Molecular physics is the study of the physical properties ofmolecules andmolecular dynamics. The field overlaps significantly withphysical chemistry,chemical physics, andquantum chemistry. It is often considered as a sub-field ofatomic, molecular, and optical physics. Research groups studying molecular physics are typically designated as one of these other fields. Molecular physics addresses phenomena due to both molecular structure and individual atomic processes within molecules. Likeatomic physics, it relies on a combination ofclassical andquantum mechanics to describe interactions betweenelectromagnetic radiation and matter. Experiments in the field often rely heavily on techniques borrowed fromatomic physics, such asspectroscopy andscattering.
In a molecule, both theelectrons andnuclei experience similar-scale forces from theCoulomb interaction. However, the nuclei remain at nearly fixed locations in the molecule while the electrons move significantly. This picture of a molecule is based on the idea thatnucleons are much heavier than electrons, so will move much less in response to the same force.Neutron scattering experiments on molecules have been used to verify this description.[1]
When atoms join into molecules, their inner electrons remain bound to their original nucleus while the outervalence electrons are distributed around the molecule. The charge distribution of these valence electrons determines the electronic energy level of a molecule, and can be described bymolecular orbital theory, which closely follows the atomic orbital theory used for single atoms. Assuming that the momenta of the electrons are on the order ofħ/a (whereħ is thereduced Planck constant anda is the average internuclear distance within a molecule, ~ 1 Å), the magnitude of the energy spacing for electronic states can be estimated at a fewelectron volts. This is the case for most low-lying molecular energy states, and corresponds to transitions in the visible andultraviolet regions of theelectromagnetic spectrum.[1][2]
In addition to the electronic energy levels shared with atoms, molecules have additionalquantized energy levels corresponding to vibrational and rotational states. Vibrational energy levels refer to motion of the nuclei about their equilibrium positions in the molecule. The approximate energy spacing of these levels can be estimated by treating each nucleus as aquantum harmonic oscillator in thepotential produced by the molecule, and comparing its associated frequency to that of an electron experiencing the same potential. The result is an energy spacing about 100× smaller than that for electronic levels. In agreement with this estimate, vibrational spectra show transitions in the near infrared (about1–5 μm).[2] Finally, rotational energy states describe semi-rigid rotation of the entire molecule and produce transition wavelengths in the farinfrared and microwave regions (about 100-10,000μm inwavelength). These are the smallest energy spacings, and their size can be understood by comparing the energy of adiatomic molecule with internuclear spacing ~ 1 Å to the energy of a valence electron (estimated above as ~ ħ/a).[1]
Actual molecular spectra also show transitions which simultaneously couple electronic, vibrational, and rotational states. For example, transitions involving both rotational and vibrational states are often referred to as rotational-vibrational or rovibrational transitions.Vibronic transitions combine electronic and vibrational transitions, androvibronic transitions combine electronic, rotational, and vibrational transitions. Due to the very different frequencies associated with each type of transition, the wavelengths associated with these mixed transitions vary across the electromagnetic spectrum.[2]
In general, the goals of molecular physics experiments are to characterize shape and size, electric and magnetic properties, internal energy levels, andionization anddissociation energies for molecules. In terms of shape and size, rotational spectra and vibrational spectra allow for the determination of molecularmoments of inertia, which allows for calculations of internuclear distances in molecules.X-ray diffraction allows determination of internuclear spacing directly, especially for molecules containing heavy elements.[2] All branches of spectroscopy contribute to determination of molecular energy levels due to the wide range of applicable energies (ultraviolet to microwave regimes).
Within atomic, molecular, and optical physics, there are numerous studies using molecules to verify fundamental constants and probe forphysics beyond the Standard Model. Certain molecular structures are predicted to be sensitive to new physics phenomena, such asparity[3] andtime-reversal[4] violation. Molecules are also considered a potential future platform fortrapped ion quantum computing, as their more complex energy level structure could facilitate higher efficiency encoding of quantum information than individual atoms.[5] From a chemical physics perspective,intramolecular vibrational energy redistribution experiments use vibrational spectra to determine how energy is redistributed between different quantum states of a vibrationally excited molecule.[6]
| International | |
|---|---|
| National | |
| Other | |
