Autler-Townes (AT) spectroscopic studies – Probing coherence and quantum interference effects in the laser molecule interaction and their applications:
In collaboration with Prof. John Huennekens, Prof. Frank Spano, Dr. Teodora Kirova

• All-optical alignment and orientation of the molecular angular momentum
  • Preparing gas-phase molecules with a pre-selected orientation is one of the main goals in chemical reaction dynamics. The ability to pre-select well-defined initial reactant quantum states eliminates ensemble averaging which greatly complicates the interpretation of dynamics effects in chemical reactions. In addition, initial state selectivity enhances the viability of quantum control schemes which seek to direct the course of chemical reactions.
  • Similarly, all-optical control of molecular orientation and alignment on surfaces may turn out in the future to be a very attractive means for providing insights into mechanisms of chemical reactions on surfaces as well as helping advance the molecular-electronics approach to device fabrication. The current efforts in the former area include surface aligned photochemistry, where the crystalline surface structures are used to align and position chemical reagents prior to reaction.
  • Furthermore, alignment of a molecule along a laser propagation direction is crucial for Coulomb explosion type molecular imaging.

• 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 (⁠𝐼_{𝑓𝑙𝑢𝑜𝑟} ∝ 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

• Control of the valence electron spin polarization through modification of singlet triplet mixing coefficients of pairs of rovibronic levels mixed by the spin-orbit interaction: prospects for an all-optical spin switch.
  • Molecular transitions are governed by the parity of the various states. In addition, only states of the same spin multiplicity combine with each other. Therefore, a transition from one singlet state to another is allowed, while a singlet to triplet transition is forbidden. This selection rule breaks down as the spin-orbit interaction becomes larger, as is the case for heavier atoms and molecules, but is quite strict for lighter molecules. We propose a novel quantum control scheme based on Autler-Townes splitting to create controllable linear combinations of singlet and triplet states, i.e dressed singlet-triplet window states. A five level molecular system interacting with three lasers is used to maximize upper level triplet state production by starting with a singlet ground state.

• High resolution (perturbation-facilitated) optical-optical double resonance [(PF)OODR] spectroscopy of highly excited (triplet) singlet states in Rb₂ and Cs₂.
• Rb₂ and Cs₂ Global Deperturbation Analysis of the A¹Σ_u⁺– b³P_u States
• Spin-orbit Interaction, Dipole Moments and Polarizabilities of Alkali Dimers
• the effect of collisions on the molecular rotational angular momentum

Molecular beam apparatus:
In collaboration with Prof.  Svetlana Kotochigova, Prof. John Huennekens

• spectroscopy of lanthanides such as Dy₂ and Er₂.

Funding from the National Science Foundation and the Lagerqvist Research Fund of Temple University.