Excited-State Photo Physics of Cryogenically Cooled Molecular Ions

The absorption of light within the visible spectrum typically results in the excitation of chromophores from their electronic ground state to either the first or the second electronically excited states. The response to photo absorption is intricately linked to the potential-energy landscape of the excited state and the couplings between states. Weak coupling with the electromagnetic field can lead to fluorescence, while non-adiabatic interactions may induce faster internal conversion. These processes are energy and vibrational-level dependent, posing challenges for resolution at room temperature, where photo excitation initiates in a warm and less well-defined ground state. To delve into 'cold spectroscopy' and establish a benchmark for such interactions, our focus is on investigating the absorption properties and molecular response times of cold chromophores in the gas phase. This involves studying them in an entirely isolated form, shielded from external perturbations originating from charges, dipoles, and hydrogen bonding.



Our approach utilizes an ion storage ring and involves cooling chromophore ions in an ion trap held at 6K before injection and storage in the ring [1]. This, combined with action spectroscopy, unveils well-resolved vibrational structures related to the excited state [2]. Furthermore, employing femtosecond laser pump-probe schemes allows us to unravel the level and spectral specific photo response [3]. Intriguingly, spectra of cold molecular chromophore ions may exhibit both well-resolved vibrational structures in certain regions and regions devoid of sharp structures.



[1] H. B. Pedersen, H. Juul, F. K. Mikkelsen, A. P. Rasmussen, and L. H. Andersen, Phys. Rev. A 106 053111 (2022)



[2] Lars H. Andersen, Anne P. Rasmussen, Henrik B. Pedersen, Oleg B. Beletsan, and Anastasia V. Bochenkova, J. Phys. Chem. Lett. 14 6395-6401 (2023)



[3] Anne P. Rasmussen, Henrik B. Pedersen, and Lars H. Andersen. Phys. Chem. Chem. Phys. 25 32868-32874 (2023)