A. Borodin

Advanced techniques for manipulation of internal states, standard in atomic physics, are demonstrated for a charged molecular species for the first time. We address individual hyperfine states of ro-vibrational levels of a diatomic ion by optical excitation of individual hyperfine transitions, and achieve controlled transfer of population into a selected hyperfine state. We use molecular hydrogen ions (HD^{+}) as a model system and employ a novel frequency-comb-based, continuous-wave 5 \mum laser spectrometer. The achieved spectral resolution is the highest obtained so far in the optical domain on a molecular ion species. As a consequence, we are also able to perform the most precise test yet of the ab-initio theory of a molecule.

We demonstrate rotational excitation of molecular ions that are sympathetically cooled by laser-cooled atomic ions to a temperature as low as ca. 10 mK. The molecular hydrogen ions HD+ and the fundamental rotational transition $(v=0,\, N=0)\rightarrow(v'=0,\, N'=1)$ at 1.3 THz, the most fundamental dipole-allowed rotational transition of any molecule, are used as a test case. This transition is here observed for the first time directly. Rotational laser cooling was employed in order to increase the signal, and resonance-enhanced multiphoton dissociation was used as detection method. The black-body-radiation-induced rotational excitation is also observed. The extension of the method to other molecular species is briefly discussed.

In this work we demonstrate vibrational spectroscopy of polyatomic ions that are trapped and sympathetically cooled by laser-cooled atomic ions. We use the protonated dipeptide tryptophane-alanine (HTyrAla+) as a model system, cooled by Barium ions to less than 800mK secular temperature. The spectroscopy is performed on the fundamental vibrational transition of a local vibrational mode at 2.74 μm using a continuous-wave optical parametric oscillator (OPO). Resonant multi-photon IR dissociation spectroscopy (without the use of a UV laser) generates charged molecular fragments, which are sympathetically cooled and trapped, and subsequently released from the trap and counted. We measured the cross section for R-IRMPD under conditions of low intensity, and found it to be approximately two orders smaller than the vibrational excitation cross section. The observed rotational bandwidth of the vibrational transition is larger than the one expected from the combined effects of 300 K black-body temperature, conformer-dependent line shifts, and intermolecular vibrational relaxation broadening (J. Stearns et al., J. Chem. Phys., 127, 154322-7 (2007)). This indicates that as the internal energy of the molecule grows, an increase of the rotational temperature of the molecular ions well above room temperature (up to on the order of 1000K), and/or an appreciable shift of the vibrational transition frequency (approx. 6-8 cm$^{-1}$) occurs.