Time-resolved Atomic Emission Research Articles

Temporal evolution of laser-induced ionization and recombination processes in argon-helium mixture

Preparation of metastable atoms (1s5) through laser-induced preionization holds the potential to mitigate the electromagnetic interference (EMI) issues associated with the large volume, atmospheric pressure discharge of traditional optically pumped rare-gas metastable laser (OPRGL). In this work, we conducted experimental investigations into the temporal evolution of the Ar 763.5 nm (2p6→1s5) spectral line in Ar-He mixture. These experiments unveiled the intricate interaction mechanism involving the laser, Ar atoms, He atoms, and free electrons within the laser-induced plasma. Our findings highlight the dual contributions of the multiphoton ionization and the inverse bremsstrahlung process to the initial plasma formation. Notably, the time-resolved atomic emission spectrum at 763.5 nm reveals two distinct regimes, namely Regime1 and Regime2. Regime1 primarily arises from the “excitation + radiation + collisional relaxation” process, wherein excited states Ar atoms, populated via multiphoton excitation and electron impact excitation, accumulate on the 2p6 level. Conversely, Regime2 is predominantly a result of the “ion-electron recombination” process. In this regime, highly excited states Ar atoms are generated through the recombination of ion and electron, subsequently populating the 2p6 level through a combination of radiation and collisional relaxation channels. The differences in the temporal evolution between 763.5 nm and 811.5 nm spectral lines can be attributed to the distinct radiation and collisional relaxation channels in the two aforementioned processes.

Population inversion mechanism of optically pumped metastable rare gas based on laser-induced preionization

A 532 nm laser was used to induce ionization of Ar–He mixture, and metastable 4s [3/2]2 level Ar atoms were prepared. The interaction mechanism between laser, Ar atoms, He atoms and free electrons in laser-induced plasma (LIP) was revealed. Through time-resolved atomic emission spectroscopy, two regimes separated by a few microseconds were found during temporal evolution of Ar I 811.5 nm (4p [5/2]3→4s [3/2]2) line. It was demonstrated that the first regime was generated by the “photoexcitation + radiation + collisional relaxation” process, while the second regime was generated by the “ion-electron recombination” process. The above conclusion was confirmed again by the temporal evolution of electron temperature and electron density, and it was also demonstrated that the ionization of Ar atoms induces the ionization of He atoms. A 811.5 nm laser was utilized to excite the metastable 4s [3/2]2 level Ar atoms to the 4p [5/2]3 level, when changing the delay time of the 811.5 nm laser lagging behind the 532 nm laser, the generated 912.3 nm (4p [1/2]1→4s [3/2]2) fluorescence intensity also showed different pattern of variations. The analysis indicated that “photoexcitation + radiation + collisional relaxation”, “ion-electron recombination” and quenching processes will affect the density of metastable 4s [3/2]2 level Ar atoms. Amplified spontaneous emission (ASE) signal at 912.3 nm was obtained along the direction of pump laser. This work verifies the feasibility of optically pumped metastable rare gas laser (OPRGL) based on laser-induced preionization, and provides an alternative scheme for the future development of OPRGL.

Laser-induced multiphoton and collisional ionizations in sodium-argon mixture

It is attractive to investigate the interaction mechanism between metal and rare gas atoms within ionization process, especially for the mixture of metal vapor and rare gas. In this paper, laser-induced breakdown spectroscopy is used to study the ionization and recombination kinetic processes in sodium-argon mixture. Sodium-argon excimer is formed by the atomic collision, and it plays an important role in the ionization process, sodium atoms on 3P state are produced after the dissociation of excimer, then multiphoton ionization happens, the electrons in plasma are accelerated to collide with argon atoms, electron-impact ionization of argon will take place, identified with time-resolved atomic emission spectrum. Population accumulate on 3D,4P, and 5S states after the recombination process of Na+ with electron, then most population will transit to 3P state mainly by collisional relaxation.

Plasma chemistry produced during laser ablation of graphite in air, argon, helium and nitrogen

Laser-induced plasma chemistry produced during the ablation of graphite targets at atmospheric pressure in air, argon, helium and nitrogen was studied through time-resolved atomic and molecular emission spectroscopy. The plasma plume and plasma chemistry were generated by focusing a 6-mm diameter, 212 mJ, 1064-nm nanosecond Nd:YAG laser to a spot size of about 1 mm diameter over graphite samples of 99.9% pureness. The atomic emissions C I 247.86 nm, N I 821.50 nm and O I 777.19 nm, and the molecular bands C2 (473.71 nm) and CN (359.04 nm and 388.30 nm) were monitored as a function of time (0.2 to 220 μs). While the C I and C2 emissions followed a near-exponential decay, the CN emission in air and nitrogen showed an emission behavior characterized by two local maxima at 0.2 μs and 20–30 μs after the plasma ignition. The first maximum was explained by the early plasma chemistry produced by the ablated carbon species and the confining background gas, whereas the second maximum was attributed to atomic recombination and shock wave-induced excitation of the plasma plume. Two main effects were observed when the ablation was produced in different background gases. First, the presence of oxygen (≈21%) in air had a negligible effect on atomic lines; however, the CN emission intensity and lifetime were significantly reduced in comparison with an atmosphere of 100% nitrogen. This was attributed to the reduction of nitrogen species as reaction partners during the plasma chemistry in air. Secondly, due to the assumed higher plasma temperature in Ar, this gas favored the emission intensity and lifetime of atomic species but hindered the formation of C2 species. Because the collection optics of the emission spectroscopy system was configured in backscatter mode, which integrates over the entire plasma volume and gate time without spatial resolution, the time-resolved behavior of the plasma emissions were only related to the number density of emitters but not to specific locations in the plasma volume.

PDN plasma diagnostics with time-resolved atomic emission spectroscopy

PDN plasma diagnostics with time-resolved atomic emission spectroscopy

Comparison of post-detonation combustion in explosives incorporating aluminum nanoparticles: Influence of the passivation layer

Aluminum nanoparticles and explosive formulations that incorporate them have been a subject of ongoing interest due to the potential of aluminum particles to dramatically increase energy content relative to conventional organic explosives. We have used time-resolved atomic and molecular emission spectroscopy to monitor the combustion of aluminum nanoparticles within the overall chemical dynamics of post-detonation fireballs. We have studied the energy release dynamics of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) charges incorporating three types of aluminum nanoparticles: commercial oxide-passivated nanoparticles, oleic acid-capped aluminum nanoparticles (AlOA), and nanoparticles in which the oxide shell of the particle has been functionalized with an acrylic monomer and copolymerized into a fluorinated acrylic matrix (AlFA). The results indicate that the commercial nanoparticles and the AlFA nanoparticles are oxidized at a similar rate, while the AlOA nanoparticles combust more quickly. This is most likely due to the fact that the commercial nano-Al and the AlFA particles are both oxide-passivated, while the AlOA particles are protected by an organic shell that is more easily compromised than an oxide layer. The peak fireball temperatures for RDX charges containing 20 wt. % of commercial nano-Al, AlFA, or AlOA were ∼3900 K, ∼3400 K, and ∼4500 K, respectively.

Algorithm of processing the sequence of time-resolved atomic emission spectra for lowering the detection limits of elements

An algorithm for the mathematical treatment of the sequence of time-resolved atomic emission spectra intended for separation of weak scintillations (flashes) of the analytical lines from noises and, as a consequence, for lowering the detection limits of the analyte elements is presented. It is shown that the application of the algorithm leads to a more than twofold increase in the signal-to-noise ratio in the time dependence graph of the intensity of an analytical peak of the analyte element. The algorithm is incorporated into the Atom Program.

Calibration-free laser-induced breakdown spectroscopy for quantitative elemental analysis of materials

The application of calibration-free laser-induced breakdown spectroscopy (CF-LIBS) for quantitative analysis of materials, illustrated by CF-LIBS applied to a brass sample of known composition, is presented in this paper. The LIBS plasma is produced by a 355 nm pulsed Nd:YAG laser with a pulse duration of 6 ns focussed onto a brass sample in air at atmospheric pressure. The time-resolved atomic and ionic emission lines of Cu and Zn from the LIBS spectra recorded by an Echelle spectrograph coupled with a gated intensified charge coupled detector are used for the plasma characterization and the quantitative analysis of the sample. The time delay where the plasma is optically thin and is also in local thermodynamic equilibrium (LTE), necessary for the elemental analysis of samples from the LIBS spectra, is deduced. An algorithm relating the experimentally measured spectral intensity values with the basic physics of the plasma is developed. Using the algorithm, the Zn and Cu concentrations in the brass sample are determined. The analytical results obtained from the CF-LIBS technique agree well with the certified values of the elements in the sample, with an accuracy error <1%.

Laser-induced plasma chemistry of the explosive RDX with various metallic nanoparticles

The feasibility of exploiting plasma chemistry to study the chemical reactions between metallic nanoparticles and molecular explosives such as cyclotrimethylenetrinitramine (RDX) has been demonstrated. This method, based on laser-induced breakdown spectroscopy, involves the production of nanoparticles in a laser-induced plasma and the simultaneous observation of time-resolved atomic and molecular emission characteristic of the species involved in the intermediate chemical reactions of the nanoenergetic material in the plasma. Using this method, it has been confirmed that the presence of aluminum promotes the ejection process of carbon from the intermediate products of RDX. The time evolution of species formation, the effects of laser pulse energy, and the effects of trace metal content on the chemical reactions were also studied.

Quantitative laser-induced breakdown spectroscopy for aerosols via internal calibration: Application to the oxidative coating of aluminum nanoparticles

We present a methodology for the quantitative use of laser-induced breakdown spectroscopy (LIBS) for the compositional characterization of nanoaerosols, using an internal standard. The approach involves finding the optimal laser delay time to collect spectra for each of the elemental species of interest, and measuring the plasma temperature, and background gas density under the same conditions. This enabled us to eliminate effects due to different experimental conditions, species excitation from different energy states, temporal evolution of the plasma volume itself and instrumental collection efficiencies and biases. We apply the method to the problem of determining the extent of oxidation or coating thickness of aluminum nanoparticles. The specific choice of system is based on a need to understand the nature of energetic properties of nano-materials and in particular the reactivity and stability of passivation coatings, such as metal oxides. The present work establishes LIBS as an effective analytical tool for quantitative estimation of the extent of oxidation in Al nanoparticles from time-resolved atomic emission spectra. More generically this approach of using the background gas as the internal standard mitigates the need for the use of materials standards and should be extendable to the characterization of other multi-component aerosol systems.

Determination of the Branching Ratios of CaBr(A2Π1/2,3/2, B2Σ+) Resulting from the Reaction of Ca[4s4p (3PJ)] with CH3Br by Time-Resolved Atomic Emission and Molecular Chemiluminescence Following Pulsed Dye-Laser Excitation of Atomic Calcium

Molecular electronic branching ratios to yield CaBr(A

Investigation of Branching Ratios into BaI(A2 Π ½,3/2,B2 Σ +) from the Reaction of Ba[6s5d(3DJ)] with CH3I by Time-resolved Atomic Emission and Molecular Chemiluminescence Following Pulsed Dye-Laser Excitation of Atomic Barium

Investigation of Branching Ratios into BaI(A² <i>Π</i> _½,3/2,B² <i>Σ</i> ⁺) from the Reaction of Ba[6s5d(³D_J)] with CH₃I by Time-resolved Atomic Emission and Molecular Chemiluminescence Following Pulsed Dye-Laser Excitation of Atomic Barium

Branching ratios for the production of the electronic states [formula omitted] from the reaction of Ba[6s5d( 3D J)] with CH 3Br investigated by time-resolved atomic emission and molecular chemiluminescence following pulsed dye laser excitation of atomic barium

We present measurements of the branching ratios for the production of the electronic states BaBr(A 2Π 1 2 3 2 , B 2Σ +) from the reaction of Ba[6s5d( 3D J )] with CH 3Br investigated by time-resolved atomic emission and molecular chemiluminescence following pulsed dye laser excitation of atomic barium at elevated temperature (900 K). Ba[6s5d( 3D J )], 1.151 eV above the 6s 2( 1S 0) ground state, was generated after the initial pulsed dye laser excitation of atomic barium via the allowed transition at λ = 553.5 nm ( Ba[6 s6 p( 1 P 1)] ← Ba[6 s 2( 1 S 0)]) in excess helium buffer gas at 900 K. A combination of radiative and collisional processes in excess helium then yielded the optically metastable Ba( 3D J ) state in the “long-time domain”. This may be monitored at λ = 791.1 nm ( Ba[6 s6 p( 3 P 1)] → Ba(6 s 2( 1 S 0)]) , employed as the spectroscopic atomic emission transition. The following long-wavelength molecular chemiluminescence transitions of BaBr were also monitored as a function of time (resulting from the collision of Ba[6s5d( 3D J )] with CH 3 Br): BaBr(A 2Π 1 2 → X 2Σ +, λ = 1002 nm, Δν = 0) ; BaBr(A 2Π 3 2 → X 2Σ +, λ = 943 nm, Δν=0) ; BaBr)B 2 Σ + → X 2 Σ +, λ = 833 nm, Δν = 0). These measurements in the time domain, particularly the A 2Π 1 2 - X 1Σ + transition, are feasible using a long-wavelength photomultiplier tube operating in this region. The exponential atomic molecular profiles are characterized by decay coefficients which are equal in given reactant mixtures, from which it may be concluded that the three molecule states, BaBr(A 2Π 1 2 3 2 , B 2Σ +) , are generated directly on collision of Ba( 3D J ) with CH 3Br via exothermic reactions. The determination of the integrated intensities for the atomic emission and molecular emissions, coupled with optical sensitivity calibrations, yielded branching ratios in the BaBr(A 2Π 1 2 3 2 , B 2Σ +) , states, with the following results: BaBr(A 2Π 1 2 ) , 3.80%; BaBr(A 2Π 1 2 ) , 3.80%; BaBr(A 2Π 3 2 ) , 1.69%; BaBr(B 2 Σ +), 0.49%. The absolute magnitudes of these branching ratios are comparable with analogous results for the reactions of Ba( 3D J ) with CH 3Cl and CH 3F reported previously. The relative variation of the branching ratios, as seen through their logarithmic variation with the energies of the states, is essentially Boltzmann in character, with an effective temperature close to the ambient temperature of the measurements, indicating the absence of propensity in the yields of these excited molecular states on collision and reflecting the role of late barriers in the potential surfaces involved.

Energy transfer and energy pooling from Ba[6s5d( 3D 1,2,3)) generated following pulsed dye-laser excitation at λ = 553.5 nm [formula omitted] and monitored by time-resolved atomic emission from Ba[6s5d( 1D 2)], Ba[6s6p( 3P 1)] and Ba[6s6p( 1P 1)] part I

We present a kinetic study of Ba[6s5d( 3D J )], 1.151 eV above the 6s 2( 1S 0) ground state ( J=1, 1.120, J=2, 1.143, J=3, 1.190 eV). This was generated indirectly following the initial pulsed dye-laser excitation of atomic barium vapour at elevated temperature at λ = 553.5 nm { Ba(6 s6 p( 1 P 1)] ← Ba[6 s 2( 1 S 0)]} in the presence of helium buffer gas in a slow flow syst kinetically equivalent to a static system. The 3D J state was subsequently populated by various emission and collisional process from Ba( 1P 1), including cascading via Ba[6s6p( 3P J )] which is generated in this system. Direct monitoring via emission in the time-domain from Ba[6 s5 d( 3 D 1)] → Ba[6 s 2( 1 S 0)] + hv ( λ = 1106.9 nm) using an interference filter and a new long wavelength response tube (S21) operating in this wavelength region did not prove sufficiently sensitive. Time-resolved emission measurements in the long-time regime (ca. 3 ms) at λ = 791.1 nm { Ba[6 s6 p( 3 P 1)] → Ba[6 s 2( 1 S 0)]} and λ = 877.4 nm { Ba[6 s5 d( 1 D 2)] → Ba[6 s 2( 1 S 0)]} were made following collisional excitation of Ba( 3D J ) to the 3P J and 1D 2 state by He and Ba and, in turn, used to monitor the decay of the 3D J state itself. Further, the long-time emission (ca. 1.5 ms) at λ = 553.5 nm { Ba[6 s6 p( 1 P 1)] → Ba[6 s 2( 1 S 0)]} (τ c = 8.37 ± 0.38 ns) resulting from excitation at this wavelength was also employed to monitor Ba( 3D J ) following energy pooling to this state accompanying the Ba( 3 D J) + Ba( 3 D J) self-annihilation. The detailed mechanism for this system was considered quantitatively in terms of the optical and collisional processes for populating the emitting states employing both the form of time profiles and integrated atomic emission intensities. The results are shown to be quantitatively in accord with the proposed mechanisms for population and excitation and indicate that these time-resolved emission measurements from states resulting from energy transfer and energy pooling act as spectroscopic markers for Ba( 3D J ).

A kinetic investigation of Sr[5s6s( 1S O)] in energy pooling from Sr[5s5p( 3P J)] studied by time-resolved atomic emission following pulsed dye-laser excitation

We present a kinetic study of Sr[5s6s( 1S o)] generated by energy pooling arising from self-annihilation of Sr[5s5p( 3P J )]. Sr[5s5p( 3P 1)] was produced by pulsed dye-laser excitation at λ = 689.3 nm (Sr[5s5p( 3P 1)] ← Sr[5s 2( 1S o)]) of strontium vapour at elevated temperatures (750–890 K) in the presence of excess helium buffer gas. Following rapid Holtzmann equilibration within the 5s5p( 3P J ) spin-orbit manifold, time-resolved emission was monitored at λ = 1124.1 nm (Sr[5s6s( 1S o)] → Sr[5s5p( 1P 1)]) using signal averaging methods. To our knowledge, this constitutes the first direct observation of this time-resolved emission, which has become feasible through the commercial development Hamamatsu) of a new long-wavelength photomultiplier tube (350–1200 nm) based on an AgOCs photocathode, whose operation and application in the present system are described in some detail. Subsequent time-resolved emission at λ = 460.7 nm Sr[5s5p( 1P 1)] → Sr[5s 2( 1S o)]) is thus seen to result from “cascading fluorescence” as originally proposed by Gallagher and coworkers. First-order decay coefficients, characterized across the above temperature range for Sr[5s6s( 1S o)], Sr[5s5p( 1P 1)] and Sr[5s5p( 3P J )], are shown to be in the ratio of 2:2:1 in accord with self-annihilation, energy pooling emission and cascading fluorescence. Integrated atomic emission intensity profiles for the transitions at λ = 1124.1 nm and 460.7 nm, coupled with optical and electronic sensitivity calibrations, yield an estimate of the Einstein coefficient for the λ = 1124.1 nm transition based on the above mechanism. Finally, the present optical system, using the new photomultiplier tube with the associated optics and gating circuitry, is seen to provide a basis for studying low-lying electronically excited states above the ground state (more than approximately 1.03 eV), particularly Ba[6s5d( 3D 1,2,3)] and Ba[6s5d( 3D 2)] by time-resol emission.

Kinetic investigation of energy pooling from Sr [5s5p(3P J )] studied by time-resolved atomic emission following pulsed dye-laser excitation

A kinetic investigation is presented of energy pooling arising from self-annihilation of the optically metastable state, Sr[5s5p(3PJ)], generated by pulsed dye-laser excitation of strontium vapour at λ= 689.3 nm {Sr[5s5p(3P1)]â†� Sr[5s2(1S0)]} at elevated temperature in the presence of excess helium buffer gas. Time-resolved emission was observed for a number of energy-pooled states, namely, Sr[5s5p(1P1)], Sr[5s6s(3S1)], Sr[4d5p(3F2, 3, 4)], Sr[4d5p(1D2)], Sr[5s6p(3P1, 2)], Sr[5s5d(3D2)] and Sr[5p2(3P2)]. This includes characterisation of the time profiles from specific spin–orbit states and significantly extends the range of atomic pooled states reported hitherto derived from the 5s5p(3PJ) energy store. Following rapid energy pooling, the profiles of the ‘pool’ and ‘store’ states are shown to be exponential in character and the first-order decay coefficients are found to be in the ratio of 2 ∶ 1 within experimental error. Further, using the appropriate sensitivity calibration of the optical system, the variation of the ratio of the integrated atomic emission intensities of the energy pooled states to that of the energy store itself were found to be linear and in accord with a mechanism for energy pooling in each case arising from 3PJ+3PJ self-annihilation. The normalised, integrated emission intensities of the energy-pooled states which involved population via endothermic processes from Sr(3PJ)+ Sr(3PJ) were determined and used to calculate the relative distributions of the energy-pooled states on collision. These were plotted semi-logarithmically as a function of the electronic state energy of the appropriate state and yielded a Boltzmann temperature higher than the ambient temperature employed, reflecting the role of atomic collisions with high kinetic energies on energy pooling.

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.

Determination of C, P and S in steels by time-resolved atomic emission spectrometry.

The time-resolved method has been applied successfully to the determination of a trace amount of elements C, P and S in steels.The following results were obtained by the use of the intensities measured after the time of 50 μs elapsed relative to spark discharge initiation:(1) A higher precision analysis was performed. The repeatabilities with standard deviations at a elemental concentration of 10 ppm were 3.7 ppm for C, 0.8 ppm for P and 0.5 ppm for S.(2) Spectral interferences were almost avoided. The concentration correction factors were 0.16×10-4 for an interference of Ni on the PI 178.29 nm line and 0.2×10-4–5.8×10-4 for that of Mn on the SI 180.73 nm line.

The collisional quenching of Sr[5s4d(1D2)] by H2and D2over the temperature range 850–1100 K studied by time-resolved atomic emission following pulsed dye-laser excitation

We present a kinetic study of the collisional quenching of Sr[5s4d(1D2)], 2.498 eV above the 5s2(1S0) electronic ground state, by H2 and D2 across the temperature range T= 850–1100 K. Sr(1D2) was generated by the repetitive pulsed dye-laser excitation of strontium vapour at λ= 496.1 nm {Sr[5s4d(1D2)]â†� Sr[5s2(1S0)]} in the presence of excess helium buffer gas and H2 or D2 in a slow flow system, kinetically equivalent to a static system. Its exponential decay was monitored in emission at the resonance wavelength using Boxcar integration coupled with computerized analysis. Sr(1D2) was also monitored using the more intense bi-exponential atomic emission profiles for Sr[5s5p(3PJ)](1.807 eV), resulting from emission and collisional quenching of Sr(1D2), at λ= 689.3 nm {Sr[5s5p(3P1)]→Sr[5s2(1S0)]+hν}. The resulting rate data, which involve considerable experimental scatter that is considered in some detail, can be expressed in the form: kH2= 7 × 10–10 exp (–13 kJ mol–1/RT) cm3 molecule–1s–1 and kD2= 3 × 10–10 exp (–17 kJ mol–1/RT) cm3 molecule–1s–1 the data clearly indicating low energy barriers and fully in accord with energy transfer on collision, as also found hitherto for Sr(3PJ)+ H2 and D2. This is in marked contrast to previous measurements on Ca[4s4p(3PJ)] and Ca[4s3d(1D2)] by H2 and D2 where collisional removal was totally dominated by chemical reaction. Finally, branching ratios for collisional removal of Sr(1D2) into the lower-lying Sr[5s4d(3DJ)](2.260 eV) and Sr[5s5p(3PJ)] states, for which the individual contributions could not be separated, are estimated and found to lie in the range ca. 0.3–0.9 . consideration of the observed low activation energies, reaction ergicities and branching ratio estimates suggest that the collisional quenching of Sr(1D2) by H2 and D2 is dominated by (E–E, V) transfer resulting in the production of Sr(3DJ) and Sr(3PJ).

Radiative lifetimes, diffusion and energy pooling of Sr(5s5p( 3P J)) and Sr(5s4d( 1D 2)) studied by time-resolved atomic emission following pulsed dye-laser excitation

A study is presented of the kinetic behaviour of the optically metastable states Sr(5s5p( 3P J )) and Sr(5s4d( 1D 2)), each separately generated by pulsed dye-laser excitation at λ = 689.3 nm (Sr(5s5p( 3P 1))←Sr(5s 2( 1S 0))) and λ = 496.1 nm (Sr(5s4d( 1D 2))←Sr(5s 2( 1S 0))) in the presence of helium buffer gas. Radiative and diffusional loss of both metastable states in helium, including the effect of radiation trapping for Sr(5s5p( 3P J )), have been investigated as a function of temperature and pressure by time-resolved emission measurements. We report the following radiative lifetimes: τ e(Sr( 3P 1)) = (19.6 −0.5 +0.6) μs for fluorescence via the spin-forbidden transition to the ground state; and τ(Sr( 1D 2)) = (412 −9 +10) μs for radiative loss to all lower-lying states. Both values are compared with the results of previous experimental work and theoretical calculations as appropriate. When expressed in the form D 12 ∝ T n , the measured temperature dependence of the diffusion coefficients D 12(Sr( 3P J )-He) and D 12(Sr( 1D 2-He) yielded n = 1.59±0.51 and n = 1.83±0.89, respectively. Rate constants for self-quenching of Sr( 3P J ) and Sr( 1D 2) atoms by ground-state Sr( 1S 0) vapour are estimated to be k Sr = (2.9−6.4)×10 −14 cm 3 atom −1 s −1 and k′ Sr = (3.4±0.9)×10 −14 cm 3 atom −1 s −1 respectively. Whilst an upper limit of k He⩽(2.6±0.6)×10 −15 cm 3 atom −1 s −1 is determined for collisional quenching of Sr( 3P J ) atoms by He, no such effect could be observed for Sr( 1D 2) atoms. Energy pooling processes involving Sr(5s5p( 3P J )) atoms to yield the higher-lying 5s5p( 1P 1), 5s4d( 1D 2) and 5s6s( 3S 1) states are also investigated in the time domain, where the mechanisms for pooling formation are established from a quantitative comparison of the time dependence of atomic emission from the energy store and pooled states. For Sr(5s4d( 1D 2)), fluorescence from some 22 energy-pooled atomic states is reported, mostly arising from bimolecular collisions between two Sr( 1D 2) atoms with subsequent emission of radiation. Approximate relative yields into all these pooled states, estimated from extrapolation of their time-dependent decay profiles coupled with appropriate corrections for Einstein coefficients and experimental sensitivity, indicate that energy pooling collisions constitute a small “bleed-off” from the store states which does not affect their time profiles. Finally, quantum yields for emission from Sr(5s5p( 3P J )) and Sr(5s4d( 1D 2)) atoms and branching ratios into the 3P J levels following laser excitation of the 1D 2 state are reported. These are shown to be quantitatively consistent with a mechanism based on removal of both excited states by radiative decay and diffusion, as seen by reference to previous calculated Einstein coefficients for the relevant radiative transitions. Using experimental data reported in an earlier publication, quantum yields for emission from Ca(4s4p( 3P J )) and Ca(4s3d( 1D 2)) atoms and the branching ratio for production of Ca(4s4p( 3P J )) following laser excitation of Ca(4s3d( 1D 2)) are also determined, from which a mechanism analogous to that for atomic strontium is established.