Measurement of the absolute magnitude of molecular electronic transition dipole moments and their dependence on the internuclear distance.
Accurate knowledge of transition dipole moment matrix elements is crucial, since important parameters associated with the interaction of light with matter, such as emission and absorption line intensities, lifetimes, and Einstein coefficients, depend on these matrix elements. We have measured absolute transition dipole matrix elements for ro-vibrational transitions of various electronic transitions in Li2 and Na2 using Autler-Townes and optical-optical double resonance spectroscopy, and we compare the results to ab initio theoretical values.
Traditionally, transition dipole matrix elements have been determined experimentally using spectral line intensities or lifetimes. However, usually only relative transition dipole moments can be determined with these methods. For example, fluorescence intensity measurements do not give absolute dipole matrix elements because absolute intensities are difficult to determine, the emission is generally not isotropic, and the wavelength and polarization dependence of the detection system must be taken into account. However, since the intensities of emission (fluorescence) lines are proportional to the square of the transition dipole matrix element and the 4th power of the transition frequency (𝐼𝑓𝑙𝑢𝑜𝑟 ∝ 𝜈4|𝜇|2), ratios of line intensities divided by 𝜈4 yield ratios of dipole matrix elements squared.
Autler-Townes (AT) spectroscopy is an alternative method that provides absolute transition dipole matrix elements (from AT splittings of spectral lines) for sufficiently strong transitions. Such measurements can be used to put relative dipole matrix elements, obtained, for example, from fluorescence measurements, on an absolute scale