AlF spectroscopy database: from molecular data to experiment-ready predictions

Turning dispersed spectroscopy into an experiment-ready resource

Aluminium monofluoride is becoming an important system for laser cooling, molecular trapping and precision spectroscopy. It was the first spin-singlet diatomic molecule loaded into a magneto-optical trap, extending direct molecular laser cooling beyond species with ²Σ electronic ground states.

Detailed information on AlF has accumulated over decades. Some electronic states still rely on systematic measurements from the 1970s, while others have been substantially refined by recent high-resolution spectroscopy. The most accurate information is therefore distributed across publications of different age, scope and precision.

For an experimental user, the quantity of interest is often not an isolated spectroscopic constant, but the transition between a specific pair of electronic, vibrational and rotational states and whether that transition is worth searching for in the laboratory.

The AlF spectroscopy database brings these data into a single online system. The current molecular model covers 15 known electronic states and uses the most reliable available measurements for each state to calculate rovibrational structure and transitions.

From molecular structure to the transition needed in the laboratory

The resource goes beyond displaying reference data. Users can select electronic, vibrational and rotational states and calculate the corresponding energy levels and transition frequencies. The target accuracy is approximately 0.1 cm⁻¹, comparable to the bandwidth of a typical narrow-band pulsed spectroscopy laser, so the predictions can guide experimental line searches and define useful scan ranges.

The website also calculates Franck–Condon factors and vibrational transition wavelengths. These quantities are directly relevant to laser cooling, optical cycling and resonance-enhanced multiphoton ionisation because they help identify promising excitation pathways and transitions likely to produce useful signal.

Rotational structure is treated with effective Hamiltonians appropriate to each electronic state. For triplet states, the system can evaluate both Hund's case (a) and case (b) representations and their state weights, helping users judge which angular-momentum coupling description is more appropriate in a given regime.

The platform also calculates thermal rovibrational populations as a function of temperature, providing a direct reference for molecular-beam and source experiments.

Combining measured spectroscopy with electronic-structure calculations

Experimental spectroscopy does not provide every quantity needed to design a new measurement. The database therefore also incorporates electronic-structure results, including potential-energy curves and transition dipole moments.

These calculations add information that can be difficult to obtain directly from experiment: the broader structure of electronic states, dissociation behaviour, interactions between nearby states and theoretical estimates of transition strengths.

Franck–Condon factors and vibrationally corrected transition dipole moments are obtained from numerical vibrational wavefunctions. Measured spectroscopy and theoretical predictions can therefore be examined within the same molecular model and used together when selecting an experimental pathway.

Making established research easier to reuse

The database turns high-value spectroscopic information scattered through the literature into a resource that can be calculated with directly, updated as new measurements appear and used in experimental planning.

Researchers do not need to reconstruct the full set of parameters and calculations from multiple papers each time they investigate a new transition. New measurements can be incorporated into the same molecular model, allowing earlier experimental and theoretical work to remain useful in subsequent studies.

Extending from AlF to other diatomic molecules

AlF includes both singlet and triplet electronic structure and spans different angular-momentum coupling regimes, covering a substantial range of the situations encountered in diatomic spectroscopy.

The rovibrational calculation framework is formulated in a general form for diatomic molecules and is not limited to AlF. Electronic-state information and spectroscopic parameters are molecule-specific inputs, while calculations of energy levels, Franck–Condon factors and transition frequencies use a common computational framework. The implementation already supports singlet and triplet systems.

Extending the resource to another diatomic molecule therefore centres on adding reliable spectroscopy, selecting the appropriate state models and validating their range of accuracy, rather than rebuilding the complete calculation and web system.

AlF is the first complete implementation of this framework. For additional diatomic molecules, the same calculation and interface structure can be retained while replacing the molecule-specific data and model parameters.

This gives the project a path beyond a single molecular database: a scientific tool developed around one experimental system can be extended into a shared computational resource for a broader range of diatomic spectroscopy and precision-molecular experiments.