Time-resolved Atomic Absorption Research Articles

Investigation of laser ablation plume dynamics by high-resolution time-resolved atomic absorption spectroscopy

The dynamics of neutral and singly ionized strontium, produced as minor components in the laser ablation of amorphous calcium carbonate (chalk), have been investigated by space- and time-resolved atomic absorption spectroscopy. Absorption of a probe beam from a single-frequency cw dye laser was used to monitor the subject species at 420.7 nm (ion) and 460.7 nm (neutral). Results including time-resolved absorption profiles, energy distributions and the effect of a cover gas are presented and discussed.

Deactivation of I(2P1/2) by CH3Cl, CH2Cl2, CHCl3, CCl3F, and CCl4

Collisional deactivation of I(2P1/2) by the title compounds was investigated through the use of the time-resolved atomic absorption of excited iodine atoms at 206.2 nm. Rate constants for atomic spin-orbit relaxation by CH3Cl, CH2Cl2, CHCl3, CCl3F, and CCl4 are 3.1±0.3×10−13, 1.28±0.08×10−13, 5.7±0.3×10−14, 3.9±0.4×10−15, and 2.3±0.3×10−15cm3 molecule−1 s−1, respectively, at room temperature (298 K). The higher efficiency observed for relaxation by CH3Cl, CH2Cl2, and CHCl3 reveals a contribution in the deactivation process of the first overtone corresponding to the C(SINGLEBOND)H stretching of the deactivating molecule (which lies close to 7603 cm−1) as well as the number of the contributing modes and certain molecular properties such as the dipole moment. It is believed that, for these molecules, a quasi-resonant (E-v,r,t) energy transfer mechanism operates. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 799–803, 1998

Simultaneous multielement graphite furnace atomic absorption measurements using a photodiode array detector

A photodiode array detector multichannel analyser system has been coupled to a graphite furnace atomizer and tested for simultaneous multielement atomic absorption analysis. Multielement hollow cathode lamps are used as light sources and spectral lines are dispersed through a spectrograph with three selectable gratings. Multiple transmitted spectra are recorded to simultaneously determine the atomic absorption profiles of the analyte elements during the atomization stage. Atomic absorbance of individual elements is obtained by integrating the respective peak areas of the appropriate time-resolved atomic absorption spectra. The obtained sensitivities for Ni-Co-Fe are within the same order of magnitude as those from conventional single element determinations using photomultiplier tube detection. The system has also been applied for simultaneous multielement flame atomic absorption spectrometry (AAS) measurements and it has been demonstrated that background absorption can be readily corrected for both flame and graphite furnace AAS by a two-line method where non-atomic absorption lines can be chosen from the simultaneously recorded spectra.

Direct Determination of Trace Elements in Graphite Matrices Using Modulated Glow Discharge Atomic Absorption Spectrometry

The trace element concentrations in high-purity graphites were determined by atomic absorption spectrometry (AAS) with the use of a modulated glow discharge atomizer. Glow discharge power was modulated at SO Hz to enhance sensitivity and to permit time-resolved atomic absorption measurements. The influence of discharge operating parameters including current, pressure, and plasma sampling on the analytical performance of this technique was investigated. This analytical technique was evaluated in terms of stability, sensitivity, reproducibility, and accuracy. The method of standard additions was used to determine the impurity levels of the standard graphite matrix. Linear analytical calibration curves were constructed for four elements of interest in samples of nuclear-grade graphite. Detection limits for these elements were calculated to be in the tens of parts-per-billion range for graphite matrices.

Quenching kinetics of I(2P½) by cyanides. Isotope effect in the quenching by CH3(D)CN

The quenching of electronically excited iodine atoms I(2P½ I*) by CH3CN, CD3CN, HCN, C2H5CN, and (CN)2 was studied. Good agreement was found for I* deactivation by acetonitrile and perdeuteroacetonitrile using two widely different techniques, namely time-resolved atomic absorption (TRAA) and chemical laser threshold gain (CLTG). The kinetic isotope effect determined, kH/kD, was 2.2 ± 0.5 for both methods. This low value would support the existence of a noticeable reactive path and a possible channel assisted by a curve-crossing mechnism. Values here reported for the second-order rate constants for I* deactivation are, respectively, (1.2 ± 0.1)× 10–13, (5.3 ± 0.5)× 10–14, (3.3 ± 0.2)× 10–14, (1.3 ± 0.1)× 10–13, (2.2 ± 0.2)× 10–15 cm3 molecule–1 s–1 for CH3CN, CD3CN, HCN, C2H5CN and (CN)2, according to the TRAA method. Results from CLTG measurements are (1.6 ± 0.1)× 10–13, (7.3 ± 0.2)× 10–14 and (8.3 ± 0.1)× 10–14 cm3 molecule–1 s–1 for CH3CN, CD3CN and HCN, respectively.

Time-Resolved Electrothermal Atomic Absorption Spectra

A wavelength-modulated continuum source atomic absorption spectrometer is used to obtain time-resolved atomic absorption spectra (absorbance vs wavelength) using an electrothermal atomizer. Absorbance spectra are acquired at a rate of 56 Hz during atomization. The operator may choose any portion of the atomization time period to be examined as well as the number of absorbance profiles to be averaged. This time-resolved spectral information facilitates the positive identification of spectral interferences as well as providing for detailed inspection of both analyte and background absorption signals. Several interferences are examined.

Production of atomic oxygen following flash photolysis of ClONO 2

The photodissociation of ClONO 2 using a broad-band ultraviolet photolysis source has been investigated using time-resolved atomic absorption spectroscopy in the vacuum ultraviolet. The predominant atomic photolysis product is O( 3P J), the quantum yield for Cl( 2P J) production being less than 4%.

O2(b 1Σg+) production and deactivation following quenching of O(1D2) in O3/O2 mixtures

The production of oxygen atoms, O(3PJ), following deactivation of photolytically generated O(2 1D2) via was monitored using time-resolved atomic absorption analysis in the vacuum ultraviolet. The measured overall rate constant for O2(b 1Σg+) deactivation by O3, 1.8±0.2×10−11 cm3 molecule−1 s−1, and total yield of ground state O(3PJ) following quenching of O2(b 1Σg+) by O3, 0.61±0.06, are in good agreement with a previous study. These results suggest that the reaction of O2(b 1Σg+) with O3 may play a role in fixing the overall density of oxygen atoms in the stratosphere.

Direct observation of S(3 1D 2) and determination of the absolute rate of reaction with OCS

We report the first direct kinetic study of electronically excited sulphur atoms in the 3 1D 2 state. The formation and decay of S(3 1D 2), following ultraviolet photolysis of OCS (λ ≈ 200 – 260 nm), were monitored using time-resolved atomic absorption photometry in the vacuum ultraviolet (λ = 167 nm). The absolute rate reaction with OCS was determinded at 290 K as k = (1.2 ± 0.3) X 10 −10 cm 3 molecule −1 s −1, close to unit collision efficiency.

Temperature dependence of the quenching of I(52P½) by HCl

The quenching of I(52P½) by HCl has been studied over the temperature range 200–400 K, using time-resolved atomic absorption spectrophotometry (λ= 206 nm). The quenching efficiency was found to increase smoothly over this range supporting a previously proposed curve crossing mechanism. The results also provide information on non-adiabatic coupling in the exit channels for reaction of H with ICl and Cl with HI.

Temperature dependence for the removal of I(5 2P [formula omitted]) by HBr

Rate data for the removal of I(5 2P 1 2 by HBr over the temperature range 253–427 K have been obtained using time-resolved atomic absorption spectrophotometry. A negative temperature dependence was observed over most of this range with evidence for a minimum at ≈400 K. The results are discussed in terms of competing reactive and inelastic channels, and are compared with data calculated from a theory based on long range E → V transfer.

Direct observation of Se(4 1D 2) and quenching by the noble gases

The first direct kinetic observation on electronically excited Se(4 1D 2) atoms, following the flash photolysis of OCSe, are reported. The kinetics of Se(4 1D 2) were monitored using time-resolved atomic absorption spectrophotometry. (λ = 185.8 nm). Quenching data for the noble gases He, Ne, Ar, Kr and Xe are reported and the data discussed in terms of singlet-triplet curve crossings which are now well established for the noble gas oxides. High efficiencies for electronic to translational energy transfer are observed despite the large electronic energy involved.

Yields and quenching of I(5 2P [formula omitted]) following photodissociation of C 6H 5I and C 6F 5I

The yields of electronically excited iodine atoms I(5 2P 1 2 ) following the UV photodissociation of C 6H 5I and C 6F 5I have been studied using time-resolved atomic absorption spectrophotometry. The yields are shown to be low and spin—orbit relaxation by both molecules is rapid ( k(C 6H 5I) = (5.2 ± 0.4) × 10 −13, k(C 6F 5I) = (2.1 ± 0.2) × 10 −13 cm 3 molecule −1 s −1). The suitability of C 6H 5I and C 6F 5I as photochemical laser materials is discussed.

Photodissociation of alkyl bromides

The branching ratios for the production of Br(4 2P 1 2 ) following the broadband flash photolysis of the alkyl bromides, CH 3Br and C 2H 5Br, and the perfluorinated molecules, CF 3Br, C 2F 5 Br and n-C 3F 7 Br, have been determined using time-resolved atomic absorption spectroscopy. The production of electronically excited bromine atoms is shown to be inefficient in the case of the alkyl bromides while the perfluorinated molecules yield decreasing amounts of Br(4 2P 1 2 ) as the molecular complexity increases, i.e., CF 3Br > C 2F 5Br > C 3F 5Br > C 3F 7. It is also shown that the hydrogenated bromides deactive electronically excited atoms almost two orders of magnitude faster than do the perfluorinated bromides.

Lineshape effects in atomic absorption spectroscopy

Deviations from Beer's Law caused by the relative lineshape functions of source and absorber in atomic absorption experiments are considered. The validity of using a modified form of the law incorporating a correction factor γ, which is particularly convenient in time-resolved atomic absorption experiments, to account for these deviations, has been critically examined for a wide range of source and absorber lineshapes using numerical evaluation of the transmission integrals involved. It is concluded that there is, in general, no theoretical justification for the use of such a γ factor, except in the case of large source linewidth/absorber linewidth ratios when the line broadening in the absorber involves a significant homogeneous (Lorentzian) contribution. The use of empirically determined γ factors, much different from unity to analyse experimental data, should be viewed with suspicion unless direct evidence is presented to show that under the experimental conditions γ happens to be constant or slowly varying.

Kinetic study of ground state antimony atoms Sb(5 4S 3/2) by atomic absorption spectroscopy

A study of the collisional behaviour of ground state antimony atoms Sb(5p 3( 4S 3/2)) in the presence of various gases in presented. The atoms were generated by the low energy pulsed irradiation of SbMe 3 and monitored photoelectrically by time-resolved atomic absorption spectroscopy in the “single shot” mode. Attenuation of the resonance transition employed (5p 26s( 4P 1 2 ) ← 5p 3( 4S 0 3/2), λ = 231.1 nm) was investigated in some detail, including measurement of the departure from the Beer—Lambert law using the standard modified expression I tr = I 0 exp {−ϵ( cl) γ} for which it was found that γ(λ = 231.1 nm) = 0.34 ± 0.03. Third order kinetics were found for the removal of Sb(5 4S 3/2) by C 2H 2 and C 2H 2 in the presence of the gases He, N 2 and SF 6 for which the following absolute rate constants are reported. For the reaction Sb + C 2H 4 + M, the values of k M C 2H 4 (cm 6 molecule −2s −1, at room temperature) obtained were (1.0 ± 0.1) × 10 −32 for M = He, (1.1 ± 0.1) × 10 −32 for M = N 2 and (1.1 5 ± 0.2) × 10 −32 for M = SF 6. For the reaction Sb + C 2H 2 + M, the values of k M C 2H 2 (cm 6 molecule −2s −1, at room temperature) obtained were (9.4 ± 0.1) × 10 −33 for M = He, (1.0 ± 0.1) × 10 −32 for M = N 2 and (1.2 ± 0.1 4) × 10 −32 for M = SF 6. Removal in the presence of NO, for which atom transfer would be endothermic, nevertheless exhibited second order kinetics for which we report k NO(He, N 2, SF 6) = (1.8 ± 0.2) × 10 −13 cm 3 molecule −1 s −1 (at room temperature). Reactions in the presence of the gases Cl 2, O 2, N 2O and SbMe 3 were also studied, leading to the following rate data: k SbMe 3 = 2.3 ± 0.2 × 10 −11 and k N 2O < 10 −16 cm 3 molecule −1 s −1 (at room temperature). Third order kinetics were observed for removal in the presence of O 2. We report the following data for the overall process: ▪ The values of k M O 2 (cm 6 molecule −2 s −1, at 300K) were (2.2 ± 0.2) × 10 −31 for M = He, (3.2 ± 0.3) × 10 −31 for M = N 2 and (3.6 ± 0.4) × 10 −31 for M = SF 6. The rate data are compared with the analogous data obtained hitherto for Bi(6 4S 3/2) and, where appropriate, rates are considered within the context of symmetry arguments employing ( J, Ω) coupling to describe the nature of the potential surfaces involved. We believe this work to constitute the first direct and detailed investigation of the collisional behaviour of the ground state antimony atom.

Production of electronically excited iodine atoms I(5p 52P [formula omitted]) by collisional release in molecular I 2

The appearance rate of 1 * (5p 5 2P 1 2 ) following laser photolysis of molecular I 2 1.2 kT below the dissociation limit o the I 2 (B 3 Π ou+) state has been monitored by time-resolved atomic absorption as a function of I 2 pressure. Data were also taken with N 2 as an added gas. The results confirm the production of I * from the B state by a collisional process and reveal an additional process by which I * continues to appear for several hundred nanoseconds after the laser pulse even at N 2 pressures as high as 750 torr.

Kinetic study of the reactions Sn+N 2O→SnO+N 2

The rate constant for the reaction Sn( 3P 0) + N 2O(X 1Σ + g) → SnO(?) + N 2(X 1Σ + g) has been determined using the techniques of flash photolysis and time-resolved atomic absorption analysis. Over the temperature range 341–377 K, this rate constant may be expressed by k T = (5.0 ± 1.0) × 10 −13 exp[−(4000 ± 200)/ RT] cm 3 molecule −1 s −1. The role of adiabatic and diabatic spin and orbital symmetry correlation in the reaction may be invoked to explain the observed energy. The small magnitude of the rate constant implies that maintenance of a population inversion on the electronic transition from excited SnO would be difficult.

Primary and secondary processes in the photolysis of GeH 3I

Time-resolved atomic absorption spectrophotometry and flash spectroscopy in the vacuum ultra-violet have been employed to investigate primary and secondary processes in the photolysis (λ > 200 nm) of GeH 3I. The initial yield of I(5 2P 1 2 ) and I(5 2P 3 2 ) atoms has been measured ([I(5 2P 1 2 ] 0/ [I(5 2P 3 2 )] 0 ≅ 1.3) and the subsequent reactions of these two states to yield HI have been investigated. Formation of HI by reaction of I(5 2P 3 2 ) atoms with GeH 3I implies a limit to the bond dissociation energy D(H-GeH 2I) ⪅298 kJ/mol. The rate constant for removal of I(5 2P 1 2 ) by GeH 3I has been determined as k 2 = (6.9 ± 1.0) X 10 −12 cm 3 molecule −1 s −1: failure to observe laser emission at 1.315 μm, in this work, is ascribed to the large value for k 2.

Relative yields of electronically excited iodine atoms, I(5 2P[formula omitted]), in the photolysis of alkyl iodides

The fraction of electronically excited iodine atoms, I(5 2P12), formed in the broadband flash photolysis of several alkyl iodides has been determined using time-resolved atomic absorption spectroscopy. By monitoring the temporal profile of the ground state, I(5 2P12), following the photolytic pulse, the initial yields of ground and electronically excited atoms may be obtained. Deactivation rates of I(5 2P12) by these alkyl iodides are also reported.