Excimer Pumped Dye Laser Research Articles

Time-Resolved Measurements and Master Equation Modelling of the Unimolecular Decomposition of CH3OCH2

Abstract The rate coefficient for the unimolecular decomposition of CH3OCH2, k 1, has been measured in time-resolved experiments by monitoring the HCHO product. CH3OCH2 was rapidly and cleanly generated by 248 nm excimer photolysis of oxalyl chloride, (ClCO)2, in an excess of CH3OCH3, and an excimer pumped dye laser tuned to 353.16 nm was used to probe HCHO via laser induced fluorescence. k 1(T,p) was measured over the ranges: 573–673 K and 0.1–4.3 × 1018 molecule cm−3 with a helium bath gas. In addition, some experiments were carried out with nitrogen as the bath gas. Ab initio calculations on CH3OCH2 decomposition were carried out and a transition-state for decomposition to CH3 and H2CO was identified. This information was used in a master equation rate calculation, using the MESMER code, where the zero-point-energy corrected barrier to reaction, ΔE 0,1, and the energy transfer parameters, ⟨ΔEdown⟩ × T n, were the adjusted parameters to best fit the experimental data, with helium as the buffer gas. The data were combined with earlier measurements by Loucks and Laidler (Can J. Chem. 1967, 45, 2767), with dimethyl ether as the third body, reinterpreted using current literature for the rate coefficient for recombination of CH3OCH2. This analysis returned ΔE 0,1 = (112.3 ± 0.6) kJ mol−1, and leads to k 1 ∞ ( T ) = 2.9 × 10 12 $k_{1}^{\infty}(T)=2.9\times{10^{12}}$ (T/300)2.5 exp(−106.8 kJ mol−1/RT). Using this model, limited experiments with nitrogen as the bath gas allowed N2 energy transfer parameters to be identified and then further MESMER simulations were carried out, where N2 was the buffer gas, to generate k 1(T,p) over a wide range of conditions: 300–1000 K and N2 = 1012–1025 molecule cm−3. The resulting k 1(T,p) has been parameterized using a Troe-expression, so that they can be readily be incorporated into combustion models. In addition, k 1(T,p) has been parametrized using PLOG for the buffer gases, He, CH3OCH3 and N2.

A linear optical trap with active medium for experiments with high power laser pulses.

A linear optical trap for circulating high power laser pulses and tuning these pulses to high repetition frequency of several tens of MHz has been developed. A ns excimer pumped dye laser pulse has been injected with help of a Wollaston prism and a synchronized Pockels cell into an optical trap formed by two highly reflecting mirrors in a linear configuration. The test was done at λ = 580 nm, but the optical trap can be used without limitations in a broad band of optical wavelengths (400-700 nm). Power considerations give an increase of the efficiency of the optical trap of about 7 times compared to single passage of the laser pulse through the experimental section. The time structure of the trapped laser pulses can be controlled by changing the distance between the two high reflecting mirrors. The efficiency of the optical trap strongly depends upon optical losses. To compensate the optical losses, an amplifying cell was introduced, and the efficiency was about 60 times higher than that by single passage of the laser pulse through the experimental section.

Femtosecond laser enhanced current in a thermionic diode with barium vapor

We studied the signal from a thermionic diode when a femtosecond laser beam was spatially overlapped by an excimer pumped dye laser beam. The nanosecond dye laser was scanned from 435 to 438nm in order to excite the autoionizing levels of barium by two photon absorption. The broadband ultrashort laser light was centered at 427nm, which is also above the first ionization limit of barium. The bias voltage between the cell body and the tungsten rod (set at either 9 or 0V) was used to collect electrons after the barium ions had been created by multiphoton (auto) ionization. The overall background of the thermionic signal was appreciably elevated due to the two photon ionization by the broadband femtosecond laser. We measured the thermionic signal with and without femtosecond laser overlap, and with a biasless and biased thermionic diode. The effect of the femtosecond laser was appreciable enhancement of the background ionization continuum. This was especially visible in the presence of noble gases at pressure of 50mbar. Argon produced the largest and helium produced the smallest enhancement in the background continuum. In addition, we observed a few broad spectral features of a presumably collision induced nature.

Temperature evaluation by simultaneous emission and saturated fluorescence measurements: A critical theoretical and experimental appraisal of the approach

Temperature is one of the most important physical parameters of plasmas induced by a focused laser beam on solid targets, and its experimental evaluation has received considerable attention. An intriguing approach, first proposed by Kunze (H.-J. Kunze, Experimental check of local thermodynamic equilibrium in discharges, Appl. Opt., 25 (1986) 13–13.) as a check of the existence of local thermodynamic equilibrium, is based upon the simultaneous measurement of the thermal emission and the optically saturated fluorescence of the same selected atomic transition. The approach, whose appealing feature is that neither the calibration of the set-up nor the spontaneous radiative probability of the transitions is needed, has not yet been applied, to our knowledge, to analytical flames and plasmas. A critical discussion of the basic requirements for the application of the method, its advantages, and its experimental limitations, is therefore presented here. For our study, Ba+ transitions in a plasma formed by focusing a pulsed Nd:YAG laser (1064nm) on a glass sample containing BaO are selected. At various delay times from the plasma initiation, a pulsed, excimer-pumped dye laser tuned at the center of two Ba transitions (6s 2S1/2→6p 2P°3/2; 455.403nm and 6p 2P°1/2→6d 2S1/2; 452.493nm) is used to enhance the populations of the excited levels (6p 2P°3/2 and 6d 2S1/2) above their thermal values. The measured ratio of the emission and direct line fluorescence signals observed at 614.171nm (6p 2P°3/2→5d 2D5/2) and 489.997nm (6d 2S1/2→6p 2P°3/2) is then related to the excitation temperature of the plasma. Our conclusion is that the approach, despite being indeed attractive and clever, does not seem to be easily applicable to flames and plasmas, in particular to transient and inhomogeneous plasmas such as those induced by lasers on solids.

NO concentrations in laminar premixed CH4/O2/N2 flames with varying equivalence ratio

Number density and concentration of nitric oxide have been investigated in laminar premixed CH4/O2/N2 flames using laser-induced fluorescence technique. Emphasis was placed on quantitative measurements, which were taken in flames at various equivalence ratios ranging from 0.9 ∼ 3.0. The flow rate was fixed at 2 SLPM. The NO A-X (0,0) vibrational band around 226 nm was excited using a XeCl excimer-pumped dye laser. By selecting an appropriate NO transition, the interferences from elastic scattering and O2 fluorescence were minimized. Results show that the maximum NO concentration increased at the range of equivalence ratio from ϕ = 0.9 to 1.6, decreased from ϕ = 1.6 to 2.0 and slowly increased with increasing equivalence ratio above ϕ = 2.0. Also, the maximum NO concentration was at around the reaction zone.

Treatment efficiency of combining photodynamic therapy and ionizing radiation for Bowen’s disease

Topical 5-aminolevulinic acid photodynamic therapy (ALA-PDT) is widely used for treating Bowen's disease (BD), but recurrence and tumour cell persistence after ALA-PDT is sometimes problematic. Radiation therapy (RT) is also effective for BD, but is limited by its side-effects, such as refractory ulcers. The objective of the study was to observe a synergic effect of combination therapy with ALA-PDT and RT for BD cases that did not respond effectively to prior ALA-PDT. Subjects were BD patients whose lesion did not show complete remission or showed recurrence after prior ALA-PDT. A total of four cases involving four lesions were studied (three male and one female, mean age 69.5). ALA ointment (20%) was applied to the lesions. After 4 to 6h, subjects received combination therapy consisting of excimer-pumped dye laser radiation at 630nm (50J/cm(2) ) followed by electron-beam radiation (3Gy). The combination therapy was repeated every 2 to 3days for a total of four treatments. The lesions were evaluated clinically or histologically after the final combination therapy session. Following combination therapy, all of the lesions disappeared. Recurrence was not detected during the observations periods, which averaged 14.0months in duration. Our results indicate that the cure rate of BD could be improved by combination therapy with ALA-PDT and RT. Compared with conventional RT, the synergetic effect of this therapy might reduce the dose of radiation required, thereby also reducing skin side-effects such as refractory ulcers.

Wavelength and Time-Resolved Imaging of Material Ejection in Infrared Matrix-Assisted Laser Desorption

The dynamics of glycerol ablation at atmospheric pressure was studied using fast photography. A mid-infrared optical parametric oscillator was used to irradiate a droplet of glycerol at normal incidence. The wavelength of the infrared source was tunable and was varied between 2.7 and 3.5 microm for the studies. After an adjustable delay, an excimer pumped dye laser was used to illuminate the expanding plume, and the 90 degrees scattered light was imaged with a high-speed CMOS camera. The time delay between the IR and UV lasers was varied up to 1 ms with a particular emphasis in the early stages of plume evolution below 1 micros. The threshold fluence for plume formation varied between 1000 and 6000 J/m(2), and the minimum fluence corresponded to the OH stretch absorption of glycerol near 3.0 microm, which also corresponded to the greatest scattered light signal and duration of material emission. The velocity of the expanding plume was measured and ranged from >300 m/s near the OH stretch absorption to <100 m/s near the 3.4 microm CH stretch. Plume modeling calculations suggest that material removal is driven by phase explosion in the stress confinement regime that is at a maximum near the wavelength of the OH stretch absorption.

Collisional shifts and broadenings of the 4sns 1S0 and 4snd 1D2 Rydberg series of calcium perturbed by inert gases

An excimer pumped dye laser was used to measure collisional shift and broadening rates of the nd 1D2 (n = 4–50) and ns 1S0 series (n = 5–30) of calcium Rydberg states. Two-photon absorption techniques were used to populate the Rydberg states. High-resolution Ca excitation spectra were measured using a diode detector while scanning the frequency of the dye laser. Inert gases Ar, Kr and Xe were used as perturbers at pressures of 100 mbar. Shifts and broadenings of spectral lines were measured as functions of the principal quantum number n. The pressure-dependence behaviour of the 4s63d 1D2 Rydberg state as well as its configuration interactions is discussed.

Shifts and broadenings of barium Rydberg states perturbed by inert gases

Collisional broadening and shift data for even parity states of neutral barium are presented. A two-photon absorption technique is used to populate Rydberg states of nd 1D2, 3D2 and 1S0 series. High-resolution Ba excitation spectra were measured using a diode detector while scanning the frequency of an excimer pumped dye laser. Inert gases Ar, Kr and Xe were used as perturbers at pressures ranging from 10 mb to 400 mb. Shifts and broadenings of spectral lines were measured as a function of pressure and principal quantum number n. Several perturbations and deviations from expected results are discussed in this paper.

Powerful fast triggerable 6 μm laser for the muonic hydrogen 2S-Lamb shift experiment

Laser spectroscopy of the 2S-Lamb shift in muonic hydrogen (μ −p) is being performed at the Paul Scherrer Institute, Switzerland, to determine the root-mean-square (rms) proton charge radius with 10 −3 precision. A multistage laser system has been developed which provides 0.2 mJ pulse energy tunable at 6 μm wavelength. An excimer pumped dye laser is used to drive a titanium sapphire (Ti:Sa) system whose wavelength is then shifted to 6 μm using a multipass Raman cell filled with hydrogen. The short cavity length of the Ti:Sa oscillator (7 cm) guarantees a pulse width of 7 ns and a pulse energy of 1.2 mJ at 708 nm, a wavelength controlled by a mono-mode cw-Ti:Sa laser. The laser is triggered at a maximum 60 s −1 repetition rate by muons entering the apparatus at random times. A new type of multipass cavity has been developed to provide a homogeneously illuminated volume (25 × 7 × 170 mm 3).

Resonance ionization spectroscopy of fermium ( Z=100)

Laser spectroscopy has been applied for the first time to measure resonant transition frequencies of fermium ( Z=100). A number of 2.7×10 10 atoms was electrodeposited on a Ta filament and covered with a 1 μm Ti layer. Fm atoms were evaporated from the filament at a temperature of approximately 1000 °C into an optical cell filled with a buffer gas and were subsequently photoionized in a two-photon 1+1′ process using a tunable excimer-pumped dye laser and the excimer (XeF) laser itself to produce the resonant (1) and non-resonant (1′) photon frequencies, respectively. The created photoions were identified by mass-selective detection. We observed resonant transitions of Fm at dye-laser energies of 25 099.8(2) and 25 111.8(2) cm −1. A possible classification of these levels is given on the basis of multiconfiguration Dirac–Fock calculations. We also interpreted the recorded time of ion drift through the buffer gas as a novel way to measure the mean ionic charge radius of the heavy elements.

Luminescence and scintillation properties of praseodymium poly and diphosphates

Abstract Single crystals of Pr(PO3)3 polyphosphate and NaPrP2O7 diphosphate were grown for the first time using the flux method. In both materials, selective excitation in the Pr3+ (4f2) 3PJ (J=0, 1 and 2) levels using an excimer-pumped dye laser results in an intense red-orange fluorescence originating mainly from transitions between 3P0 and 4f2 lower lying levels. The observed emissions decay exponentially whatever the temperature may be. The 3P0 fluorescence decay time τ experiences a significant lengthening as the temperature is decreased from 300 to 6 K. This is especially true in the case of NaPrP2O7 for which τ increases from 25 up to 275 ns, while τ increases from 23 up to 76 ns only for Pr(PO3)3. Both compounds exhibit intense ultraviolet 4f15d1 → 4f2 band emissions in the 200–300 nm spectral range under X-ray excitation both at room and low temperatures. Comparing the intensities of these ultraviolet emissions with that of the bismuth germanate Bi4Ge3O12 fluorescence leads to relative light yields in the order of 30% and 11% for Pr(PO3)3 and NaPrP2O7, respectively.

Scintillation properties of CsPrP 4O 12 and RbPrP 4O 12

Single crystals of CsPrP 4O 12 and RbPrP 4O 12 cyclotetraphosphates have been grown using the flux method. Selective excitations in the Pr 3+ (4f 2) 3P 0 and 3P 2 levels using an excimer-pumped dye laser mainly result in intense line emissions in both materials. Though originating from transitions within the Pr 3+ 4f 2 ground configuration, these emissions have very short decay times, on the order of 50 ns. Both compounds also exhibit intense ultraviolet band emissions in the 200–300 nm spectral range when using either a xenon lamp or a X-ray source for optical excitation. The decays of these emissions under pulsed X-ray excitation were measured using the single-photon counting technique. They exhibit a main fast component (decay time τ f) and a minor slower one (decay time τ s), the decay times of which being τ f≅9 and 5 ns, τ s≅30 and 20 ns, in the Cs and Rb compounds, respectively.

Measurement of smoke concentration usingdegenerate four-wave mixing

Degenerate four-wave mixing (DFWM) was successfullyused to monitor a wide range of smoke concentrations (0.1-10mg m-3) in sample cells. To the authors' knowledge, thisis the first measurement of smoke by DFWM. The DFWM method isvery sensitive, measuring down to ~0.1 ppb soot volumefraction. To verify the visible laser DFWM system, NO2concentrations from 2 to 200 ppm were measured with a sensitivityof 2 ppm of NO2. Both smoke and NO2 species wereresonantly pumped at 500 nm by an excimer-pumped dye laser(~2 mJ/pulse). The DFWM signal from bothNO2 and smoke showed a squared dependence on concentration.Single-pulse measurements of NO2 and smoke in sample cellsindicate that temporally and spatially resolved in situDFWM measurements are feasible for engine exhausts.

Atmospheric trace gas analysis with cavity ring‐down spectroscopy

AbstractCavity ring‐down spectroscopy (CRDS) is a highly sensitive laser absorption method. It can be used for quantitative analysis of molecular species at the sub‐ppb level. The absorption cell (cavity) is sealed by two high‐reflective mirrors on each side, which results in an effective absorption path‐length of some kilometers. Our experiments for atmospheric gas analysis have been carried out so far with an Excimer pumped dye laser in the UV‐VIS and a CO overtone sideband laser in the wavelength region around 3 μm. Experiments with an all solid‐state difference frequency laser system will follow. In the UV‐VIS region, we measured trace gas molecules like SO2, NO2, and CH2O. In the mid‐infrared, around 3 μm, we measured hydrocarbons like CH4, C2H6, and C2H4 with a detection limit of less than 1 ppb. The noise equivalent absorption coefficients in the MIR are in the order of 1.7·10−9 cm−1. Due to the high data acquisition rate and the high sensitivity, CRDS enables real‐time detection of trace gases in ambient air.

Luminescence properties of CsPrP 4O 12 and RbPrP 4O 12

Single crystals of CsPrP 4O 12 and RbPrP 4O 12 cyclotetraphosphates have been grown using the flux method. Selective excitations in the Pr 3+ (4f 2) 3P 0 and 3P 2 levels using an excimer-pumped dye laser mainly result in intense line emissions in both materials. Although originating from transitions within the Pr 3+ 4f 2 ground configuration, these emissions have very short decay times, of the order of 50 ns. Both compounds also exhibit intense ultraviolet band emissions in the 200–300 nm spectral range when using either a xenon lamp or an X-ray source for optical excitation. The decays of these emissions under pulsed X-ray excitation were measured using the start–stop technique. They exhibit a main fast component (decay time τ f) and a minor slower one (decay time τ s), the decay times of which are τ f≅9 and 5 ns, τ s≅30 and 20 ns, in the Cs and Rb compounds, respectively.

Density of N2 (X 1Σg+; v = 18) molecules in a dc glowdischarge measured by cavity ringdown spectroscopy at 227 nm; validitydomain of the technique

The high sensitivity cavity ringdown spectroscopy (CRDS)absorption technique is used to measure the absolute density of the groundstate nitrogen molecules in the v'' = 18 vibrational level in a dc glowdischarge. The experimental conditions are 2.3 Torr nitrogen pressure,100 mA current and a discharge tube of 3.6 cm diameter and 80 cm length.The excimer-pumped dye laser is tuned on theLyman-Birge-Hopfield, N2 (a 1Πg; 8-X 1Σg; 18) band at227 nm and absorption rates on spectrally-resolved rotational lines areobtained. The gas temperature deduced from the rotational distribution is500 K. We measure a total density of (9±3.5)×1019 molecules m-3 in the v'' = 18 vibrational level, equivalent to0.2% of the total nitrogen density. So high density suggests that thev'' = 18 level should be located in the Treanor plateau of the vibrationaldistribution function. We also analyse the validity domain of the CRDSmeasurements, regarding the optical saturation due to the high intensity ofthe laser radiation inside the CRDS cavity and depletion of the absorbingspecies. If, for our measurements, the linear absorption conditions arefulfilled, we show that very often the optical saturation becomes dominant,inducing a non-single exponential decay of the ringdown signal and leadingto an underestimated absorber density.

Selective ultra-trace detection of NO and NO2 in complex gas mixtures using broad-bandwidth REMPI mass spectrometry.

In this paper we demonstrate the feasibility of ultra-trace resonance enhanced multiphoton ionization (REMPI) detection employing a small broad-bandwidth solid state laser system. The results reported here are compared with measurements carried out with a conventional excimer pumped dye laser combination. Mass selected broad-bandwidth REMPI spectra for the environmentally relevant nitrogen oxides NO and NO2 are presented. Tunable broad-bandwidth laser radiation with a spectral resolution of > 10 cm(-1) in the wavelength range 560-400 nm was employed for the detection of NO2. For NO detection, the range 230-224 nm was covered. Laser radiation was generated using an optical parametric oscillator pumped by an unseeded Nd:YAG laser. A mobile time-of-flight mass spectrometer equipped with an atmospheric pressure laser ionization source allowed for mass selective parent ion detection at m/z 30 for NO and m/z 46 for NO2. The limit of detection was 10 pptV for NO and 20 pptV for NO2. A selectivity of > 2000 for both compounds with respect to N2O5, organic nitrates and NO2 in the case of NO is reported. An improved laser system currently under construction is expected to provide detection limits below pptv mixing ratios for both nitrogen oxides in a 20 s integration interval.

Optical Analysis of Pulse Combustion Using Shadowgraph and Planar CH-LIF Imaging Technique

Planar imaging of laser-induced fluorescence of CH radical is made to examine combustion processes in a valveless pulse combustor. An excimer-pumped dye laser tuned to a wavelength of 387 nm is used to excite the R1N″=6 line of (0,0) band of the B2Σ−−X2Π system of CH radical, and an image-intensified CCD camera system is used to detect the (0,1) band emission at around 435 nm. According to the CH-LIF images, it is found that the progress in combustion during a pulsation period is expressed by the enlargement and breakup of the earlobe-shaped flame front along the outline of a pair of large-scale eddies of fresh mixture.

A REMPI method for the ultrasensitive detection of NO and NO2 using atmospheric pressure laser ionization mass spectrometry.

We report on the development of a quasi-simultaneous highly selective method for NO and NO2 detection at the ultratrace level. Atmospheric pressure laser ionization (APLI), recently introduced by our group, is used to detect both compounds at low parts per trillion by volume (pptv) mixing ratios. APLI is based on resonance-enhanced multiphoton ionization mass spectrometry. Two-color pump-probe experiments employing a single excimer pumped dye laser combination allow for the ultrasensitive measurement of NO and NO2 within a narrow range of maximum pumping efficiency of the laser dye Coumarin 120. NO is detected via excitation of the long-lived A 2sigma+ (nu' = 1) level at 215.36 nm and subsequently ionized with 308-nm radiation provided by the excimer pump laser. NO2 is ionized after double resonant excitation of the A2B1 and 3psigma manifolds in a (1 + 1' + 1(')) process using 431.65 + 308 nm. The selectivity of the NO measurement exceeds 2,000 with respect to NO2 and N2O5. For NO2, a selectivity of >3,000 with respect to N2O5 and organic nitrates is observed. The current APLI detection limit of NO and NO2 is 0.5 and 5 pptv, respectively, with a 20-s integration time.