Laser-induced plasma is a universal plasma source for spectral diagnostics of processes under extreme conditions. Due to possibility to freely vary laser energy, ambient pressure, compositions of the ablation target and surrounding environment plasma parameters can also be varied within a wide range. Typical temperatures (0.2-4 eV) and electron number densities (10¹⁵-10¹⁹ cm⁻³) allow observation of both atomic emission and emission of small, predominantly diatomic, molecules. In spite of spatial inhomogeneity of laser plasma, its certain symmetry opens up space for spatially resolved studies. Laser-induced fluorescence in plasma appears to be one of the most promising tools for spatially resolved plasma diagnostics. All these unique properties of laser plasma, combined with research interest in combustion processes during the meteor events in the Earth’s atmosphere, led us to the study of Ca and CaO distribution in laser plasma under low ambient pressure. We measured emission spectra of atomic calcium and calcium monoxide varying delay after laser pulse and ambient pressure from 0.16 Torr to atmospheric. Plasma temperature and electron number density were calculated where possible. By comparison of experimental spectra and spectra of Benešov bolide at different heights we showed that the emitting bolide wake is formed under 7-10 times higher pressure than the one at the corresponding altitude. The obtained data lead us to suggestion that the formation of CaO in plasma occurs primarily using oxygen from atmosphere. Therefore, abundance of CaO should have a strong dependency on the pressure of the surrounding media. Also, we performed plasma elemental imaging (resolution of 200μm along each of 2 axes) by the means of Ca and CaO fluorescence in laser plasma. Ca atomic lines Ca I 428.30 nm and Ca I 430.52 nm and bands of CaO red system were used for this purpose. The estimated spatial distribution of Ca atoms and CaO molecules in laser plasma proves our suggestion that CaO is formed both in laser plasma and in the meteor wake primarily using oxygen from ambient air on the periphery of the cloud and this process almost does not involve oxygen from the ablated material (CaCO₃). This work was supported by the Russian Science Foundation (grant 18-13-00269-Π)

The properties of laser plasma vary significantly depending on the pressure and composition of the environment, thus it a promising emission source to imitate of radiation from various objects in atmosphere (meteor wake, airglow) and in outer space. We aimed to register spectra of FeO and CaO bands in laser plasma as close as possible to the ones observed during the Benešov bolide event to reconstruct the composition and behavior of meteor wake. We fit synthetic spectra of spontaneous vatying temperatures in the region of 1000-8000 K for the infrared system of CaO molecules to those measured in laser-induced plasma. It was found that the excitation (atomic species), vibrational and rotational temperatures of the experimental spectra indicate the absence of local thermodynamic equilibrium (LTE) and does not coincide with each other. The atomic excitation temperature are close to 10000 , vibrational temperature varies in the range of 3500–5000 K, while the rotational temperature is noticeably lower than  2000–3000 K. Moreover, the specific values of rotational temperatures vary greatly from band to band. We also found the valuable deviation of lines wavelengths and transition probabilities between model spectra based on EXOMOL data. Calculations of the chemical composition of laser-produced clouds formed by laser heating of Fe and CaCO₃ targets were performed. Timescales of main reactions with participation of Fe- and Ca- containing species were calculated using rate constants of the reactions. Results of calculations of equilibrium composition of laser-produced and impact-produced clouds are presented. Quenching conditions of chemical reactions in laser-produced and impact-produced clouds are found. This work was supported by the Russian Science Foundation (grant 18-13-00269-Π).

Two color planar laser induced fluorescence (PLIF) is a robust combustion diagnostics technique to flame temperature field. Widely used OH-PLIF can measure the high temperature post flame front zone, but cannot accurately measure the intermediate temperature pre-flame front region where OH radical concentration is low. Here, the rotational resolved absorption cross section of formaldehyde in this region was analyzed and two peaks at 28183.5 cm⁻¹and 28184.5 cm⁻¹were selected as the line pair to determine flame temperature. The wavelength region can be easily accessed using the 3^rd harmonics of Nd:YAG lasers at 355 nm. We demonstrate 20 kHz two dimensional flame temperature field measurement of a laminar coflow diffusion flame, a free jet flame and a reacting jet in hot crossflow using a wavelength-switching injection seeding burst mode laser and a single high speed camera.

The thermal decomposition mechanism of trimethylchlorosilane at temperatures up to 1400 K was investigated using flash pyrolysis microreactor coupled with vacuum ultraviolet (118.2 nm) photoionization time-of-flight mass spectrometry. The main initiation reaction of the parent molecule was identified to be molecular elimination producing HCl and Me₂Si=CH₂. Other initiation pathways such as chlorine-atom loss, methyl radical loss, and methane elimination were also observed. Density function theory (DFT) calculations at UB3LYP/6-311++G(d,p) level of theory, with Grimme’s empirical dispersion correction GD3, were performed to study the energetics of the possible initiation pathways. The theoretical calculations revealed that the HCl elimination channel via a van der Waals intermediate was the most energetically favored pathway among all initiation channels, in agreement with the experimental observations. Some secondary reactions of the initial products were identified, and their possible mechanisms were proposed.

Vacuum ultraviolet spectroscopy was used to investigate the lowest-lying electronic state band edge of subcritical ethanol as a function of temperature from 25-200 ^°C, and for supercritical ethanol as a function of density at 250 ^°C. For subcritical ethanol, the band edge is observed to red shift with increasing temperature. Supercritical spectra clearly demonstrate a gradual transition from gas-phase to liquid-phase behavior with increasing density, as evidenced by a gradual blue shift and loss of spectral detail. We discuss both effects regarding the extent of hydrogen bonding in the system and Rydbergization effects, similar to those observed for subcritical and supercritical water.

Vibrational energy transfer is a fundamental process in molecules which is closely related to chemical reactivity. Supersonic jet expansions have been an important tool in spectroscopy and chemical physics. These expansions are used to produce cold molecules under collision-free conditions. Among the various degrees of freedom that are collisionally relaxed, our focus is on vibrationally inelastic collisions between the analyte molecule and the carrier gas. A chirped-pulse Fourier-transform millimeter wave spectrometer (CP-FTmmW) is employed to observe vibrational relaxation (VR) of low-frequency vibrational modes in small molecules SO₂, CHF₃, CH₃CN and a medium sized molecule CH₂CHCN. Systematic study of several supersonic expansion parameters extracts empirical relationships between VR and collision conditions. This includes a study of VR in molecules seeded in helium considering different valve types (Even-Lavie valve vs. General Valve), instrumental parameters (nozzle temperature, stagnation pressure, orifice dimensions), and variation of the seeded molecule concentration. The identity of the collision partner is explored using several carrier gases (neon, argon, nitrogen, and hydrogen) and comparing the observed VR with that of helium. A universal inverse-linear relationship between the extent of VR and the frequency of the vibrational mode has been revealed by the experiments using helium. This was strikingly different from what was observed for other choices of carrier gases, where mode-specific VR was observed. For CH₃CN (which has a degenerate bending mode, 2v₈^0,2), efficient l-relaxation was observed. Separate use of two complementary laser-based techniques, laser induced fluorescence and millimeter wave optical double resonance, led to characterization of the velocity slip effect, the onset of clustering, and effects of Van der Waals bonding, studied as analyte concentrations were increased. Apart from demonstrating the power of a multiplexed form of rotationally resolved spectroscopy (CP-FTmmW), a ‘roadmap’ is generated to aid the design of future experiments by tailoring the choices of supersonic conditions. Empirical and intuitive approximate models are assembled that will aid in understanding vibrationally inelastic scattering and VR across a wide range of expansion parameters.

The spectroscopic investigation of the hydrogen molecule and its isotopologues has played a crucial role in the advancement of quantum mechanics in the molecular domain. Particularly, highly accurate measurements of rovibrational transitions allow for various tests of fundamental physics including searches for physics beyond the Standard ModelW. Ubachs, J.C.J. Koelemeij, K.S.E. Eikema, E.J. Salumbides, J. Mol. Spectr. 320, 1-12 (2016) Recent Doppler-free measurements performed at room temperatureF.M.J. Cozijn, P. Dupre, K.S.E. Eikema, E.J. Salumbides, W. Ubachs,, Phys. Rev. Lett. 120, 153002 (2018)M.L. Diouf , F.M.J. Cozijn, B. Darquie, E.J. Salumbides, W. Ubachs, Opt. Lett. 44, 4733 (2019)M.L. Diouf, F.M.J. Cozijn, K.F. Lai, E.J. Salumbides, W. Ubachs , Phys. Rev. Res. 2, 023209 (2020) in the (2,0) overtone band of the hydrogen deuteride molecule spurred a stimulating debate on the interpretation of the spectra which significantly differ from typical Lamb-dips. New measurements were performed in a cryogenically cooled cavity with the hope of resolving the underlying hyperfine structure. However the resulted spectra observed shared the same unusual lineshapes which still hinder the extraction of the absolute rovibrational positions. With the goal of extracting the rotational interval with a better accuracy, pairs of P and R transitions were considered. This leads to accurate values for rotational energy intervals and to a precise test of molecular QED theoryP. Czachorowski, M. Puchalski, J. Komasa, K. Pachucki, Phys. Rev. A 98, 052506 (2018)

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Footnotes:

W. Ubachs, J.C.J. Koelemeij, K.S.E. Eikema, E.J. Salumbides, J. Mol. Spectr. 320, 1-12 (2016). F.M.J. Cozijn, P. Dupre, K.S.E. Eikema, E.J. Salumbides, W. Ubachs,, Phys. Rev. Lett. 120, 153002 (2018)

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Footnotes:

M.L. Diouf, F.M.J. Cozijn, K.F. Lai, E.J. Salumbides, W. Ubachs , Phys. Rev. Res. 2, 023209 (2020), P. Czachorowski, M. Puchalski, J. Komasa, K. Pachucki, Phys. Rev. A 98, 052506 (2018).

Fluorescence Quantum Yield allows scientists to both quantify spectroscopic properties of dyes and compare to literature references. With the growing interest in NIR-emissive dyes for biological imaging, it is of great importance to reliably measure the fluorescent quantum yield of these novel dyes. Using a broadband excitation source and liquid nitrogen cooled InGaAs detector, steady state emission of four novel pentamethine indolizine cyanine dyes synthesized with N,N-dimethylaniline-based substituents on the indolizine periphery at varied substitutions sites is recorded.

The design and characterization of organic dyes emitting in the near infrared (NIR) and short-wave infrared (SWIR) regions are of a great interest to the research community for several applications including bio-imaging. These abstract reports the results of photo-physical studies of a set of four newly-designed and synthesized SWIR emissive RhodIndolizine dyes. All the dyes were found to absorb and emit well within the SWIR domain (reaching emission maximum up to 1256 nm) with an onset beyond 1400 nm and Stokes shifts varying between 140-170 nm. The quantum yields of these dyes were estimated relative to the emission standard of IR1061 with a quantum yield of 0.0059 or 0.59% in dichloromethane. Further, nanoencapsulation studies in a water-soluble surfactant demonstrate their efficiency towards biological imaging.

As part of the efforts to determine the applications of molecular rotational resonance (MRR) technique to SO₂ monitoring in ambient air, a K-band MRR analyzer has been employed to record the MRR signature of multiple synthetic air samples containing SO₂ pollutant as well as that of standard SO₂ samples. The observed MRR features reveal a rich rotational pattern due to MRR’s sensitivity. The interfering matrix (i.e., air moisture), which typically challenges other conventional techniques, showed no impact on MRR signatures of SO₂. The validity of MRR for SO₂ monitoring has been examined by measuring MRR signal response of a set of standard SO₂ samples over a range of sampling pressures (5-15). The obtained linear correlations allowed the determination of recovery percentage (97-100%) and low detection limit of better than 1mg/m³. Work to improve this analytical procedure is underway and will be reported in this talk.

Space-time variations of fundamental constants that are assumed in theories beyond the Standard Model may be investigated by precision molecular spectroscopy. The molecular energy levels are intrinsically sensitive to a variation of the proton-to-electron mass ratio μ and the transitions between closely-spaced energy levels display an enhanced sensitivityV.V. Flambaum and M.G. Kozlov, Phys. Rev. Lett. 99, 150801 (2007). The lasers stabilized to isotopic acetylene lines probed by saturated absorption spectroscopy provided secondary frequency references in the 1.5 μm spectral region. The acetylene optical clock enables now access to fractional frequency stability of 3×10⁻¹³ at one second in a compact and robust setup that ensures optical frequency referencing with drifts lower than 1 Hz/day, as it is indicated in ref.T. Talvard et al, Optics Express 25, 2259-2269 (2017). This contribution discusses the potential in constraining time variation of μ from precision spectroscopy of ¹²C₂H₂ transitions pertaining to the v₁+v₃ and v₁+v₂+v₄+v₅ combination bandsF.L. Constantin, Vibrational Spectroscopy 85, 228-234 (2016). The acetylene energy levels are modeled with a state-of-the-art Hamiltonian that takes into account different rovibrational interactions and the sensitivities of the reference acetylene transitions are calculated. The frequency splittings between near resonant transitions, that may arise from the cancellation of the rotational intervals with frequency shifts associated to the origins of the vibrational bands, the anharmonicity, and the rovibrational interactions, display sensitivity coefficients up to ±10³ level. The systematic frequency shifts are conservatively evaluated for intracavity spectroscopy setups. The constraint to the time variation of μ derived from absolute frequency measurements of acetylene optical clocks is estimated at the 10⁻¹³/yr level.

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Footnotes:

V.V. Flambaum and M.G. Kozlov, Phys. Rev. Lett. 99, 150801 (2007).. T. Talvard et al, Optics Express 25, 2259-2269 (2017).. F.L. Constantin, Vibrational Spectroscopy 85, 228-234 (2016)..