Members:
- Livio Gianfrani
- Antonio Castrillo
- Eugenio Fasci
- Stefania Gravina
- Luigi Moretti
Research Profile:
The AMP group at the Department of Mathematics and Physics of University of Campania “Luigi Vanvitelli” is active in the field of light-matter interaction, quantitative laser spectroscopy and high-precision studies on atomic and molecular systems. It boasts a long-standing and well-recognized expertise in precision spectroscopic measurements, cavity enhanced techniques, nonlinear optics and spectroscopy, trace gas detection.
Nowadays, precisely controlled lasers enable deep communication with the quantum world of atoms and molecules. By using probe lasers with the highest coherence properties, often combined with the optical frequency comb technology, the AMP group is highly motivated to perform fundamental tests and measurements, as well as to provide high-quality spectroscopic data for atoms and molecules of particular interest to astrophysics, plasma physics or terrestrial and planetary atmospheric sciences.
The main research activities are described hereafter.
- Doppler-broadening gas thermometry
The well-known expression of the Doppler width of a spectral line, valid for a gas sample at thermodynamic equilibrium, represents a powerful tool to link the thermodynamic temperature to an optical frequency. This is the basis of a relatively new method of primary gas thermometry, known as Doppler broadening thermometry. The AMP team is widely recognized as the world-leading group in this field. Past work on the spectroscopic determination of the Boltzmann constant is particularly well known. More recently, the team has contributed to the success of the European network InK, Implementing the new Kelvin. In that context, Doppler broadening thermometry (DBT) made a significant progress for the aims of low uncertainty thermodynamic temperature determinations, by use of a comb-assisted dual-laser absorption spectrometer to probe an acetylene line-doublet in the near-infrared with high resolution and high spectral fidelity. Current research efforts are devoted to a new DBT implementation that is based upon the precise observation of the shape of the mercury intercombination transition in the deep-ultraviolet region, with the ambitious goal of approaching an experimental accuracy at the part-per-million level. To this aim, a comb-locked frequency chain has been developed, adopting the most advanced schemes of nonlinear frequency mixing, thus producing an absolute frequency axis nearby the wavelength of 253 nm.
- QED tests from molecular hydrogen infrared spectra
Molecular hydrogen is a benchmark system to test quantum electrodynamics. The ability to perform ab-initio calculations of energy levels in hydrogen isotopologues (H2, HD and D2) is constantly growing, to the point where all known quantum electrodynamic (QED) corrections (relativistic, vacuum polarization and self-energy) can be quantified. Therefore, comparison of these theoretical results with highly accurate experimental values resulting from precision spectroscopic investigations provide an effective tool to test the QED theory, potentially looking for new physics beyond the Standard Model. Even when a perfect agreement is encountered, the experimental results can be interpreted to put constraints on the strength of a putative fifth force in nature. The AMP group is active in this field by measuring the near-infrared spectrum of HD. Deuterated molecular hydrogen is the simplest heteronuclear (and neutral) diatomic molecule, which has gained a growing interest in recent years for a variety of reasons. In particular, HD has been the subject of many astrophysical studies. Detection of HD lines from the interstellar medium in galaxies provides useful constraints on physical conditions, due to the fact that HD is fragile and easily photodissociated. By using the technique of frequency-stabilized comb-calibrated cavity ring-down spectroscopy, the AMP team has detected the weak R(1) transition of the HD first overtone band at the wavelength of 1.4 micron, thus determining the unperturbed line-center frequency with an absolute global uncertainty of 76 kHz. Current activities are aimed to improve the measurement uncertainty, as well as to extend the investigation to other wavelengths of interest.
- Experimental tests of quantum chemistry calculations for simple molecules
Atmospheric monitoring applications often require high quality spectroscopic data for a variety of simple molecules. This is particularly true for greenhouse gases, such as carbon dioxide (CO2) and nitrous oxide (N2O). As widely experienced in many laboratories around the world, measuring molecular parameters (especially line intensity factors) with a low uncertainty (less than 1%) is not an easy task. Accurate experimental studies, however, are usually limited to a small number of vibration-rotation lines. On the theory side, there have been a number of attempts in the last three decades, but only recently an accurate theoretical solution to the problem of line intensity determinations has been provided, using quantum chemistry calculations. Error analysis of the calculated transition strengths, based on purely theoretical considerations, indicates that the majority of strong bands have uncertainties smaller than 1%. Hence, extensive line lists have been produced for a large number of molecules of special interest for spectroscopic studies of Earth and planetary atmospheres, exoplanets and other astronomical bodies. Theoretical datasets may include, besides the transition strengths, the partition functions, spontaneous emission Einstein coefficients, temperature-dependent cross sections, pressure-broadening parameters. Experimental validations of these calculations are of the utmost importance to quantify the achievable level of accuracy. The AMP group is active in this field since long time, also making significant contributions in the development of innovative spectroscopic methods and spectral analysis strategies.
- Ultra-sensitive cavity ring-down spectroscopy
Cavity ring-down spectroscopy (CRDS) is a highly sensitive laser absorption technique that has found wide use in molecular spectroscopy since its invention in 1988, with a number of applications in many other research fields. CRDS relies on the measurement of the decay rate (the so called “ring-down time”) of the coherent radiation trapped in a high finesse optical cavity. This latter typically consists of two highly reflective mirrors (R>99.99%). This approach allows for a prolonged interaction between the radiation and the intracavity gas. CRDS retrieve the intracavity absorption (namely, the absorption coefficient of the gas target) from the variation of the ring-down times when the cavity is empty and is filled with an absorbing atomic/molecular sample. Thanks to the very long achievable pathlength (up to several tens of km), the minimum detectable absorption coefficient can be as low as 10-13 cm-1, thus making CRDS particularly suitable for studies of weak spectral lines (such as those that are forbidden under the electric dipole approximation), rare isotopologues detection, or trace gas measurements. The AMP group has contributed significantly to the progress in this research field, also developing other cavity-based techniques, such as CEAS (Cavity Enhanced Absorption Spectroscopy), NICE-OHMS (Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy), OF-CEAS (Optical Feedback Cavity Enhanced Absorption Spectroscopy). Recently, the AMP group has specialized in precision spectroscopic measurements in weakly absorbing gases, combining CRDS with the technology of optical frequency combs. Current activities in this field include Lamb-dip CRDS of acetylene, trace-water determinations in ultra-pure gases and detection of selected hydrocarbons in ultra-high vacuum environments.
- Precision spectroscopy of muonic hydrogen
The AMP group contributes to the FAMU (Fisica degli Atomi MUonici) collaboration, within the framework of a National project funded by INFN, the National Institute for Nuclear Physics. The FAMU team is setting up an experiment for the first hyperfine splitting measurement of the muonic hydrogen (μp) ground state at the muon beam facility of the Rutherford-Appleton Laboratory (RAL), UK. It will lead to a determination of the proton Zemach radius, which in turn can be used to infer the proton size. FAMU intends to measure the proton Zemach radius with higher precision (<1%) than previously possible, disentangling discordant theoretical values and quantifying any level of discrepancy that may exist between values as extracted from normal and muonic hydrogen atoms. In this framework, the AMP group is in charge of a twofold task: development, optimization and use of a multipass optical cavity (MOC) to enhance the strength of the transition probability; development of a strategy for the frequency calibration of the FAMU experiment at RAL, characterized by an effective traceability to the primary frequency standard. The MOC is designed to guarantee: (i) high number of reflections and small losses; (ii) homogeneous illumination of most of the cavity volume; (iii) small illuminated surface; (iv) open sides to allow the muon beam to flow inside the MOC and prevent the X-ray detection signal from being blocked. The first measurements’ run is expected by the end of 2023.
