Angle-resolved photoelectron spectroscopy of the unpaired electron in sodium-doped water, methanol, ammonia, and dimethyl ether clusters is presented. The experimental observations and the complementary calculations are consistent with surface electrons for the cluster size range studied. Evidence against internally solvated electrons is provided by the photoelectron angular distribution. The trends in the ionization energies seem to be mainly determined by the degree of hydrogen bonding in the solvent and the solvation of the ion core. The onset ionization energies of water and methanol clusters do not level off at small cluster sizes but decrease slightly with increasing cluster size.

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.

Aluminium monofluoride (AlF) is a promising candidate for laser cooling and trapping at high densities. We show efficient production of AlF in a bright, pulsed cryogenic buffer gas beam, and demonstrate rapid optical cycling on the Q rotational lines of the A 1Π ↔ X 1Σ+ transition. We measure the brightness of the molecular beam to be >1012 molecules per steradian per pulse in a single rotational state and present a new method to determine its velocity distribution in a single shot. The photon scattering rate of the optical cycling scheme is measured using three different methods, and is compared to theoretical predictions of the optical Bloch equations and a simplified rate equation model. Despite the large number of Zeeman sublevels (up to 216 for the Q(4) transition) involved, a high scattering rate of at least 17(2) × 106 s−1 can be sustained using a single, fixed-frequency laser without the need to modulate the polarisation. We deflect the molecu-lar beam using the radiation pressure force and measure an acceleration of 8.7(1.5) × 105 m s−2. Losses from the optical cycle due to vibrational branching to X 1Σ+, v″ = 1 are addressed efficiently with a single repump laser. Further, we investigate two other loss channels, parity mixing by stray electric fields and photo-ionisation. The upper bounds for these effects are sufficiently low to allow loading into a magneto‐optical trap.

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.

A direct simulation Monte Carlo (DSMC) method is applied to model collisions between He buffer gas atoms and ammonia molecules within a buffer gas cell. State-to-state cross sections, calculated as a function of the collision energy, enable the inelastic collisions between He and NH 3 to be considered explicitly. The inclusion of rotational-state-changing collisions affects the translational temperature of the beam, indicating that elastic and inelastic processes should not be considered in isolation. The properties of the cold molecular beam exiting the cell are examined as a function of the cell parameters and operating conditions; the rotational and translational energy distributions are in accord with experimental measurements. The DSMC calculations show that thermalisation occurs well within the typical 10-20 mm length of many buffer gas cells, suggesting that shorter cells could be employed in many instances-yielding a higher flux of cold molecules.