Single-photon Laser-induced Fluorescence Research Articles

In-depth structural and compositional assessment of aged terpenoid varnish layers by nonlinear optical microscopy

Varnishes are employed in painted artworks for protection from atmospheric pollution and oxidation and for improving the esthetic appearance by providing an even and brilliant surface finish. Also, in some cases, colored varnishes are applied intentionally to modify the chromatic appearance of paintings. Varnishes undergo complex and differentiated structural and chemical changes over time depending on their composition and conservation conditions. The present work investigates the extent of degradation of the outermost layers of varnish coatings due to aging as a function of depth by using nonlinear optical microscopy (NLOM) in the modality of multiphoton excitation fluorescence (MPEF). This non-invasive technique has been employed for the determination of the thickness, with a microscopic axial and lateral resolution, of the affected regions of pictorial terpenoid varnish layers, such as dammar, mastic, shellac and sandarac, subjected to various types and degrees of aging, artificial, natural and a combination of both. A homemade nonlinear optical microscope, based on a tightly focused pulsed femtosecond laser emitting at 800 nm, was used for the investigation. Single-photon laser-induced fluorescence (LIF) measurements served to determine the degree of surface aging and the optimum NLOM-MPEF operating conditions and helped to interpret the results obtained by applying the latter. These results signpost the correlations between the extent of degradation, non-invasively determined by NLOM-MPEF, the type and length of aging, the nature of the varnish and the initial thickness of the varnish layer.

Crosstalk-Free Excitation Scheme for Quantitative OH Laser-Induced Fluorescence in Environments Containing Excited CO

Spectral overlap in the single-photon laser-induced fluorescence between the 3064 Å system of OH and the third positive system of CO is detected in a highly-excited environment, namely, a CO2-H2O plasma. The overlap is distorting excitation and fluorescence spectra as well as fluorescence time decays of commonly used excitation transitions of OH. As a consequence, systematic errors are introduced into the determination of temperatures, gas compositions, and absolute number densities. The P1(2) transition is proposed to circumvent the distortion while still allowing for quantitative measurements due to the availability of non-radiative rate coefficients.

Single-photon laser-induced fluorescence detection of nitric oxide at sub-parts-per-trillion mixing ratios

Abstract. We describe a newly developed single-photon laser-induced fluorescence sensor for measurements of nitric oxide (NO) in the atmosphere. Rapid tuning of a narrow-band laser on and off of a rotationally resolved NO spectral feature near 215 nm and detection of the red-shifted fluorescence provides for interference-free direct measurements of NO with a detection limit of 1 part per trillion by volume (pptv) for 1 s of integration, or 0.3 pptv for 10 s of integration. Uncertainty in the sensitivity of the instrument is typically ±6–9 %, with no known interferences. Uncertainty in the zero of the detector is shown to be <0.2 pptv. The instrument was deployed on the NASA DC-8 aircraft during the NASA/NOAA FIREX-AQ experiment (Fire Influence on Regional to Global Environments Experiment – Air Quality) during July–September 2019 and provided more than 140 h of NO measurements over 22 flights, demonstrating the ability of this instrument to operate routinely and autonomously. Comparisons with a seasoned chemiluminescence sensor during FIREX-AQ in a variety of chemical environments provides validation and confidence in the accuracy of this technique.

Hydroxyl Radical Kinetics in Repetitively Pulsed Hydrogen–Air Nanosecond Plasmas

Absolute hydroxyl radical (OH) concentration is determined in stoichiometric hydrogen-air mixtures at P = 54-94 torr and initial temperature of T = 100°C-200°C, which are both functions of time, after the application of a single approximately 25-ns-duration approximately 20-kV discharge pulse and 60 μs after the final pulse of a variable-length burst of pulses, using single-photon laser-induced fluorescence (LIF). Relative LIF signal levels are put on an absolute number density scale by means of calibration with a standard atmospheric-pressure near-adiabatic Hencken flat-flame burner. By obtaining OH LIF data in both the plasma and the flame and correcting for differences in the collisional quenching and vibrational energy transfer rates, absolute OH number density has been determined. For a single discharge pulse, the absolute OH temporal profile is found to rise rapidly during the initial ~0.1 ms after discharge initiation and decay relatively slowly, with a characteristic time scale of ~1 ms. In repetitive burst mode, the absolute OH number density is observed to rise rapidly during the first approximately ten pulses (0.25 ms) and then level off to a near steady-state plateau. In all cases, a large secondary rise in OH number density is also observed, which is clearly indicative of ignition, with ignition time ranging from 5 to 10 ms, for initial temperatures of 100 °C and 200°C and pressures in the range of 54-94 torr. Plasma kinetic modeling predictions capture this trend quantitatively, using both a full 22-hydrogen-air-chemical-reaction set and a reduced 9-reaction set.

Formation of Hydroxyl Radical from the Photolysis of Salicylic Acid

Photodissociation dynamics of salicylic acid (SA) in the gas phase at different photolysis wavelengths (266, 315-317 nm) is investigated by probing the nascent OH photoproduct employing the single-photon laser-induced fluorescence (LIF) technique. At all the photolysis wavelengths it is found that the nascent OH radicals are produced mostly in a vibrationally ground state (υ'' = 0) and have similar rotational state distributions. The two spin-orbit and Λ-doublet states of the OH fragment formed in the dissociation are measured to have a nonstatistical distribution at each photolysis wavelength. The LIF signal of the OH could be observed upon photolysis at 317 nm but not at 317.5 nm. The threshold of OH formation from SA photodissociation is estimated to be 98.2 ± 0.9 kcal/mol. The effect of the phenolic OH group on the dissociation of SA is discussed.

Direct Detection of NO Produced by High-Temperature Surface-Catalyzed Atom Recombination

The surface-catalytic recombination of oxygen and nitrogen atoms to form nitric oxide was confirmed by the direct detection of product NO molecules, using single-photon laser-induced fluorescence spectroscopy. Experiments were performed from room temperature to 1200 K in a quartz diffusion-tube sidearm reactor enclosed in a high-temperature tube furnace. Atomic nitrogen was generated using a microwave discharge, and atomic oxygen was produced via the rapid gas-phase titration reaction N + NO → O + N 2 . The use of isotopically labeled titration gases 15 N 16 O and 15 N 18 O allowed for the unambiguous identification of nitric oxide produced by the O + N surface reaction. The absolute number densities of surface-produced NO were determined from separate calibration experiments using 14 N 16 O. Observed variations of the NO number density with temperature and varying O/N atomic ratios at the sidearm entrance are generally consistent with the predictions of a simple reaction-diffusion model of the sidearm reactor that includes surface-catalyzed NO production as a species boundary condition.

OH produced from o-nitrophenol photolysis: A combined experimental and theoretical investigation

Photodissociation dynamics of o-nitrophenol in the gas phase at different photolysis wavelengths (361-390 nm) is investigated, and the nascent OH radical is observed by the single-photon laser-induced fluorescence technique. At all the photolysis wavelengths, the OH radicals are formed in vibrationally cold state (upsilon(")=0) and have similar rotational state distributions. The average rotational temperature for all the photolysis wavelengths is approximately 970+/-120 K, corresponding to a rotational energy of 1.9+/-0.2 kcal mol(-1). The spin orbit and Lambda-doublet states of the OH fragments formed in the dissociation are measured to have nonstatistical distributions. To get an insight into the dissociative mechanism leading to OH formation in the photolysis of o-nitrophenol, the potential energy surfaces of the OH-forming channels are mapped by ab initio theoretical calculations. According to both experimental and theoretical results, a possible mechanism for OH formation is proposed.

Photolysis of o‐Nitrobenzaldehyde in the Gas Phase: A New OH. Formation Channel

Photolysis of gaseous o-nitrobenzaldehyde (o-NBA) with selected different excitation wavelengths (355-400 nm) is investigated, and the nascent OH radical is detected by the single-photon laser-induced fluorescence (LIF) technique. The relative quantum yield and rotational excitation of OH formation are found to be dependent on the excitation energy. The distributions of rotational, spin-orbit, and Lambda-doublet states are obtained at 355-400 nm by analyzing the experimental data. The OH radicals are found to be vibrationally cold at all photolysis wavelengths. The spin-orbit and Lambda-doublet states have nonstatistical distributions. To understand the dissociative process involved in the OH-generating channel, DFT calculations are performed. Based on both experimental and theoretical results, possible photolysis channels of o-NBA leading to the OH fragment are proposed and discussed.

Dynamics of OH Formation in the Photodissociation of o-Nitrobenzoic Acid at 295 and 355 nm

Photodissociation dynamics of o-nitrobenzoic acid at 295 and 355 nm is studied by probing the nascent OH photoproduct employing the single-photon laser-induced fluorescence technique. At both of the photolysis wavelengths, the OH fragments are found to be vibrationally cold but have different rotational state distributions. Upon photolysis at 295 nm, the relative population of OH in different rotational states does not follow the Boltzmann equilibrium distribution, whereas upon photolysis at 355 nm, a Boltzmann distribution is observed with a rotational temperature of 1010 +/- 100 K. Between the two spin-orbit states, (2)Pi(3/2) and (2)Pi(1/2), the former is found to be preferentially populated, and the distribution of the Pi(A') state for the Lambda-doublet is dominant at both of the wavelengths studied. Several possible dissociation pathways of o-nitrobenzoic acid leading to formation of the OH fragment are investigated computationally. On the basis of the theoretical and experimental studies, a possible mechanism of OH formation from the photodissociation of o-nitrobenzoic acid at 295 and 355 nm is proposed.

Photodissociation dynamics of benzenesulfonic acid at 266 nm: OH detection by laser-induced fluorescence

Photodissociation dynamics of benzenesulfonic acid at 266 nm was studied by probing the nascent OH photoproduct using the single-photon laser-induced fluorescence technique. Measurements on the nascent OH fragment revealed that it was produced vibrationally cold and its rotational state distribution was Boltzmann-like, characterized by a rotational temperature of 840 ± 70 K. The ratios of 2Π 3/2 to 2Π 1/2 states of the OH fragment were found to be more than unity and the population of the Π + state for the Λ-doublet was dominant. Based on both experimental and theoretical results, a probable mechanism for the formation of OH fragment produced from the photodissociation of BSA is proposed.

Optical–optical double resonance LIF detection of in high pressure gas discharges

An optical–optical double resonance (OODR) laser induced fluorescence (LIF) for the detection of the metastable is described. OODR employs the double laser excitation of A–B and B–C nitrogen transitions. Here it is proposed as a sensitive approach to detect in high-pressure discharges. We have tested its sensitivity with respect to conventional single photon LIF by an excitation/detection scheme in which the two wavelengths required are generated by a provisional single laser setup. Other single laser configurations that might allow a sensitivity close to two orders of magnitude larger than that of conventional LIF are proposed. Such gain can be particularly useful for the detection of in non-equilibrium, high-pressure discharges.

Photodissociation of nitrobenzene at 266 nm: Experimental and theoretical approach

The 266 nm photodissociation of gaseous nitrobenzene has been studied by monitoring the NO X 2Π product using the single-photon laser-induced fluorescence technique. The rotational population and internal energy distributions of fragment NO product were determined. In the experiment, we only observed the rotational state distributions of the NO fragment for v″=0. The rotational state distributions of the nascent NO photofragment can be characterized by a Boltzmann temperature of 3300±300 K. Ab initio calculations were performed to characterize the transition state and to determine the barrier height for the rearrangement of nitrobenzene to phenylnitrite. A clear photodissociation picture has been proposed theoretically.

Gas phase elemental mercury: a comparison of LIF detection techniques and study of the kinetics of reaction with the hydroxyl radical

We have examined the sensitivity of single and sequential two-photon laser-induced fluorescence (LIF) techniques for the detection of elemental mercury, Hg(0), in the gas phase. Single photon LIF involves excitation of the 6 3 P 1 –6 1 S 0 transition at 253.7 nm, followed by observation of resonance fluorescence. Sequential two-photon techniques follow the initial 6 3 P 1 –6 1 S 0 excitation with a second excitation step to either the 7 1 S 0 or 7 3 S 1 levels followed by observation of blue or red shifted fluorescence. We have examined four variants of these approaches which all exceed the sensitivity of single photon LIF. The most sensitive detection approach involves the initial 253.7 nm excitation followed by excitation of the 7 1 S 0 –6 3 P 1 transition at 407.8 nm. Fluorescence is observed on the 6 1 P 1 –6 1 S 0 transition at 184.9 nm. Using this approach, our limits of detection are 0.1 ng m −3 with a 10 s integration time in air. We have also examined the effects of saturation, quenching and line-width on detection sensitivity. We have used the pulsed laser photolysis–pulsed laser-induced fluorescence (PLP-PLIF) technique to study the kinetics of the reaction of elemental mercury with the hydroxyl radical under atmospheric conditions at 298 K. We see no evidence for reaction and obtain an upper limit of 1.2×10 −13 cm 3 molecule −1 s −1 for the rate coefficient.

Application of a compact all solid-state laser system to the in situ detection of atmospheric OH, HO2, NO and IO by laser-induced fluorescence

A tuneable, high pulse-repetition-frequency, solid state Nd:YAG pumped titanium sapphire laser capable of generating radiation for the detection of OH, HO2, NO and IO radicals in the atmosphere by laser induced fluorescence (LIF) has been developed. The integration of the laser system operating at 308 nm into a field measurement apparatus for the simultaneous detection of hydroxyl and hydroperoxy radicals is described, with detection limits of 3.1 x 10(5) molecule cm(-3) (0.012 pptv in the boundary layer) and 2.6 x 10(6) molecule cm(-3) (0.09 pptv) achieved for OH and HO2 respectively (30 s signal integration, 30 s background integration, signal-to-noise ratio = 1). The system has been field tested and offers several advantages over copper vapour laser pumped dye laser systems for the detection of atmospheric OH and HO2 radicals by LIF, with benefits of greater tuning range and ease of use coupled with reduced power consumption, instrument footprint and warm-up time. NO has been detected in the atmosphere at approximately 1 ppbv by single photon LIF using the Alpha 2Sigma+ <-- Chi 2Pi1/2 (0,0) transition at 226 nm, with absolute concentrations in good agreement with simultaneous measurements made using a chemiluminescence analyser. With some improvements in performance, particularly with regard to laser power, the theoretical detection limit for NO is projected to be approximately 2 x 10(6) molecule cm(-3) (0.08 pptv). Whilst operating at 445 nm, the laser system has been used to readily detect the IO radical in the laboratory, and although it is difficult to project the sensitivity in the field, an estimate of the detection limit is < 1 x 10(5) molecule cm(-3) (< 0.004 pptv), well below previously measured atmospheric concentrations of IO.

Internal-state distribution and polarization in nascent NO [formula omitted] from the photodissociation of C 6H 5NO

The 266 nm photodissociation of gaseous C 6H 5NO has been studied by monitoring the nascent NO ( X 2Π v″=0–3 ) product using the single-photon laser-induced fluorescence technique. The rotational-state distributions of the nascent NO photofragment have been measured and can be characterized by Bolzmann temperatures. The vibrational-state and Λ-doublet populations were also determined. Energy disposal into the internal degrees of freedom of NO ( X 2Π ) photofragment is about 18% of the total available energy. Some planarity in the fragmentation process was observed.

Laser-induced fluorescence of N2(X 1Σg+) and electron-impact excited N2+ (X 2Σg+) in a pulsed supersonic beam: Rotational distributions

Abstract Rotational population distributions have been obtained for N 2 and N 2 + in a pulsed supersonic expansion. For N 2 , the populations were determined from two-photon laser-induced fluorescence spectra of the a 1 Π g ← X 1 Σ g + transition, while rotational populations for N 2 + (X 2 Σ g + ) were determined from single-photon laser-induced fluorescence spectra of the B 2 Σ u + ← X 2 Σ g + transition. The N 2 + produced by electron impact exhibits much higher rotational excitation than predicted by either the dipole or dipole plus n quadrupole model, and indicates the ionization—excitation process is more complex than has previously been realized.

Single-photon laser-induced fluorescence detection of NO and SO_2 for atomospheric conditions of composition and pressure: errata

Reported here are laboratory results from a laser-induced fluorescence (LIF) study of the molecules NO and SO2 in which both the selectivity and sensitivity of the LIF method are examined. The laser excitation of these molecules occurred at 226 and 222 nm, respectively. The laser system employed consisted of a Nd: YAG-driven Quanta-Ray PDL dye laser, the fundamental of which was frequency doubled, and this output, in turn, was then frequency mixed with the Nd:YAG fundamental at 1064 nm. Two different dyes were required for generating the 226- and 222-nm wavelengths. To make these results as relevant as possible to the ultimate development of an atmospheric airborne field sampling system all experiments were carried out in atmospheric conditions of pressure and composition. In addition to the experimental data provided there has also been presented a theoretical assessment of the signal strength for both the NO and SO2 LIF systems, and these results have been compared with the experimentally measured values. Current state-of-the-art technology would suggest that both NO and SO2 can be measured by the LIF technique in atmospheric conditions at concentration levels of a few pptv.

Sequential two-photon-laser-induced fluorescence: a new method for detecting atmospheric trace levels of NO

Sequential two-photon-laser-induced fluorescence (TP-LIF) in NO has been achieved by using 226-nm radiation to excite the X(2)Pi ? A(2)Sigma transition followed by excitation of the A(2)Sigma ? D(2)Sigma transition using the fundamental output of a Nd:YAG laser at 1064 nm. The resulting fluorescence was monitored at wavelengths as low as 187 nm (D(2)Sigma ? X(2)Pi transition), thus permitting major discrimination against conventional noise sources as encountered in single-photon-laser-induced fluorescence (SP-LIF) systems. Also, since the 226-nm radiation used in the TPLIF method is generated in the same manner as in the SP-LIF technique (i.e., the sum-frequency mixing of 288-with 1064-nm radiation), the two-photon sequential-pumping scheme requires no major extra optical hardware. Although the final optimization of the TP-LIF NO method has not yet been completed, extrapolations based on currently measured signal and noise levels suggest a detection sensitivity level of 3 parts in 10(13) (0.3 pptv) under atmospheric conditions. This would represent approximately a 25-fold increase in sensitivity over the SP-LIF NO technique.