Time-resolved spectroscopy measurements of hydrogen-alpha, -beta, and -gamma emissions (original) (raw)
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In this study, we examine the atomic and molecular signatures in laser-induced plasma. Abel inversions of measured line-of-sight data reveal insight into the radial plasma distribution. Laser-plasma is generated with 6 ns, Q-switched Nd:YAG radiation with energies in the range of 100 to 800 mJ. Temporally- and spatially-resolved emission spectroscopy investigates expansion dynamics. Specific interests include atomic hydrogen (H) and cyanide (CN). Atomic hydrogen spectra indicate axisymmetric shell structures and isentropic expansion of the plasma kernel. The recombination radiation of CN emanates within the first 100 nanoseconds for laser-induced breakdown in a 1:1 mole ratio CO2:N2 gas mixture. CN excitation temperatures are determined from fitting recorded and computed spectra. Chemical equilibrium mole fractions of CN are computed for air and the CO2:N2 gas mixture. Measurements utilize a 0.64-m Czerny–Turner type spectrometer and an intensified charge-coupled device.
Direct evidence of mismatching effect on H emission in laser-induced atmospheric helium gas plasma
Journal of Applied Physics, 2013
A time-resolved orthogonal double pulse laser-induced breakdown spectroscopy (LIBS) with helium surrounding gas is developed for the explicit demonstration of time mismatch between the passage of fast moving impurity hydrogen atoms and the formation of thermal shock wave plasma generated by the relatively slow moving major host atoms of much greater masses ablated from the same sample. Although this so-called "mismatching effect" has been consistently shown to be responsible for the gas pressure induced intensity diminution of hydrogen emission in a number of LIBS measurements using different ambient gases, its explicit demonstration has yet to be reported. The previously reported helium assisted excitation process has made possible the use of surrounding helium gas in our experimental setup for showing that the ablated hydrogen atoms indeed move faster than the simultaneously ablated much heavier major host atoms as signaled by the earlier H emission in the helium plasma generated by a separate laser prior to the laser ablation. This conclusion is further substantiated by the observed dominant distribution of H atoms in the forward cone-shaped target plasma. V
This work communicates the connection of measured shadowgraphs from optically induced air breakdown with emission spectroscopy in selected gas mixtures. Laser-induced optical breakdown is generated using 850 mJ and 170 mJ, 6-ns pulses at a wavelength of 1064 nm, the shadowgraphs are recorded using time-delayed 5-ns pulses at a wavelength of 532 nm and a digital camera, and emission spectra are recorded for typically a dozen of discrete time-delays from optical breakdown by employing an intensified charge-coupled device. The symmetry of the breakdown event can be viewed as close-to spherical symmetry for time-delays of several 100 ns. Spectroscopic analysis explores well-above hypersonic expansion dynamics using primarily the diatomic molecule cyanide and atomic hydrogen emission spectroscopy. Analysis of the air breakdown and selected gas breakdown events permits the use of Abel inversion for inference of the expanding species distribution. Typically, species are prevalent at higher...
Time-Resolved Emission Spectroscopy of Atomic and Molecular Species in Laser-Induced Plasma
2018
This work examines atomic and molecular signatures in laser-induced plasma in standard ambient temperature and pressure environments, including background contributions to the spectra that depend on the laser pulse-width. Investigations include solids, gases, and nano-particles. Abel inversions of measured line-of-sight data reveal insight into the radial plasma distribution. For nominal 6 nanosecond laser pulses and for pulse-energies in the range of 100 to 800 milli-Joules, expansion dynamics and turbulence due to shock phenomena are elucidated to address local equilibrium details that are frequently assumed in spatially averaged emission spectroscopy. Chemical equilibrium computations reveal temperature dependence of selected plasma species. Specific interests include atomic hydrogen (H) and cyanide (CN). The atomic H spectra, collected following optical breakdown in ultra-high-pure hydrogen and 9:1 mixtures of ultra-pure hydrogen and nitrogen gases, indicate spherical shel...
Explosion Dynamics of Methane Clusters Irradiated by 38 nm XUV Laser Pulses
CLEO: 2015, 2015
We have studied the explosion dynamics of methane clusters irradiated by intense, femtosecond, 38 nm (32.6 eV) XUV laser pulses. The ion time-of-flight spectrum measured with a Wiley-McLarentype time-of-flight spectrometer reveals undissociated molecular CH + 4 ions, fragments which are missing hydrogen atoms due to the breakage of one or more C-H bonds (CH + 3 , CH + 2 and CH +) and the recombination product CH + 5. Also visible on the time-of-flight traces are atomic and molecular hydrogen ions (H + and H + 2), carbon ions, and larger hydrocarbons such as C2H + 2 and C2H + 3. No doubly-charged parent ions (CH 2+ 4) were detected. The time-of-flight results show that total and relative ion yields depend strongly on cluster size. The absolute yields of CH + 5 and H + scale linearly with the yields of the other generated fragments up to a cluster size of N = 70, 000 molecules, then begin to decrease, whereas the yields of the CH + n (n = 1 − 4) fragments plateau at this cluster size. The behavior of H + may be understood through the electron recombination rate, which depends on the electron temperature and the cluster average charge. Moreover, the CH + 5 behavior is explained by the depletion of both CH + 4 and H + via electron-ion recombination in the expanding nanoplasma.
Observation of exclusively He-induced H emission in cooled laser plasma
Journal of Applied Physics, 2011
An experiment was performed for the observation of H emission induced in a cooled laser-induced atmospheric pressure gas plasma of He atoms in their metastable excited state. The strong H emission detected clearly established, to the exclusion of other well known major excitation processes, the exclusive contribution of the He-induced excitation (HIE) mechanism. The process is suggested to take place by means of energy transfer from the excited He atoms to the H atoms via Penning collision induced ionization involving electron exchange. The result further shows that this mechanism may also work for elements other than H and thereby strongly suggests the use of ambient He gas to broaden and complement the applications of standard laser-induced breakdown spectroscopy.
2015
This work discusses time-resolved measurements of atomic and diatomic spectra following laserinduced optical breakdown. Spatiallyand temporallyresolved spectroscopy is employed to characterize micro-plasma generated in laboratory air. Starkbroadened atomic emission profiles for hydrogen alpha and beta are utilized to determine plasma characteristics for specific time delays. The plasma dynamics include: variations in Stark-broadening emission of hydrogen alpha and beta lines, occurrence of molecular spectra due to recombination radiation, and change in line shape and appearance of atomic and molecular spectra due to collision and plasma oscillations. Comparisons of electron densities determined from hydrogen alpha and nitrogen II lines allow one to evaluate hydrogen self-absorption effects within the laser-induced plasma. Of interest are laser ablation measurements of atomic and diatomic emission spectra from aluminium (Al), aluminium monoxide (AlO), titanium (Ti), titanium monoxide...
Fragmentation dynamics of methane induced by femtosecond laser pulses
Applied Physics B, 2016
In this study it has been carried out theoretical simulations of ab-initio molecular dynamics of the C-H photo-dissociation of methane induced by femtosecond laser pulses. Our discussion about the reaction mechanism leading to the formation of the H and CH 3 fragments is based on the rectification of the Lorentz force. Such an electric force rectification occurs via a periodical charge transfer between H and CH 3 induced by the strong infrared laser pulse.
Journal of Physics D: Applied Physics
The temporal dynamics of the spatial distribution of atomic hydrogen and oxygen in a lean methane–air flame, forced by a nanosecond repetitively pulsed discharge-induced plasma, are investigated via femtosecond two-photon absorption laser-induced fluorescence technique. Plasma luminescence that interferes with the fluorescence from H and O atoms was observed to decay completely within 15 ns, which is the minimum delay required for imaging measurements with respect to the discharge occurrence. During discharge, H atoms in the excited state rather than the ground state, produced by electron-impact dissociation processes, are detected at the flame front. It was found that the temporal evolution of H and O fluorescence intensity during a cycle of 100 µs between two discharge pulses remains constant. Finally, the decay time of O-atoms produced by the discharge in the fresh methane–air mixture was about 2 µs, which suggests a faster reaction between O-atoms and methane than in air.
Spatially Resolved Laser-Induced Breakdown Spectroscopy in Methane-Air Diffusion Flames
Applied Spectroscopy, 2011
A new setup for spatially resolved laser-induced breakdown spectroscopy (SR-LIBS) is used for the first time to analyze methane-air diffusion flames. Using this configuration, background continuum emission is reduced, signal-to-background noise ratio is increased up to eight times, and spatial resolution is enhanced. The local equivalence ratio is also quantitatively estimated and the width of the secondary combustion region at a specified height above the burner is determined for two different methane flow rates. Furthermore, the threshold energy for spark formation is measured for regions inside and outside the flame. The results show that threshold energy is larger at the secondary combustion region, near the border of the flame, than inside the flame.