Action spectroscopy at T ≈ 30 K, as a proxy for the visible absorption band, and the branching between electron detachment and dissociation in the cyan fluorescent protein chromophore anion are reported. The cryogenic action spectra, which show the presence of several rotamers, serve as a reference point for interpreting the effect of nano-environmental interactions in complex protein environments. The adiabatic detachment energy for the lowest energy geometric isomer (Z1) is 19 531 ± 40 cm−1, with the vertical S1 ← S0 transition energy at 23 734 ± 40 cm−1. For Z1, the propensity for internal conversion followed by dissociation is low (<10%) compared with autodetachment as the S1 ← S0 absorption band is entirely situated in the detachment continuum and is classified as a shape resonance.

Indene (C9H8) is the only polycyclic pure hydrocarbon identified in the interstellar medium to date, with an observed abundance orders of magnitude higher than predicted by astrochemical models. The dissociation and radiative stabilization of vibrationally hot indene cations are investigated by measuring the time-dependent neutral particle emission rate from ions in a cryogenic ion-beam storage ring for up to 100 ms. Time-resolved measurements of the kinetic energy released upon hydrogen atom loss from C 9 H 8 + , analyzed in view of a model of tunneling through a potential energy barrier, provide the dissociation rate coefficient. Master equation simulations of the dissociation in competition with vibrational and electronic radiative cooling reproduce the measured dissociation rate. We find that radiative stabilization arrests one of the main C9H8 destruction channels included in astrochemical models, helping to rationalize its high observed abundance.

Infrared (IR) cooling of polycyclic aromatic hydrocarbon (PAH) molecules is a major radiative stabilization mechanism of PAHs present in space and is the origin of the aromatic infrared bands (AIBs). Here, we report an anharmonic cascade model in a master equation framework to model IR emission rates and emission spectra of energized PAHs as a function of internal energy. The underlying (simple harmonic cascade) framework for fundamental vibrations has been developed through the modeling of cooling rates of PAH cations and other carboneaous ions measured in electrostatic ion storage ring experiments performed under “molecular cloud in a box” conditions. The anharmonic extension is necessitated because cyano-PAHs, recently identified in Taurus Molecular Cloud-1 (TMC-1), exhibit strong anharmonic couplings, which make substantial contributions to the IR emission dynamics. We report an experimental mid-IR (650-3200 cm-1) absorption spectrum of 2-cyanoindene (2CNI), which is the smallest cyano-PAH that has been identified in TMC-1 and model its IR cooling rates and emission properties. The mid-IR absorption spectrum is reasonably described by anharmonic calculations at the B3LYP/N07D level of theory that include resonance polyad matrices, although the CN-stretch mode frequency continues to be difficult to describe. The anharmonic cascade framework can be readily applied to other neutral or charged PAHs and is also readily extended to include competing processes, such as recurrent fluorescence and isomerization.

An electrospray ion source has been coupled to a cryogenic electrostatic ion-beam storage ring to enable experimental studies of the fundamental properties of biomolecular ions and their reactions in the gas phase on longer timescales than with previous instruments. Using this equipment, we have measured the vibrational radiative cooling rate of the deprotonated anion of the chromophore of the cyan fluorescent protein, a color-shifted mutant of the iconic green fluorescent protein. Time-resolved dissociation rates of collisionally activated ions are first measured to benchmark a model of the dissociation rate coefficient. Storage time-dependent laser-induced dissociation rates are then measured to probe the evolution of the internal energy distribution of the stored ion ensemble. We find that significant heating of the electrosprayed ions occurs upon their extraction from the ion source, and that the radiative cooling rate is consistent with the prediction of a simple harmonic cascade model of vibrational relaxation.

Several small polycyclic aromatic hydrocarbons (PAHs) with closed-shell electronic structure have been identified in the cold, dark environment Taurus Molecular Cloud 1. We measure efficient radiative cooling through the combination of recurrent fluorescence (RF) and IR emission in the closed-shell indenyl cation (C9H7+), finding good agreement with a master equation model including molecular dynamics trajectories to describe internal-energy-dependent properties for RF. We find that C9H7+ formed with up to Ec=5.85 eV vibrational energy, which is ≈2 eV above the dissociation threshold, radiatively cool rather than dissociate. The efficient radiative stabilization dynamics are likely common to other closed-shell PAHs present in space, contributing to their abundance.

Abstract: In this roadmap article, we consider the main challenges and recent breakthroughs in understanding the role of carbon molecular nanostructures in space and propose future avenues of research. The focus lies on small carbon-containing molecules up to fullerenes, extending to even larger, more complex organic species. The roadmap contains forty contributions from scientists with leading expertize in observational astronomy, laboratory astrophysics/chemistry, astrobiology, theoretical chemistry, synthetic chemistry, molecular reaction dynamics, material science, spectroscopy, graph theory, and data science. The concerted interdisciplinary combination of the state-of-the-art of these astronomical, laboratory, and theoretical studies opens up new ways to advance the fundamental understanding of the physics and chemistry of cosmic carbon molecular nanostructures and touches on their wider relevance and impact in nanotechnology and catalysis.

2-Cyanoindene is one of the few specific aromatic or polycyclic aromatic hydrocarbon (PAH) molecules positively identified in Taurus molecular cloud-1 (TMC-1), a cold, dense molecular cloud that is considered the nearest star-forming region to Earth. We report cryogenic mid-infrared (550-3200 cm-1) and visible (16,500-20,000 cm-1, over the D2 ← D0 electronic transition) spectra of 2-cyanoindene radical cations (2CNI+), measured using messenger tagging (He and Ne) photodissociation spectroscopy. The infrared spectra reveal the prominence of anharmonic couplings, particularly over the fingerprint region. There is a strong CN-stretching mode at 2177 ± 1 cm-1 (4.593 μm), which may contribute to a broad plateau of CN-stretching modes across astronomical aromatic infrared band spectra. However, the activity of this mode is suppressed in the dehydrogenated (closed shell) cation, [2CNI-H]+. The IR spectral frequencies are modeled by anharmonic calculations at the B3LYP/N07D level of theory that include resonance polyad matrices, demonstrating that the CN-stretch mode remains challenging to describe with theory. The D2 ← D0 electronic transition of 2CNI+, which is origin dominated, occurs at 16,549 ± 5 cm-1 in vacuum (6041.8 Å in air). There are no correspondences with reported diffuse interstellar bands.

We measured the spontaneous and photoinduced decays of anionic gold clusters, , with sizes ranging from 𝑁=2 to 13 and 15. After production in a sputter ion source, the size-selected clusters were stored in the cryogenic electrostatic ion-beam storage ring DESIREE, and their neutralization decays were measured for storage times between 0.1 and 100 s. The dimer was observed to decay by electron emission in parallel to neutral atom emission at long times, implying a breakdown of the Born-Oppenheimer approximation, analogous to the behavior of copper and silver dimers. Radiative cooling is observed for all other cluster sizes. The decays of clusters 𝑁=3,6,8–13,15 show only a single radiative cooling time. For 𝑁=6–13 the cooling times have a strong odd-even oscillation with an amplitude that decrease with cluster size and with the even 𝑁 having the faster cooling. We compare our results with previous measurements of radiative cooling rates of the corresponding cationic gold clusters, , which also show an odd-even effect with a similar oscillation amplitude but at orders of magnitude shorter timescales and out of phase with the anions. The tetramer and pentamer both show two cooling times, which we tentatively ascribe to different structural forms at different ranges of high angular momenta of the ions in the and beams. For , the shape of the decay curve suggests that the cluster cools by emission of low-energy photons. The calculated limit on photon energies strongly suggests that cooling is by vibrational transitions in this case. For , time-resolved studies of photoinduced decays were performed to track the evolution of the internal energy distribution. We conclude that the radiative cooling is dominated by sequences of vibrational transitions in the IR. The laser-enhanced neutralization rate of was exponential, in contrast to its spontaneous decay rate, indicating that the cluster had already been cooled to a very narrow internal energy distribution at 120 ms as the total (integrated) laser-enhanced intensity was independent of the laser firing time at later times. The unimolecular rate constants decreased from 500 s⁻¹ when laser excited at 0.12 s to 40 s⁻¹ when laser excited at 0.62 s.

The radiative cooling of naphthalene dimer cations, (C₁₀H₈)₂⁺ was studied experimentally through action spectroscopy using two different electrostatic ion-beam storage rings, DESIREE in Stockholm and Mini-Ring in Lyon. The spectral characteristics of the charge resonance (CR) band were observed to vary significantly with a storage time of up to 30 seconds in DESIREE. In particular, the position of the CR band shifts to the blue, with specific times (inverse of rates) of 0.64 s and 8.0 s in the 0–5 s and 5–30 s storage time ranges, respectively. These long-time scales indicate that the internal energy distribution of the stored ions evolves by vibrational radiative cooling, which is consistent with the absence of fast radiative cooling via recurrent fluorescence for (C₁₀H₈)₂⁺. Density functional based tight binding calculations with local excitations and configuration interactions (DFTB-EXCI) were used to simulate the absorption spectrum for ion temperatures between 10 and 500 K. The evolution of the bandwidth and position with temperature is in qualitative agreement with the experimental findings. Furthermore, these calculations yielded linear temperature dependencies for both the shift and the broadening. Combining the relationship between the CR band position and the ion temperature with the results of the statistical model, we demonstrate that the observed blue shift can be used to determine the radiative cooling rate of (C₁₀H₈)₂⁺.