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Register a new userDiagnostics of collisions between electrons and water molecules in near-ultraviolet and visible wavelengths
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We studied dissociation reactions of electron impact on water vapor for several fragment species at optical and near ultraviolet wavelengths (200 - 850 nm). The resulting spectrum is dominated by the Hydrogen Balmer series, by the OH (A $^2Sigma^+$ - X $^2Pi$) band, and by the emission of ionic H$_2$O$^+$ (A $^2$A$_1$ - X $^2$B$_1$) and OH$^+$ (A $^3Pi$ - X $^3Sigma^-$) band systems. Emission cross sections and reaction channel thresholds were determined for energies between 5 - 100 eV. We find that electron impact dissociation of H$_2$O results in an emission spectrum of the OH (A $^2Sigma^+$ - X $^2Pi$) band that is distinctly different than the emission spectra from other excitation mechanisms seen in planetary astronomy. We attribute the change to a strongly non-thermal population of rotational states seen in planetary astronomy. This difference can be utilized for remote probing of the contribution of different physical reactions in astrophysical environments.
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Polytetrafluoroethylene (PTFE) is an excellent diffuse reflector widely used in light collection systems for particle physics experiments. However, the reflectance of PTFE is a function of its thickness. In this work, we investigate this dependence in air for light of wavelengths 260 nm and 450 nm using two complementary methods. We find that PTFE reflectance for thicknesses from 5 mm to 10 mm ranges from 92.5% to 94.5% at 450 nm, and from 90.0% to 92.0% at 260 nm. We also see that the reflectance of PTFE of a given thickness can vary by as much as 2.7% within the same piece of material. Finally, we show that placing a specular reflector behind the PTFE can recover the loss of reflectance in the visible without introducing a specular component in the reflectance.
The treatment of the inelastic collisions with electrons and hydrogen atoms are the main source of uncertainties in non-Local Thermodynamic Equilibrium (LTE) spectral line computations. We report, in this research note, quantum mechanical data for 369 collisional transitions of ion{Mg}{I} with electrons for temperatures comprised between 500 and 20000~K. We give the quantum mechanical data in terms of effective collision strengths, more practical for non-LTE studies.
Understanding collisions between ultracold molecules is crucial for making stable molecular quantum gases and harnessing their rich internal degrees of freedom for quantum engineering. Transient complexes can strongly influence collisional physics, but in the ultracold regime, key aspects of their behavior have remained unknown. To explain experimentally observed loss of ground-state molecules from optical dipole traps, it was recently proposed that molecular complexes can be lost due to photo-excitation. By trapping molecules in a repulsive box potential using laser light near a narrow molecular transition, we are able to test this hypothesis with light intensities three orders of magnitude lower than what is typical in red-detuned dipole traps. This allows us to investigate light-induced collisional loss in a gas of nonreactive fermionic $^{23}$Na$^{40}$K molecules. Even for the lowest intensities available in our experiment, our results are consistent with universal loss, meaning unit loss probability inside the short-range interaction potential. Our findings disagree by at least two orders of magnitude with latest theoretical predictions, showing that crucial aspects of molecular collisions are not yet understood, and provide a benchmark for the development of new theories.
We study the visible and near-infrared (NIR) spectral properties of different ACO populations and compare them to the independently determined properties of comets. We select our ACOs sample based on published dynamical criteria and present our own observational results obtained using the 10.4m Gran Telescopio Canarias (GTC), the 4.2m William Herschel Telescope (WHT), the 3.56m Telescopio Nazionale Galileo (TNG), and the 2.5m Isaac Newton Telescope (INT), all located at the El Roque de los Muchachos Observatory (La Palma, Spain), and the 3.0m NASA Infrared Telescope Facility (IRTF), located at the Mauna Kea Observatory, in Hawaii. We include in the analysis the spectra of ACOs obtained from the literature. We derive the spectral class and the visible and NIR spectral slopes. We also study the presence of hydrated minerals by studying the 0.7 $mu$m band and the UV-drop below 0.5 $mu$m associated with phyllosilicates. We present new observations of 17 ACOs, 11 of them observed in the visible, 2 in the NIR and 4 in the visible and NIR. We also discuss the spectra of 12 ACOs obtained from the literature. All but two ACOs have a primitive-like class spectrum (X or D-type). Almost 100% of the ACOs in long-period cometary orbits (Damocloids) are D-types. Those in Jupiter family comet orbits (JFC-ACOs) are $sim$ 60% D-types and $sim$ 40% X-types. The mean spectral slope $S$ of JFC-ACOs is 9.7 $pm$ 4.6 %/1000 AA and for the Damocloids this is 12.2 $pm$ 2.0 %/1000 AA . No evidence of hydration on the surface of ACOs is found from their visible spectra. The slope and spectral class distribution of ACOs is similar to that of comets. The spectral classification, the spectral slope distribution of ACOs, and the lack of spectral features indicative of the presence of hydrated minerals on their surface, strongly suggest that ACOs are likely dormant or extinct comets.
We demonstrate a versatile, rotational-state dependent trapping scheme for the ground and first excited rotational states of $^{23}$Na$^{40}$K molecules. Close to the rotational manifold of a narrow electronic transition, we determine tune-out frequencies where the polarizability of one state vanishes while the other remains finite, and a magic frequency where both states experience equal polarizability. The proximity of these frequencies of only 10 GHz allows for dynamic switching between different trap configurations in a single experiment, while still maintaining sufficiently low scattering rates.
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