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1. Temperature dependence of collisional broadening and shift for the Kr 4 p 6 S 01→5 p [3/2] 2 electronic transition

   https://doi.org/10.1364/AO.380102  

   Sahoo, Abinash ; Zelenak, Dominic ; Narayanaswamy, Venkateswaran ( February 2020 , Applied Optics)

   Temperature scaling of collisional broadening parameters for krypton (absorber)$4p^6{S}_0^1\to <\#comment/>5p[3/2{]}_2$electronic transition centered at 107.3 nm in the presence of major combustion species (perturber) is investigated. The absorption spectrum in the vicinity of the transition is obtained from the fluorescence due to the two-photon excitation scan of krypton. Krypton was added in small amounts to major combustion species such as${\mathrm{C}\mathrm{H}}_4$,${\mathrm{C}\mathrm{O}}_2$,${\mathrm{N}}_2$, and air, which then heated to elevated temperatures when flowed through a set of heated coils. In a separate experimental campaign, laminar premixed flat flame product mixtures of methane combustion were employed to extend the investigations to higher temperature ranges relevant to combustion. Collisional full width half maximum (FWHM) ($w_C$) and shift (${\mathrm{\delta <\#comment/>}}_C$) were computed from the absorption spectrum by synthetically fitting Voigt profiles to the excitation scans, and their corresponding temperature scaling was determined by fitting power-law temperature dependencies to the$w_C$and${\mathrm{\delta <\#comment/>}}_C$data for each perturber species. The temperature exponents of$w_C$and${\mathrm{\delta <\#comment/>}}_C$for all considered combustion species (perturbers) were$-<\#comment/>0.73$and$-<\#comment/>0.6$, respectively. Whereas the temperature exponents of$w_C$are closer to the value ($-<\#comment/>0.7$) predicted by the dispersive interaction collision theory, the corresponding exponents of${\mathrm{\delta <\#comment/>}}_C$are in between the dispersive interaction theory and the kinetic theory of hard-sphere collisions. Comparison with existing literature on broadening parameters of NO, OH, and CO laser-induced fluorescence spectra reveal interesting contributions from non-dispersive interactions on the temperature exponent.

    

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2. Phase-factor spectra of turbulent phase screens

   https://doi.org/10.1364/JOSAA.429928  

   Muschinski, Andreas ( August 2021 , Journal of the Optical Society of America A)

   The optical phase$\mathrm{\varphi <\#comment/>}$is a key quantity in the physics of light propagating through a turbulent medium. In certain respects, however, the statistics of the phasefactor,$\mathrm{\psi <\#comment/>}=\exp <\#comment/>(i\mathrm{\varphi <\#comment/>})$, are more relevant than the statistics of the phase itself. Here, we present a theoretical analysis of the 2D phase-factor spectrum$F_{\mathrm{\psi <\#comment/>}}(\mathit{\kappa <\#comment/>})$of a random phase screen. We apply the theory to four types of phase screens, each characterized by a power-law phase structure function,$D_{\mathrm{\varphi <\#comment/>}}(r)=(r/r_c{)}^{\mathrm{\gamma <\#comment/>}}$(where$r_c$is the phase coherence length defined by$D_{\mathrm{\varphi <\#comment/>}}(r_c)=1{\mathrm{r}\mathrm{a}\mathrm{d}}^2$), and a probability density function$p_{\mathrm{\alpha <\#comment/>}}(\mathrm{\alpha <\#comment/>})$of the phase increments for a given spatial lag. We analyze phase screens with turbulent ($\mathrm{\gamma <\#comment/>}=5/3$) and quadratic ($\mathrm{\gamma <\#comment/>}=2$) phase structure functions and with normally distributed (i.e., Gaussian) versus Laplacian phase increments. We find that there is a pronounced bump in each of the four phase-factor spectra$F_{\mathrm{\psi <\#comment/>}}(\mathrm{\kappa <\#comment/>})$. The precise location and shape of the bump are different for the four phase-screen types, but in each case it occurs at$\mathrm{\kappa <\#comment/>}\sim <\#comment/>1/r_c$. The bump is unrelated to the well-known “Hill bump” and is not caused by diffraction effects. It is solely a characteristic of the refractive-index statistics represented by the respective phase screen. We show that the second-order$\mathrm{\psi <\#comment/>}$statistics (covariance function, structure function, and spectrum) characterize a random phase screen more completely than the second-order$\mathrm{\varphi <\#comment/>}$counterparts.

    

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3. Spectroscopic study of the 4 f 7 6 s 2 8 S 7/2∘−4 f 7 ( 8 S ∘ )6 s 6 p ( 1 P ∘ ) 8 P 9/2 transition in neutral europium-151 and europium-153: absolute frequency and hyperfine structure

   https://doi.org/10.1364/JOSAB.467968  

   Herd, M. T. ; Maruko, C. ; Herzog, M. M. ; Brand, A. ; Cannon, G. ; Duah, B. ; Hollin, N. ; Karani, T. ; Wallace, A. ; Whitmore, M. ; et al ( September 2022 , Journal of the Optical Society of America B)

   We report on spectroscopic measurements on the$4f^{7}6s^2{}^8{S}_{7/2}^{\circ <\#comment/>}\to <\#comment/>4f^7{(}^8{S}^{\circ <\#comment/>})6s6p{(}^1{P}^{\circ <\#comment/>}){}^8{P}_{9/2}$transition in neutral europium-151 and europium-153 at 459.4 nm. The center of gravity frequencies for the 151 and 153 isotopes, reported for the first time in this paper, to our knowledge, were found to be 652,389,757.16(34) MHz and 652,386,593.2(5) MHz, respectively. The hyperfine coefficients for the$6s6p{(}^1{P}^{\circ <\#comment/>}){}^8{P}_{9/2}$state were found to be$\mathrm{A}(151)=-<\#comment/>228.84(2)\mathrm{M}\mathrm{H}\mathrm{z}$,$\mathrm{B}(151)=226.9(5)\mathrm{M}\mathrm{H}\mathrm{z}$and$\mathrm{A}(153)=-<\#comment/>101.87(6)\mathrm{M}\mathrm{H}\mathrm{z}$,$\mathrm{B}(153)=575.4(1.5)\mathrm{M}\mathrm{H}\mathrm{z}$, which all agree with previously published results except for A(153), which shows a small discrepancy. The isotope shift is found to be 3163.8(6) MHz, which also has a discrepancy with previously published results.

    

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4. Emissions in short-gated ns/ps/fs-LIBS for fuel-to-air ratio measurements in methane-air flames

   https://doi.org/10.1364/AO.418453  

   Gragston, Mark ; Hsu, Paul ; Jiang, Naibo ; Roy, Sukesh ; Zhang, Zhili ( May 2021 , Applied Optics)

   A study of short-gated 10 nanosecond (ns), 100 picosecond (ps), and 100 femtosecond (fs) laser induced breakdown spectroscopy (LIBS) was conducted for fuel-to-air ratio (FAR) measurements in an atmospheric Hencken flame. The intent of the work is to understand which emission lines are available near the optical range in each pulse width regime and which emission ratios may be favorable for generating equivalence ratio calibration curves. The emission spectra in the range of 550–800 nm for ns-LIBS and ps-LIBS are mostly similar with slightly elevated atomic oxygen lines by ps-LIBS. Spectra from fs-LIBS show the lowest continuum background and prominent individual atomic lines, though have significantly weaker ionic emission from nitrogen. A qualitative explanation based on assumed local thermodynamic equilibrium and electron temperatures calculated by the${\mathrm{N}}_{\mathrm{I}\mathrm{I}}(565\mathrm{n}\mathrm{m})$and${\mathrm{N}}_{\mathrm{I}\mathrm{I}}(594\mathrm{n}\mathrm{m})$emissions is presented. In studying line emission ratios for FAR calculation, it is found that${\mathrm{H}}_{\mathrm{\alpha <\#comment/>}}(656\mathrm{n}\mathrm{m})/{\mathrm{N}}_{\mathrm{I}\mathrm{I}}(568\mathrm{n}\mathrm{m})$is best for FAR measurements with ns-LIBS and remains viable for ps-LIBS, while${\mathrm{H}}_{\mathrm{\alpha <\#comment/>}}(656\mathrm{n}\mathrm{m})/{\mathrm{O}}_{\mathrm{I}}(777\mathrm{n}\mathrm{m})$is optimal for the ps-LIBS and fs-LIBS cases. Due to low continuum background and short time delay for spectra collection, fs-LIBS is very promising for high-speed FAR measurements using short-gated LIBS.

    

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5. Variability of relationship between the volume scattering function at 180° and the backscattering coefficient for aquatic particles

   https://doi.org/10.1364/AO.383229  

   Hu, Lianbo ; Zhang, Xiaodong ; Xiong, Yuanheng ; Gray, Deric J. ; He, Ming-Xia ( February 2020 , Applied Optics)

   Properly interpreting lidar (light detection and ranging) signal for characterizing particle distribution relies on a key parameter,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$, which relates the particulate volume scattering function (VSF) at 180° (${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$) that a lidar measures to the particulate backscattering coefficient ($b_{\mathit{\text{bp}}}$). However,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$has been seldom studied due to challenges in accurately measuring${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$and$b_{\mathit{\text{bp}}}$concurrently in the field. In this study,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$, as well as its spectral dependence, was re-examined using the VSFs measuredin situat high angular resolution in a wide range of waters.${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$, while not measured directly, was inferred using a physically sound, well-validated VSF-inversion method. The effects of particle shape and internal structure on the inversion were tested using three inversion kernels consisting of phase functions computed for particles that are assumed as homogenous sphere, homogenous asymmetric hexahedra, or coated sphere. The reconstructed VSFs using any of the three kernels agreed well with the measured VSFs with a mean percentage difference$<<\#comment/>5\mathrm{\%<\#comment/>}$at scattering angles$<<\#comment/>{170}^{\circ <\#comment/>}$. At angles immediately near or equal to 180°, the reconstructed${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$depends strongly on the inversion kernel.${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$derived with the sphere kernels was smaller than those derived with the hexahedra kernel but consistent with${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$estimated directly from high-spectral-resolution lidar andin situbackscattering sensor. The possible explanation was that the sphere kernels are able to capture the backscattering enhancement feature near 180° that has been observed for marine particles.${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$derived using the coated sphere kernel was generally lower than those derived with the homogenous sphere kernel. Our result suggests that${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$is sensitive to the shape and internal structure of particles and significant error could be induced if a fixed value of${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$is to be used to interpret lidar signal collected in different waters. On the other hand,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$showed little spectral dependence.

    

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