Reactive molecular ions play a central role in the chemistry of the interstellar medium and in planetary atmospheres. Spectroscopic studies of these often elusive ions yield fundamental insights on their geometrical and electronic structure, and provide vibrational and rotational signatures needed for their identification in space. Cryogenic ion traps have proven to be ideal tools for the development of sensitive spectroscopic schemes of mass-selected, cold, and isolated molecular ions. Recent progress on these so-called action spectroscopic methods allows not only to probe electronic and vibrational excitation processes, but also to record high-resolution purely rotational molecular spectra S. Brünken, L. Kluge, A. Stoffels, O. Asvany, and S. Schlemmer, Astrophys. J. Lett., 783, L4 (2014); A. Stoffels, L. Kluge, S. Schlemmer, and S. Brünken, A&A 593, A56 (2016); S. Brünken, L. Kluge, A. Stoffels, J. Pèrez-Rios, and S. Schlemmer, J. Mol. Spectrosc., 332, 67 (2017) which are a direct prerequisite for radio-astronomical detections of new species in space as will be demonstrated with selected examples. In addition, details of broadband infrared experiments on several astrophysically important hydrocarbon cations ranging in size from comparatively small systems (e.g., C₂H⁺, C₃H₂⁺, and C₃H⁺) to PAH cations will be given, using the unique combination of a cryogenic ion trap instrument O. Asvany, S. Bünken, L. Kluge, and S. Schlemmer, Appl. Phys. B, 114, 203 (2014)nterfaced to the free electron lasers at the FELIX Laboratory. S. Brünken, L. Kluge, A. Stoffels, O. Asvany, and S. Schlemmer, Astrophys. J. Lett., 783, L4 (2014); A. Stoffels, L. Kluge, S. Schlemmer, and S. Brünken, A&A 593, A56 (2016); S. Brünken, L. Kluge, A. Stoffels, J. Pèrez-Rios, and S. Schlemmer, J. Mol. Spectrosc., 332, 67 (2017), O. Asvany, S. Bünken, L. Kluge, and S. Schlemmer, Appl. Phys. B, 114, 203 (2014)i

The methods of ultrafast UV-visible and mid-IR transient absorption spectroscopy are powerful probes of dynamical processes occurring in solution [1]. They can be used to determine the rates and mechanisms of chemical and photochemical reactions, identify short-lived reactive intermediates, and examine how the dynamics are modified by solute-solvent interactions. The mechanistic insights which derive from the application of transient absorption spectroscopy will be illustrated by recent studies of solvent effects on electronically non-adiabatic pathways in photoexcited molecules, and direct observation of the intermediates involved in radical reactions controlled by the use of organic photoredox catalysts [2]. [1] Taking the plunge: chemical reaction dynamics in liquids, A.J. Orr-Ewing, Chem. Soc. Rev. 2017, 46, 7597-7614. DOI: 10.1039/C7CS00331E. [2] Ultrafast observation of a photoredox reaction mechanism: photo-initiation in organocatalyzed atom-transfer radial polymerization, D. Koyama, H.J.A. Dale and A.J. Orr-Ewing, J. Am. Chem. Soc. 2018, 140, 1285-1293. DOI: 10.1021/jacs.7b07829.

C–O bond breaking is an important process in the activation of CO₂ that can be catalyzed by the presence of a metal. In this talk, we investigate the factors that lead to bond insertion in [M(CO₂)_y]⁻ gas phase clusters, specifically addressing differences amongst the metals M = Ni, Fe, and Ti. Gas phase anionic clusters were generated using laser ablation of a metal target in the presence of a CO₂ expansion, and the infrared photodissociation spectra were measured from 950–2400 cm⁻¹. Metal carbonyl vibrational signatures were used to infer bond insertion, and computational chemistry simulations were used to assess the feasibility of bond breaking in these systems.

Attosecond time-resolved spectroscopy is beginning to provide experimental access to the most fundamental time scales of molecules, on which the electronic dynamics take place. A few recent experiments that access purely electronic dynamics, as well as coupled electronic and nuclear dynamics in molecules will be discussed. The theoretical developments that accompanied the experimental work will also be presented. The ionization of most molecules on the sub-femtosecond time scale prepares the molecular cation in a superposition of several electronic states that supports charge migration. Detailed measurements of the phase and amplitude of high-harmonic emission from spatially oriented iodoacetylene molecules have enabled the reconstruction of sub-femtosecond charge migration in the iodoacetylene cation (see figure)P. M. Kraus et al., Science 350, 790 (2015) r0pt Figure The ionization of molecules by attosecond pulses and a synchronized infrared field was used to measure photoionization time delays between the two highest-lying occupied valence orbitals of H₂O and N₂O. These measurements revealed delays of up to  ∼ 160 as in the case of N₂O, which are characteristic of the transient trapping of the photoelectron by shape resonancesM. Huppert et al., Phys. Rev. Lett. 117, 093001 (2016) Finally, the extension of attosecond spectroscopy to the soft-X-ray domain (water window) will be discussed. The broad spectral bandwidth available in this domain has been exploited to synthesize one of the shortest attosecond pulses to date (43 as)T. Gaumnitz et al., Opt. Exp. 25, 27506 (2017) Transient absorption spectroscopy at the carbon K-edge has been used to study the photodissociation dynamics of CF₄⁺, revealing the rearrangement of the electronic structure during this ultrafast ( ∼ 40 fs) processY. Pertot et al., Science 355, 264 (2017) An outlook on attosecond spectroscopy of both isolated and solvated molecules will be given.

---

Footnotes:

P. M. Kraus et al., Science 350, 790 (2015). M. Huppert et al., Phys. Rev. Lett. 117, 093001 (2016). T. Gaumnitz et al., Opt. Exp. 25, 27506 (2017). Y. Pertot et al., Science 355, 264 (2017).