Whereas atom-molecule collisions have been studied with complete quantum-state resolution, interactions between two state-selected molecules have proven much harder to probe. Here, we report the measurement of state-resolved inelastic scattering cross sections for collisions between two open-shell molecules that are both prepared in a single quantum state. Stark-decelerated hydroxyl (OH) radicals were scattered with hexapole-focused nitric oxide (NO) radicals in a crossed-beam configuration. Rotationally and spin-orbit inelastic scattering cross sections were measured on an absolute scale for collision energies between 70 and 300 cm(-1). These cross sections show fair agreement with quantum coupled-channels calculations using a set of coupled model potential energy surfaces based on ab initio calculations for the long-range nonadiabatic interactions and a simplistic short-range interaction. This comparison reveals the crucial role of electrostatic forces in complex molecular collision processes.

Experimental studies of the electronic structure of excess electrons in liquids—archetypal quantum solutes—have been largely restricted to very dilute electron concentrations. We overcame this limitation by applying soft x-ray photoelectron spectroscopy to characterize excess electrons originating from steadily increasing amounts of alkali metals dissolved in refrigerated liquid ammonia microjets. As concentration rises, a narrow peak at ~2 electron volts, corresponding to vertical photodetachment of localized solvated electrons and dielectrons, transforms continuously into a band with a sharp Fermi edge accompanied by a plasmon peak, characteristic of delocalized metallic electrons. Through our experimental approach combined with ab initio calculations of localized electrons and dielectrons, we obtain a clear picture of the energetics and density of states of the ammoniated electrons over the gradual transition from dilute blue electrolytes to concentrated bronze metallic solutions.

Here we report on spectroscopic measurements of the aluminum monofluoride molecule (AlF) that are relevant to laser cooling and trapping experiments. We measure the detailed energy level structure of AlF in the X 1 Σ + electronic ground state, in the A 1 Π state, and in the metastable a 3 Π state. We determine the rotational, vibrational and electronic branching ratios from the A 1 Π state. We also study how the rotational levels split and shift in external electric and magnetic fields. We find that AlF is an excellent candidate for laser cooling on any Q-line of the A 1 Π -X 1 Σ + transition and for trapping at high densities.The energy levels in the X 1 Σ + , v = 0 state and within each Ω-manifold in the a 3 Π, v = 0 state are determined with a relative accuracy of a few kHz, using laser-radio-frequency multiple resonance and ionization detection schemes in a jet-cooled, pulsed molecular beam. To determine the hyperfine and Λ-doubling parameters we measure transitions throughout the 0.1 MHz -66 GHz range, between rotational levels in the X 1 Σ + , v = 0 state and between rotational and Λ-doublet levels in all three spin-orbit manifolds of the a 3 Π, v = 0 state. We measure the hyperfine splitting in the A 1 Π state using continuous wave (CW) laser-induced fluorescence spectroscopy of the A 1 Π, v = 0 ← X 1 Σ + , v = 0 band. The resolution is limited by the short radiative lifetime of the A 1 Π, v = 0 state, which we experimentally determine to be 1.90 ± 0.03 ns. The hyperfine mixing of the lowest rotational levels in the A 1 Π state causes a small loss from the the main laser cooling transition of 10 −5 . The off-diagonal vibrational branching from the A 1 Π, v = 0 state is measured to be (5.60 ± 0.02) × 10 −3 in good agreement with theoretical predictions. The strength of the spin-forbidden A 1 Π, v = 0 → a 3 Π, v = 0 transition is measured to be seven orders of magnitude lower than the strength of the A 1 Π, v = 0 → X 1 Σ + , v = 0 transition. We determine the electric dipole moments µ(X) = 1.515 ± 0.004 Debye, µ(a) = 1.780 ± 0.003 Debye and µ(A) = 1.45 ± 0.02 Debye in X 1 Σ + , v = 0, a 3 Π, v = 0 and A 1 Π, v = 0, respectively, by recording CW laser excitation spectra in electric fields up to 150 kV/cm.

We report coherent reflection of thermal He atom beams from various microscopically rough surfaces at grazing incidence. For a sufficiently small normal component kz of the incident wavevector of the atom the reflection probability is found to be a function of k z only. This behavior is explained by quantum-reflection at the attractive branch of the Casimir-van der Waals interaction potential. For larger values of kz the overall reflection probability decreases rapidly and is found to also depend on the parallel component kx of the wave-vector. The material specific kx dependence for this classic reflection at the repulsive branch of the potential is discussed in terms of an averagingout of the surface roughness under grazing incidence conditions. atom beams of thermal energies. In these experimental studies classic reflection at the repulsive branch of the potential was considered to be negligible, either because of deexcitation of the metastable atoms [3,4], inelastic scattering or adsorption [5], or surface roughness [6]. Quantum reflection was also theoretically studied, using the long-range Casimir-van der Waals atom-surface potential, indicating that the reflection probability is only a function of k z , the component of the incident wave-vector that is perpendicular to the surface [2].

We present an experimental and theoretical investigation of rotationally inelastic transitions of OH, prepared in the X 2 Π, v = 0, j = 3/2 F 1 f level, in collisions with molecular hydrogen (H 2 and D 2 ). In a crossed beam experiment, the OH radicals were state selected and velocity tuned over the collision energy range 75-155 cm −1 using a Stark decelerator. Relative parity-resolved state-to-state integral cross sections were determined for collisions with normal and para converted H 2 . These cross sections, as well as previous OH-H 2 measurements at 595 cm (2010)], were compared with the results of quantum scattering calculations using recently determined ab initio potential energy surfaces [Ma et al., J. Chem. Phys. 141, 174309 (2014)]. Good agreement between the experimental and computed relative cross sections was found, although some structure seen in the OH( j = 3/2 F 1 f → j = 5/2 F 1 e) + H 2 ( j = 0) cross section is not understood. C 2015 AIP Publishing LLC. [http://dx

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We present chemical kinetics measurements of the luminol oxidation chemiluminescence (CL) reaction at the interface between two aqueous solutions, using liquid jet technology. Free-flowing liquid microjets are a relatively recent development that have found their way into a growing number of applications in spectroscopy and dynamics. A variant thereof, called flat-jet, is obtained when two cylindrical jets of a liquid are crossed, leading to a chain of planar leaf-shaped structures of the flowing liquid. We here show that in the first leaf of this chain, the fluids do not exhibit turbulent mixing, providing a clean interface between the liquids from the impinging jets. We also show, using the example of the luminol CL reaction, how this setup can be used to obtain kinetics information from friction-less flow and by circumventing the requirement for rapid mixing by intentionally suppressing all turbulent mixing and instead relying on diffusion.

Recently, we determined the detailed energy level structure of the X 1 + , A 1 and a 3 states of AlF that are relevant to laser cooling and trapping experiments [Truppe et al., Phys. Rev. A. 100 (5), 052513 (2019)]. Here, we investigate the b 3 + , v = 0 state of the AlF molecule. A rotationally resolved (1 + 2)-REMPI spectrum of the b 3 + , v = 0 ← a 3 , v = 0 band is presented and the lifetime of the b 3 + , v = 0 state is measured to be 190(2) ns. Hyperfine-resolved, laser-induced fluorescence spectra of the b 3 + , v = 0 ← X 1 + , v = 1 and the b 3 + , v = 0 ← a 3 , v = 0 bands are recorded to determine fine-and hyperfine structure parameters. The interaction between the b 3 + , v = 0 and the nearby A 1 state is studied and the magnitude of the spin-orbit coupling between the two electronic states is derived using three independent methods to give a consistent value of 10(1) cm −1. The triplet character of the A state causes an A → a loss from the main A−X laser cooling cycle below the 10 −6 level.