Atomic absorption spectroscopy for online laser monitoring of triatomic metal species

Initial sequential CPFAAS experiments with PbCl2 were performed in controlled laboratory conditions to validate the theoretical derivation and obtain absorption cross-section values for the PbCl2 fragments. The experimental arrangement for PbCl2 detection is presented in Figure 1, consisting of two pulsed UV lasers and a CW narrow-linewidth diode laser (Sure Lock, Ondax, Santa Clara (CA), United States of America). The first fragmentation process was induced by the third harmonic (355 nm) of an Nd:YAG pulsed laser (Ultra Big Sky series, Quantel, France) emitting 5 ns wide pulses with a repetition rate of 10 Hz. The PbCl2 molecules were excited to a dissociative state due to absorption of the 355 nm pulses producing PbCl + Cl fragments. The second fragmentation process of PbCl into Pb* + Cl fragments was induced by the absorption of the second laser pulse from the fourth harmonic (266 nm) of an Nd:YAG laser (FQSS-266-200, CryLas GmBh, Berlin, Germany) having a temporal width of 1 ns. The simplified energy level diagram of the fragmentation process is presented in Figure 2 demonstrating that the produced Pb atoms were in excited (*) state. Hence, the probe laser wavelength was chosen accordingly to be 405.789 nm. The probe laser output was divided into the probe and reference beams using a wedge window (BS, WG10050-A, Thorlabs, Newton (NJ), United States of America). The reference beam was directed through a lead hollow cathode lamp (Heraeus, Hanau, Germany) containing vaporized Pb* atoms. The wavelength of the CW probe laser was tuned and actively locked to the Pb* absorption profile at 405.789 nm [34] by monitoring the reference beam transmission through the lamp. The beam transmitted through BS was used as the probe beam in the CPFAAS arrangement. To verify the probe beam wavelength, it was measured with a wavelength-meter (WA-1500-VIS, EXFO Burleigh) with an accuracy of 0.1 pm. The three laser beams, two pulsed and one CW, were aligned in a collinear optical path through the sample volume. Their cross-section areas were expanded and limited to a circular aperture of 7 mm in diameter. The beams were aligned on the collinear path and separated after passing the sample by using appropriate dichroic mirrors (DM, Semrock, Rochester (NY), United States of America). Pulse energies were monitored with photodiode energy sensors (EM, PD-10, and PE-9, Ophir, Jerusalem, Israel, and two J25MB-LE, Coherent, Santa Clara (CA), United States of America) at the input and output of the sample volume. The probe laser intensity transmitted through the sample was monitored using a biased silicon photodiode (DET, DET10A, Thorlabs, Newton (NJ), United States of America) allowing a fast response in the ns-scale for CPFAAS signal detection. A digital oscilloscope (OSC, HDO6054, LeCroy, Chestnut Ridge (NY), United States of America) was used to record the signal for further analysis. Customized software was used to control the measurements from a portable computer. The software controlled and monitored the probe wavelength locking, synchronized the interpulse delay firing of the laser pulses, and the measurement of pulse energies (Pulsar-2 interface, Ophir, Jerusalem, Israel) and acquired the CPFAAS signal.
Figure 1. Experimental setup for the gas-phase detection of PbCl2. The transmission of the probe laser (405 nm) through a sample volume (SV) was monitored by a photodiode (DET) for collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) signal upon double photofragmentation process of 355 nm and 266 nm laser pulses. Pulse energies were monitored by energy meters (EM) at the input and output. Beam splitters (BS) and dichroic mirrors (DM) were used to split, direct, and co-align the beams. An oscilloscope (OSC) was used for recording the signal for further analysis.
Figure 2. Simplified energy levels diagram for PbCl molecules and Pb atoms adapted from [35]. The Pb transition associated with the optical absorption of light at 405.79 nm involved the excited states 6p2 3P0 and 6p7s 3P1. According to the diagram, the 6p2 3P0 level of Pb atoms can be achieved upon a double photodissociation process of PbCl2 molecules using laser pulses at 355 nm and 266 nm wavelengths.
In laboratory studies, the PbCl2 vapor was produced by heating solid PbCl2. The lead (II) chloride powder (268,690, Sigma Aldrich) was contained in a quartz tube that was placed into an electrical tube oven. Optical path length through the sample volume was 0.6 m. The experiments were carried out at N2 or at N2-O2 atmosphere and at atmospheric pressure. The amount of PbCl2 at gas phase was varied during the experiment by controlling the temperature of the oven. The set temperature during experiments in constant temperature was 493 °C. To reach the desired measurement conditions, the sample tube was flushed with the premixed gas mixture for 300 s. After flushing, the sample volume was let 120 s to settle into an equilibrium before recording the desired measurement signal. It was experimentally validated that 120 s was sufficient to reach static temperature and PbCl2 vapor concentration above the solid PbCl2 sample. The equilibrium concentrations of PbCl2 vapor at each temperature were computed using commercial thermochemical database HSC 5.1 (Outokumpu Research). The database has been previously utilized, for example, to validate quantitative KCl measurement with CPFAAS. The KCl results were compared also with differential optical absorption spectroscopy (DOAS) showing high correspondence. [29] Therefore, HSC 5.1 database was relied on also in the case of PbCl2.

4.2. CPFAAS Signal Formation and Limit of Detection

Examples of the temporal probe beam transmission curves after PbCl2 photodissociation are presented in Figure 4. The example curve is an average of 100 signals. The recorded photodetector signal shows a transmission decay of the transmitted probe beam at t = 0 that corresponds to the launch of the second fragmenting laser pulse. The relative depth of the temporal minimum in transmission corresponds to the absorbance 𝛼Lmax after consecutive fragmenting laser pulses that were fired through the sample volume. The signal dip resulted from a sudden increase of Pb* atoms on the optical path inside the sample chamber and their ability to absorb light at the wavelength of the probe beam. Hence, the photon energy associated to the first fragmentation laser pulse with wavelength of 355 nm dissociated resonantly the PbCl2 molecules into PbCl in a ground state X as depicted in the diagram of Figure 2. The second absorption process of PbCl molecules was initiated by absorption of the second fragmenting laser pulse with wavelength of 266 nm that populated the electronic state B of the PbCl molecule. A predissociation process was achieved along the C curve into Pb* in the excited state 6p2 3P2 [38]. Here, the transmission of the probe beam was reduced due to optical absorption into Pb*, producing the signal related to 𝛼Lmax. The transmission signal decays to the base level after hundreds of nanoseconds following an exponential form, being in agreement with Equation (8). The signal of the transmitted beam had similar behavior to the CPFAAS signal observed earlier for KCl and KOH molecules [14,15,16].
Figure 4. Examples of CPFAAS signal that shows the effect of O2 environment on the magnitude of a signal.
Process gas environments, such as combustion flue gas, are reactive gas mixtures. Therefore, it is important to know how changes in gas composition affect the obtained CPFAAS signal due to its dependence on chemical reaction kinetics. O2 concentration was found to be the most critical factor on quantitative PbCl2 measurement with CPFAAS due to its reactivity and good availability on combustion gases. The lifetime of the PbCl molecules after the first fragmentation event was analyzed experimentally for N2 and N2-O2 (96–4%) environments by tuning the interpulse delay time, Δt. The decay time of the PbCl radical concentration followed an exponential decay for both environments. At pure N2 atmosphere, the decay time was estimated to be 30 μs. For the N2-O2 atmosphere, the PbCl lifetime decreased to 14 μs. Thus, the Δt played a major role on the signal formation in O2 environments since shorter Δt led to stronger CPFAAS signal. In our experiments, Δt was varying due to the accumulative effect of the laser jitters. Hence, to obtain credible quantitative data, Δt was controlled to be below 1 μs by neglecting the signals recorded with interpulse delay exceeding this threshold value. Interpulse delay Δt = 1 μs led to quantitative measurement uncertainty of 4% and 7% in N2 and in used N2-O2 atmospheres, respectively. In addition to PbCl lifetime, the presence of O2 affected the lifetime of Pb* by reducing it to nanosecond time scale. Figure 4 shows the combined effect of PbCl and Pb* lifetimes at different atmospheres on CPFAAS signal. Typically, the concentrations of major gases are fairly well known in industrial processes and, therefore, the effect of O2 concentration on quantitative PbCl2 monitoring can be mitigated by taking it into consideration in signal processing. However, this demonstrates how sequential CPFAAS can aid chemical kinetic research by providing direct measurement data on chemical reaction rates that previously had not been accessible. Chemical models and simulations rely often on computational kinetic values, especially in the case of radical intermediates. Providing feedback on such models is a topic of another research.

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