Frequency-doubled Dye Laser Research Articles

Optimal approaches to formaldehyde planar laser-induced fluorescence

We assessed approaches to i) detecting formaldehyde (CH2O) via planar laser-induced fluorescence (PLIF) in atmospheric-pressure premixed flames, including those with pre-vaporized jet fuels, and ii) improving signal-to-noise ratio (SNR) of CH2O PLIF more generally. We compared excitation wavelengths and schemes – employing the third-harmonic output of a Nd:YAG laser vs. the frequency-doubled output of a dye laser. Specifically, we identified the optimal excitation line accessible via the frequency-doubled dye laser at 353.07 nm, which was ∼4× more efficient than tripled-Nd:YAG excitation, but considering that the tripled-Nd:YAG laser output is generally ∼10× more energetic, it will produce more CH2O-LIF signal. We then determined the optimal detection setup to include a 100-mm focal length (f/1.4) objective lens coupled to a standard achromatic lens, which provided ∼4× greater signal relative to our baseline lens configuration, and subsequently we demonstrated that a Gen III GaAsP intensifier produced remarkably high PLIF SNR (>15). Measurements in premixed turbulent flames using pre-vaporized kerosene fuels showed that interference from fuel-based LIF was largely absent in all cases and that high-quality CH2O PLIF was efficacious.

One-dimensional air temperature measurements by air resonance enhanced multiphoton Ionization thermometry (ART).

In this work, a detailed calibration study is performed to establish non-intrusive one-dimensional (1D) rovibrational temperature measurements in unseeded air, based on air resonance enhanced multiphoton ionization thermometry (ART). ART is generated by REMPI (resonance enhanced multi-photon ionization) of molecular oxygen and subsequent avalanche ionization of molecular nitrogen in a single laser pulse. ART signal, the fluorescence from the first negative band of molecular nitrogen, is directly proportional to the 2-photon transition of molecular oxygen C3Π (v = 2) ← X3Σ (v'=0), which is used to determine temperature. Experimentally, hyperfine structures of the O2 rotational branches with high temperature sensitivity are selectively excited through a frequency-doubled dye laser. Electron-avalanche ionization of N2 results in the fluorescence emissions from the first negative bands of N2 + near 390, 425, and 430nm, which are captured as a 1D line by a gated intensified camera. Post processing of the N2 + fluorescence yields a 1D thermometry line that is representative of the air temperature. It is demonstrated that the technique provides ART fluorescence of ∼5cm in length in the unseeded air, presenting an attractive thermometry solution for high-speed wind tunnels and other ground test facilities.

500 Hz ultraviolet dye laser with pulse energy 1.7 mJ and potential for PLIF imaging

ABSTRACTThis paper describes a high-pulse-energy frequency-doubled ultraviolet dye laser operating at a repetition rate of 500 Hz. The pump source is a laser-diode side-pumped Q-switched Nd:YAG laser with a pulse energy of 29 mJ at 532 nm. A master oscillator power amplifier is employed to amplify the output pulse of the dye laser to 8.1 mJ at 566 nm, and by frequency doubling with BBO crystal a pulse energy of 1.7 mJ at 283 nm is achieved with a pulse width of 8 ns. This is more than four times the largest reported pulse energies generated by other fixed-frequency dye lasers when operating at repetition rates of more than 1 kHz. The conversion efficiency and stability of dye laser are discussed, which show the potential for high-speed laser diagnostics in the fields of combustion and turbulent flow detection.

Flame imaging using planar laser induced fluorescence of sulfur dioxide

Laser induced fluorescence of sulfur dioxide (SO2-PLIF) has been demonstrated as a useful tool for flame imaging. Advantage was taken from the strong temperature dependence of the SO2 fluorescence signal. SO2 fluorescence intensity increases by more than one order of magnitude if the temperature changes from ambient conditions to adiabatic flame temperatures of stoichiometric methane–air flames. This results in a steep gradient of SO2-PLIF intensities at the reaction zone and therefore can be used as a reliable flame marker. SO2 can be excited electronically using the fourth-harmonic of an Nd:YAG laser at 266 nm. This is an attractive alternative to OH-LIF, a well-recognized flame front marker, because no frequency-doubled dye lasers are needed. This simplifies the experimental setup and is advantageous for measurements at high repetition rates where dye bleaching can become an issue. To prove the performance of this approach, SO2-PLIF measurements were performed simultaneously with OH-PLIF on laminar premixed methane-air Bunsen flames for equivalence ratios between 0.9 and 1.25. These measurements were compared to 1D laminar flamelet simulations. The SO2 fluorescence signal was found to follow the temperature rise of the flame and is located closer to the steep temperature gradient than OH. Finally, the combined SO2- and OH-PLIF setup was applied to a spark ignition IC-engine to visualize the development of the early flame kernel.

Krypton tagging velocimetry of an underexpanded jet.

In this work, we present the excitation/emission strategy, experimental setup, and results of an implementation of krypton tagging velocimetry (KTV). KTV is performed as follows: (i)seed a base flow with krypton; (ii)photosynthesize metastable krypton atoms with a frequency-doubled dye laser to form the tagged tracer; (iii)record the translation of the tagged metastable krypton by imaging the laser-induced fluorescence (LIF) that is produced with an additional dye laser. The principle strength of KTV, relative to other tagging velocimetry techniques, is the use of a chemically inert tracer. KTV results are presented for an underexpanded jet of three mixtures of varying Kr/N2 concentration. It is demonstrated that KTV can be used in gas mixtures of relatively low krypton mole fraction (0.5% Kr/99.5% N2), and the KTV data from that experiment are found to be in good agreement with an empirical fit found in the literature. We find that KTV is useful to perform instantaneous velocity measurements with metastable krypton as a chemically inert, dilute, long-lifetime tracer in gas-phase flows.

Production of a rovibrationally selected O2+beam for dissociative recombination studies

In order to study the reactivity of particular excited states of O2+, we have studied the resonance enhanced multiphoton ionization of O2 via the 1,3Πg states with a frequency-doubled pulsed dye laser in the range 296.5–303.5 nm. We managed to produce a large ensemble of O2+ ions in selected v+,N+ states, with v+ = 0 or 1, and N+ up to 25. Such an ion source, with tunability and selectivity of the output rovibrational state, can be coupled to a storage device in order to study the collisional and radiative properties of infrared inactive species.

Simultaneous Visualization of Water and Hydrogen Peroxide Vapor Using Two-Photon Laser-Induced Fluorescence and Photofragmentation Laser-Induced Fluorescence

A concept based on a combination of photofragmentation laser-induced fluorescence (PF-LIF) and two-photon laser-induced fluorescence (LIF) is for the first time demonstrated for simultaneous detection of hydrogen peroxide (H2O2) and water (H2O) vapor. Water detection is based on two-photon excitation by an injection-locked krypton fluoride (KrF) excimer laser (248.28 nm), which induces broadband fluorescence (400-500 nm) from water. The same laser simultaneously photodissociates H2O2, whereupon the generated OH fragments are probed by LIF after a time delay of typically 50 ns, by a frequency-doubled dye laser (281.91 nm). Experiments in six different H2O2/H2O mixtures of known compositions show that both signals are linearly dependent on respective species concentration. For the H2O2 detection there is a minor interfering signal contribution from OH fragments created by two-photon photodissociation of H2O. Since the PF-LIF signal yield from H2O2 is found to be at least ∼24,000 times higher than the PF-LIF signal yield from H2O at room temperature, this interference is negligible for most H2O/H2O2 mixtures of practical interest. Simultaneous single-shot imaging of both species was demonstrated in a slightly turbulent flow. For single-shot imaging the minimum detectable H2O2 and H2O concentration is 10 ppm and 0.5%, respectively. The proposed measurement concept could be a valuable asset in several areas, for example, in atmospheric and combustion science and research on vapor-phase H2O2 sterilization in the pharmaceutical and aseptic food-packaging industries.

Continuous hydroxyl radical planar laser imaging at 50 kHz repetition rate.

This study demonstrates high-repetition-rate planar laser-induced fluorescence (PLIF) imaging of hydroxyl radicals (OH) in flames at a continuous framing rate of 50kHz. A frequency-doubled dye laser is pumped by the second harmonic of an Nd:YAG laser to generate laser radiation near 283nm with a pulse width of 8ns and rate of 50kHz. Fluorescence is recorded by a two-stage image intensifier and complementary metal-oxide-semiconductor camera. The average power of the 283nm beam reaches 7W, yielding a pulse energy of 140μJ. Both a Hencken burner and a DC transient-arc plasmatron are used to produce premixed CH4/air flames to evaluate the OH PLIF system. The average signal-to-noise ratio for the Hencken burner flame is greater than 20 near the flame front and greater than 10 further downstream in a region of the flame near equilibrium. Image sequences of the DC plasmatron discharge clearly illustrate development and evolution of flow features with signal levels comparable to those in the Hencken burner. The results are a demonstration of the ability to make high-fidelity OH PLIF measurements at 50kHz using a Nd:YAG-pumped, frequency-doubled dye laser.

Nitric-oxide planar laser-induced fluorescence at 10 kHz in a seeded flow, a plasma discharge, and a flame

This study demonstrates high-repetition-rate planar laser-induced fluorescence (PLIF) imaging of both cold (~300 K) and hot (~2400 K) nitric oxide (NO) at a framing rate of 10 kHz. The laser system is composed of a frequency-doubled dye laser pumped by the third harmonic of a 10 kHz Nd:YAG laser to generate continuously pulsed laser radiation at 226 nm for excitation of NO. The laser-induced fluorescence signal is detected using a high-frame rate, intensified CMOS camera, yielding a continuous cinematographic propagation of the NO plume where data acquisition duration is limited only by camera memory. The pulse energy of the beam is ~20 μJ with a spectral width ~0.15 cm(-1), though energies as high as 40 μJ were generated. Hot NO is generated by passing air through a DC transient-arc plasma torch that dissociates air. The plasma torch is also used to ignite and sustain a CH(4)/air premixed flame. Cold NO is imaged from a 1% NO flow (buffered by nitrogen). The estimated signal-to-noise ratio (SNR) for the cold seeded flow and air plasma exceeds 50 with expected NO concentrations of 6000-8000 parts per million (ppm, volume basis). Images show distinct, high-contrast boundaries. The plasma-assisted flame images have an SNR of less than 10 for concentrations reaching 1000 ppm. For many combustion applications, the pulse energy is insufficient for PLIF measurements. However, the equipment and strategies herein could be applied to high-frequency line imaging of NO at concentrations of 10-100 ppm. Generation of 226 nm radiation was also performed using sum-frequency mixing of the 532 nm pumped dye laser and 355 nm Nd:YAG third harmonic but was limited in energy to 14 μJ. Frequency tripling a 532 nm pumped dye laser produced 226 nm radiation at energies comparable to the 355 nm pumping scheme.

Photoionization studies from the 3p P2 excited state of neutral lithium

In the present work, the experimentally determined oscillator strengths of the 3p P2→nd D2 (16≤n≤43) Rydberg transitions of lithium have been reported for the first time. The experiments were performed using two dye lasers simultaneously pumped by the second harmonic (532 nm) of an Nd:YAG laser, and the vapor containment and detection system was a thermionic diode ion detector. The first frequency-doubled dye laser excites the ground-state atoms to the 3p P2 excited state at 325.35 nm, and the second dye laser was scanned up to the first ionization threshold. The f-values of the aforementioned transitions have been determined using the threshold value of the photoionization cross section (30±4.8 Mb) from the 3p P2 excited state. A complete picture of the oscillator strengths from n=3 to 43 has been presented, including the previously known oscillator strength and determined in this work.

Oscillator strength measurements of the 5s5p 3P 1 → 5s nd 3D 2 Rydberg transitions of cadmium

We report oscillator strengths distribution in the bound region of cadmium corresponding to the 5s5p 3P 1 → 5s nd 3D 2 (21 ≤ n ≤ 52) Rydberg transitions. The experiment was performed using two frequency doubled dye lasers simultaneously pumped by a common Nd:YAG laser in conjunction with an atomic beam set-up. The absolute photoionization cross section of the 5s5p 3P 1 intermediate state at the 5s 2S 1/2 ionization threshold has been measured as 20 (4) Mb using the saturation technique. The measured value of the photoionization cross section at threshold is used to extract the f-values of the above mentioned Rydberg transitions.

Chiral Recognition in Cinchona Alkaloid Protonated Dimers: Mass Spectrometry and UV Photodissociation Studies

Chiral recognition in protonated cinchona alkaloid dimers has been studied in mass spectrometry experiments. The experimental setups involved a modified 7T FT-ICR (Fourier transform-ion cyclotron resonance) mass spectrometer (MS) and a modified Paul ion trap both equipped with an electrospray ionization source (ESI). The Paul ion trap has been coupled to a frequency-doubled dye laser. The fragmentation of protonated dimers made from cinchonidine (Cd) and the two pseudoenantiomers of quinine, namely, quinine (Qn) and quinidine (Qd), has been assessed by means of collision-induced dissociation (CID) as well as UV photodissociation (UVPD). Whereas CID fragmentation of the dimers only leads to the evaporation of the monomers, UVPD results in the additional loss of a neutral radical fragment corresponding to the quinuclidinyl radical. The effect of the excitation wavelength and of complexation with H(2)SO(4) has been studied to cast light on the reaction mechanism. Complexation with H(2)SO(4) modifies the photoreactivity of the dimers; only evaporation of the monomeric fragments, quinine, and cinchonidine is observed. Comparison between the mass spectra of the cinchona alkaloid (CdQnH(+)) or (CdQdH(+)) dimers resulting from the UVPD of (CdQnH(2)SO(4)H(+)) and that of bare (CdQnH(+)) helps propose a fragmentation mechanism, which is thought to involve fast proton transfer from the quinuclidine part of a molecular subunit to the quinoline ring. CID and UV fragmentation experiments show that the homochiral dimer is more strongly bound than the heterochiral adduct.

Optical spectroscopy of rubidium Rydberg atoms with a 297 nm frequency-doubled dye laser

We demonstrate Doppler-free, purely optical detection of laser-excited rubidium Rydberg atoms in a room-temperature gas cell. The Rydberg atoms are excited with a frequency-doubled dye laser at 297 nm in a single-excitation step from the ground state; the detection is performed with a 780 nm diode laser in a scheme similar to electron shelving. Laser spectroscopy of Rydberg transitions is demonstrated, and a frequency-doubled dye is stabilized to one Rydberg transition in the UV. The performance of this stabilization is measured with an atomic-beam apparatus and a time of flight experiment.

Polarisation of a spin--NMR probe 11Be using collinear laser optical pumping

Spin ½ probes have special importance in the field of NMR, since they have no electric quadrupole moment and are thus pure magnetic sensors. The absence of quadrupolar splittings greatly simplifies the NMR frequency spectrum. That the muon is spin ½is one of the reasons it has become so useful as a magnetic probe. Until now there has been no spin ½probe suitable for beta-detected NMR. Such a probe would be particularly useful in complex materials, which are in general non-cubic. In this paper we describe the development of a low energy beam of spin-polarised Be, which is the lightest spin ½ isotope suitable for β-NMR. The intensity of the 11 + beam―generated by a laser ionisation source― is about 10 6 /s. The Be nuclei are polarised by collinear laser optical pumping with an externally frequency doubled dye laser tuned to the 2 S 1/2 - 2 P 1/2 transition of Be + at 313 nm.

Collisional Deactivation of Ba 5d7p 3D1 by Noble Gases

Collisional deactivation of the 5d7p (3)D1 state of Ba by noble gases is studied by time- and wavelength-resolved fluorescence techniques. A pulsed, frequency-doubled dye laser at 273.9 nm excites the 5d7p (3)D1 level from the ground state, and fluorescence at 364.1 and 366.6 nm from the 5d7p (3)D1 --> 6s5d (3)D1 and 5d7p (3)D1 --> 6s5d (3)D2 transitions, respectively, is monitored in real time to obtain the deactivation rate constants. At 835 K these rate constants are as follows: He, (1.69 +/- 0.08) x 10(-9) cm(3) s(-1); Ne, (3.93 +/- 0.14) x 10(-10) cm(3) s(-1); Ar, (4.53 +/- 0.15) x 10(-10) cm(3) s(-1); Kr, (4.64 +/- 0.13) x 10(-10) cm(3) s(-1); Xe, (5.59 +/- 0.22) x 10(-10) cm(3) s(-1). From time-resolved 5d7p (3)D1 emission in the absence of noble gas and from the intercepts of the quenching plots, the lifetime of this state is determined to be 100 +/- 1 ns. Using time- and wavelength-resolved Ba emission with a low background pressure of noble gas, radiative lifetimes of several near-resonant states are determined from the exponential rise of the fluorescence signals. These results are as follows: 5d6d (3)D3, 28 +/- 3 ns; 5d7p (3)P1, 46 +/- 2 ns; 5d6d (3)G3, 21.5 +/- 0.8 ns; 5d7p (3)F3, 48 +/- 1 ns. Integrated fluorescence signals are used to infer the relative rate constants for population transfer from the 5d7p (3)D1 state to eleven near-resonant fine structure states.

Effect of Glow-to-Arc Transition on Loss Mechanism of Ba Atoms from Electrode of Fluorescent Lamp

The loss of Ba atoms from the electrode of a fluorescent lamp was measured while the lamp was operated in the glow and arc discharge modes at 60 Hz. A laser-induced fluorescence (LIF) technique was applied to the measurements of the temporal and spatial distributions of Ba atoms in the vicinity of the electrode. Ground-state (61S0) Ba atoms were excited to a 51P1 level by a frequency-doubled dye laser beam (350.1 nm), and the subsequent fluorescence (51P1–51D2, 582.6 nm) was detected. The temporal and spatial distributions of Ba atoms were found to be completely different in the two discharge modes. Temporally; in the arc discharge mode, the density of the Ba atoms was found to have two peaks, and the number of Ba atoms emitted in the anode half-cycle was about twofold larger than that emitted in the cathode half-cycle. In the glow discharge mode, the number of Ba atoms emitted in the anode half-cycle was found to be negligible compared with that emitted in the cathode half-cycle. Spatially; in the arc discharge mode, Ba atoms were found to be emitted mainly from the hot spot of the filament electrode. In the glow discharge mode, Ba atoms were found to be emitted from all parts of the filament electrodes homogeneously. The mechanism of Ba atom loss in both modes was discussed.

Intrinsic fluorescence of B and Z forms of poly d(G-m5C)·poly d(G-m5C), a synthetic double-stranded DNA: spectra and lifetimes by the maximum entropy method

A study has been made of the fluorescence of poly d(G-m5C).poly d(G-m5C), a synthetic double-stranded DNA, in buffered neutral aqueous solution at room temperature, excited by synchrotron radiation at 280 nm and 250 nm and by a frequency-doubled pulse dye laser at 290 nm. Exciting at 280 nm, the B form shows a uni-modal UV spectrum with lambdaf(max) approximately 340 nm. The Z form has in addition a visible emission lambdaf(max) at 450 nm. The spectral positions remain unchanged on exciting at 250 nm but the relative intensities change considerably. Decay profiles have been obtained at 360 nm and 450 nm for both the B and Z forms and have been analyzed by fitting to a pseudo-continuous distribution of 100 (and occasionally 200) exponentials, ranging from 10 ps to 20 ns, by optimizing the 'entropy' of the signal (the method of maximum entropy). We find the mean lifetimes for both wavelengths of emission and for both structural forms fall into three well-separated regions in the ranges indicated tau1 approximately 0.04-0.21 ns, tau2 approximately 0.9-1.26 ns, and tau3 approximately 5.1-6.5 ns. The UV emission, from its spectral position and half-width, correlates with monomeric emission from m5C (and from C for poly d(G-C)). However the lifetime tau1 is approximately 2 orders of magnitude longer than the monomers and points to an involvement of protonated guanosine (GH+, tauf approximately 200 ps) in the overall absorption/emission sequence. In the UV the tau3 emission is predominant, with fractional time-integrated emission approximately 86% for B DNA and approximately 64% for Z. We suggest it results from exciton (stacked) absorption followed by dissociative emission. For Z DNA the visible (450 nm) emission is dominated by a tau3 species (approximately 91%) with a lifetime of 6.5 ns and we suggest it represents a hetero-excimer emission consequent upon absorption by the strongly overlapped base-stacking, which differs from that in B DNA. The weak emission corresponding to tau2 is made more apparent by scanned gated detection of the emission from laser excitation (290 nm) of single-crystal d(m5C-G)3. A central role is attributed to the tight stacking of the bases in the Z form which correlates with enhanced hypochromism at 250 nm vs. 280 nm and with the reversal of the fluorescence intensity ratios UV-visible between these wavelengths.

Observation of OH Radical Reactions with Atomic Hydrogen in He/H2O Plasmas by Laser-Induced Fluorescence Spectroscopy

A novel atmospheric pressure plasma device that generates atomic hydrogen has been developed. The reaction of OH radicals with atomic hydrogen supplied from the device was investigated in a He/H2O mixture at a pressure of 530 Torr by laser-induced fluorescence (LIF) spectroscopy. OH radicals were generated using a dielectric barrier discharge (DBD) plasma and excited to A2Σ+(v'=1) from X2Π(v''=0) by a frequency-doubled dye laser (282 nm). When the device was operated, OH-LIF intensity was reduced near the DBD plasma. The spatial density profile of the OH radicals was also obtained when the device was turned on and off. The reaction mechanism was briefly discussed.

Study on Temporal and Spatial Distributions of Ba Atoms in Fluorescent Lamp Discharge Using Laser-Induced Florescence

A laser-induced fluorescence (LIF) technique was applied to the measurements of the temporal and spatial distributions of Ba atoms in the vicinity of the electrode of a fluorescent lamp operated at 60 Hz. Ground-state (61S0) Ba atoms were excited to a 51P1 level (350.1 nm) by a frequency-doubled dye laser beam, and the subsequent fluorescence (51P1–51D2, 582.6 nm) was detected. Over a whole periodic time (16.67 ms), the density of the Ba atoms was found to have two peaks, and the number of Ba atoms emitted in the anode half-cycle was about twofold larger than that emitted in the cathode half-cycle. This difference between the Ba atoms emitted during the anode half-cycle and those emitted during the cathode half-cycle was studied for lamps with different gas pressures. Ba atoms were found to be emitted mainly from the hot spot of the filament electrode. It is suggested that the main factor for Ba atom emission from the electrode is not sputtering by ion bombardment but thermal evaporation.

Precision spectroscopy at heavy ion ring accelerator SIS300

Unique spectroscopic possibilities open up if a laser beam interacts with relativistic lithium-like ions stored in the heavy ion ring accelerator SIS300 at the future Facility for Antiproton and Ion Research FAIR in Darmstadt, Germany. At a relativistic factor γ = 36 the 2P1/2 level can be excited from the 2S1/2 ground state for any element with frequency doubled dye-lasers in collinear geometry. Precise transition energy measurements can be performed if the fluorescence photons, boosted in forward direction into the X-ray region, are energetically analyzed with a single crystal monochromator. The hyperfine structure can be investigated at the 2P1/2−2S1/2 transition for all elements and at the 2P3/2−2S1/2 transition for elements with Z≤50. Isotope shifts and nuclear moments can be measured with unprecedented precision, in principle even for only a few stored radioactive species with known nuclear spin. A superior relative line width in the order of 5·10−7 may be feasible after laser cooling, and even polarized external beams may be prepared by optical pumping.