(7 3P 1 ← Pb(6 3P 0)). The construction of an intense resonance source for atomic lead involved further application of the microwave-powered cavity originally developed in the Philips Laboratories at Eindhoven which was constructed in these laboratories and used earlier for the kinetic study of atomic bismuth. The photoelectric signals were recorded in the “single-shot” mode using pretrigger photomultiplier gating and were analysed in digital form by computer. The kinetic study included an investigation of the diffusion of Pb(6 3P 0) in helium together with the third-order reactions with oxygen and NO (plus helium) for which we report the following absolute rate constants (300 K): k(Pb + O 2 + He) = (2.5 ± 0.2) × 10 −32 cm 6 molecules −2 s −1 k(Pb + NO + He) = (9 ± 3) × 10 −31 cm 6 molecules −2 s −1 These are compared with previous estimates reported from time-resolved atomic resonance absorption measurements on Pb(6 3P 0). Finally, we report fluorescence quenching cross sections for Pb(7 3P 1) of σ NO 2 = 38 ± 3 Å 2 and σ O 2 2 = 12 ± 8 Å 2, which are in agreement with data reported earlier from atomic emission measurements on flames.

Quantum interference effect for two atoms radiating a single photon.

We present experimental evidence for a modulation in the time-resolved atomic fluorescence light following the photodissociation of ${\mathrm{Ca}}_{2}$ molecules by a pulsed laser. This modulation is due to an interference effect involving two atoms recoiling in opposite directions, while only one photon is emitted. The period of the modulation is equal to the time for the interatomic distance to increase by an optical wavelength.