Giuseppe A. Petrucci

Ground state saturated population distribution of OH in an acetylene-air flame measured by two optical double resonance pump-probe approaches

Two optical double-resonance pump-probe techniques were used to determine the ground-state rotational population distributions of OH in an acetylene-air flame when a saturating laser beam is tuned to the Q(1)4 transition of the (0, 0) Sigma-II band. The saturated absorption technique is based on the detection of absorption by a probe laser under conditions of saturation with a pump laser and no saturation. In the fluorescence technique, a probe laser is scanned through the (1, 0) band, while a saturating pump laser, tuned to the (0, 0) band, is on or off. We found that approximately 15% of the total population of the ground state was transferred to the excited state. Perturbation of the rotational population distribution was greater for rotational levels close to the directly excited laser-coupled level. The rotational energy transfer rate in the ground state was somewhat greater than in the excited state. The assumption of the balanced cross-rate model was verified as a means of determining the absoslute OH number density with adequate accuracy.

Laser-excited atomic-fluorescence spectrometry with electrothermal tube atomization

The performance of graphite-tube electrothermal atomizers is evaluated for laser-excited atomic-fluorescence spectrometry for several elements. Three pulsed laser systems are used to pump tunable dye lasers which subsequently are used to excite Pb, Ga, In, Fe, Ir, and Tl atoms in the hot graphite tube. The dye laser systems used are pumped by nitrogen, copper vapour and Nd:YAG lasers. Detection limits in the femtogram and subfemtogram range are typically obtained for all elements. A commercial graphite-tube furnace is important for the successful utilization of the laser-based method when the determination of trace elements is intended, especially when complicated matrices may be present.

Microdetermination of fluoride by laser-excited molecular fluorescence spectroscopy in a graphite furnace☆

Abstract Fluoride at picogram levels is determined by using laser-excited molecular fluorescence of MgF produced in a graphite atomizer by adding magnesium to the fluoride sample. A pulsed dye-laser, tuned at 268 nm, is used to excite MgF and the resulting fluorescence is measured at 358 nm. Experimental parameters, including graphite furnace conditions and magnesium concentration, are optimized. The effect of other halides and anions on MgF fluorescence intensity have also been explored.

Determination of chloride at picogram levels by molecular fluorescence in a graphite furnace

Chloride was determined at nanogram levels by adding excess of indium to the sample introduced into a graphite furnace and measuring the laser induced molecular fluorescence of indium chloride. The diatomic molecules of indium chloride were excited by a pulsed dye laser at 267 nm and fluorescence was measured at 359 nm. The effects of various parameters including amount of indium added, furnace thermal conditions and presence of concomitants were also studied. A linear calibration in the range of 0.025-1.25 ng and a detection limit of 17 pg of chloride were obtained under optimum conditions. The analytical usefulness of the method was checked by determining the chloride content in National Institute of Standards and Technology, Standard Reference Materials 1571a and 1571b Orchard Leaves.

Determination of Phosphorous by ICP-AES Using Solid-Phase Hydride Generation

Abstract A reliable and sensitive analytical optical method for the determination of phosphorous in oceanographic and environmental materials does not exist. In this work, a novel hydride generation method was used to form phosphine from a sample containing phosphate mixed with 7% sodium borohydride. By heating the mixture to 500 °C, phosphine was reproducibly generated and collected in a cold trap. Phosphine was then introduced into an ICP where several atomic emission lines for phosphorous were observed. Copper emission did not spectrally interfere with phosphorous atomic emission with this hydride generation-ICP-AES method. This hydride generation method allows the use of small sample volumes (< 5 μL).

High-Spatial-Resolution OH Rotational Temperature Measurements in an Atmospheric-Pressure Flame Using an Indium-Based Resonance Ionization Detector

The use of a resonance ionization photon detector (RID) is described for the measurement of flame temperatures with a spatial resolution of less than 100 μm. The detector, based on the two-step excitation of indium atoms, with subsequent collisional ionization, was used to record rotational excitation scans of OH in an atmospheric-pressure acetylene/air flame. The OH excitation spectra were recorded by scanning an “excitation” laser in the A2σ+ ← X2II i (1, 0) vibronic band in the wavelength range, 281–288 nm, while simultaneously illuminating the same flame region with the “detection” laser, tuned to the 6p2P3/2 → 10d2. D5/2 excited-state transition of In at 786.44 nm. The excitation and detection laser beams were made orthogonal in the flame, defining the resolution to be limited by the waist of the excitation beam (100 μm), whose diameter was always smaller than the detection laser beam. A temperature profile of the flame is recorded with the use of both the RID approach and a more conventional laser-induced fluorescence (LIF) approach for comparison. A more structured temperature profile is recorded with the RID owing to its high spatial resolution, whereas the LIF method, which is inherently a line-of-sight method, produces a rather featureless temperature distribution across the flame. Anomalously high flame temperatures were recorded at the flame edge with the RID. The cause of these high flame temperatures has not been determined.

The double-resonance optogalvanic effect of neon as a sensitive photon detector

Abstract The double-resonance optogalvanic effect (OGE) of neon in a commercial hollow cathode lamp is used as a sensitive photon detector. The signal photons, emulated by a highly attenuated laser, served as the first step of the two-step, sequential excitation, while a high intensity laser saturated the second, connected transition. The limiting experimental noise was found to be the shot noise on the lamp operating current. The detector exhibited a high quantum efficiency (> 1) over approximately 5 orders of magnitude of energy. The minimum detectable peak energy was 1.3× 10 −15 J (4.2 × 10 3 photons at 640.225 nm).

Experimental evaluation of the autoionization cross section of the magnesium transition at 300.9 nm by laser induced fluorescence

Abstract The decrease in the laser induced resonance fluorescence of magnesium atoms at 285.213 nm, caused by photoionization of the 3 p −1 P 1 0 state with a second laser tuned at an autoionizing resonance at 300.9 nm, has been used to evaluate the autoionization cross section. This autoionizing resonance was found to have a cross section of 3 (±0.5) × 10 −16 cm 2 and a halfwidth of about 2.5 nm. The agreement with previous literature data, obtained by measuring the total number of charges produced, testifies to the reliability of our fluorescence method. The precision of the technique is also discussed and it is shown that our method works best for low-to-moderate values of the product of the autoionization cross section and the laser fluence.

Detection of OH in an atmospheric pressure flame via laser enhanced ionization of indium

Abstract A resonance ionization detector (RID) based on the two-step laser enhanced ionization of indium metal in an atmospheric pressure air/acetylene flame is studied and characterized. Practical utilization of the RID is shown by recording a partial excitation spectrum of the OH radical in the same air/acetylene flame used as the detector cell and measurement of the flame temperature by the Boltzmann plot method. OH transitions were excited in the X 2 Π → A 2 Σ + (1,0) vibronic band in the wavelength range 281–288 nm. Two possible mechanisms, fluorescence capture and collisional energy transfer, for the observation of the OH excitation spectrum are put forth and discussed. No conclusive evidence supporting either mechanism as predominant was found.

Study of the double-resonance ionization spectrum of indium with emphasis on the resonance detection of photons

Abstract The double-resonance ionization spectrum of indium has been recorded in an air/acetylene flame. The first step in the double-resonance scheme was tuned to the 303.94 nm resonance transition of In between the ground state and the 5 d 2 D 3/2 excited state. The double-resonance ionization spectrum was obtained by scanning a second laser in the wavelength range 740–796 nm. A total of 29 transitions were observed and identified. The observed transitions have been assigned to the three principal series: 5 d 2 D → nf 2 F , 6 p 2 P → nd 2 D and 6 p 2 P → ns 2 S . The latter two series originate in an atomic energy level of In that is collisionally populated from the laser pumped 5 d 2 D states. The transition 6 p 2 P 3/2 → 10 d 2 D 5/2 was subsequently studied as the second step in a resonance ionization photon detector (RID). Figures of merit for the RID based on the detection of photons at 303.94 nm are presented. A quantum efficiency of 0.76 electrons/photon and a minimum detectable number of photons of 1600 were found for the selected scheme.