Near-infrared absorption and radiative cooling of naphthalene dimers (C10H8)2.

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+.

Mutual Neutralization of NO

We have studied the mutual neutralization reaction of vibronically cold NO^{+} with O^{-} at a collision energy of ≈0.1 eV and under single-collision conditions. The reaction is completely dominated by production of three ground-state atomic fragments. We employ product-momentum analysis in the framework of a simple model, which assumes the anion acts only as an electron donor and the product neutral molecule acts as a free rotor, to conclude that the process occurs in a two-step mechanism via an intermediate Rydberg state of NO which subsequently fragments.

The mutual neutralization of hydronium and hydroxide.

Mutual neutralization of hydronium (H3O+) and hydroxide (OH-) ions is a very fundamental chemical reaction. Yet, there is only limited experimental evidence about the underlying reaction mechanisms. Here, we report three-dimensional imaging of coincident neutral products of mutual-neutralization reactions at low collision energies of cold and isolated ions in the cryogenic double electrostatic ion-beam storage ring (DESIREE). We identified predominant H2O + OH + H and 2OH + H2 product channels and attributed them to an electron-transfer mechanism, whereas a minor contribution of H2O + H2O with high internal excitation was attributed to proton transfer. The reported mechanism-resolved internal product excitation, as well as collision-energy and initial ion-temperature dependence, provide a benchmark for modeling charge-transfer mechanisms.

Experimental radiative cooling rates of a polycyclic aromatic hydrocarbon cation.

Several small Polycyclic Aromatic Hydrocarbons (PAHs) have been identified recently in the Taurus Molecular Cloud (TMC-1) using radio telescope observations. Reproducing the observed abundances of these molecules has been a challenge for astrochemical models. Rapid radiative cooling of PAHs by Recurrent Fluorescence (RF), the emission of optical photons from thermally populated electronically excited states, has been shown to efficiently stabilize small PAHs following ionization, augmenting their resilience in astronomical environments and helping to rationalize their observed high abundances. Here, we use a novel method to experimentally determine the radiative cooling rate of the cation of 1-cyanonaphthalene (C10H7CN, 1-CNN), the neutral species of which has been identified in TMC-1. Laser-induced dissociation rates and kinetic energy release distributions of 1-CNN cations isolated in a cryogenic electrostatic ion-beam storage ring are analysed to track the time evolution of the vibrational energy distribution of the initially hot ion ensemble as it cools. The measured cooling rate is in good agreement with the previously calculated RF rate coefficient. Improved measurements and models of the RF mechanism are needed to interpret astronomical observations and refine predictions of the stabilities of interstellar PAHs.

Cooling dynamics of energized naphthalene and azulene radical cations.

Naphthalene and azulene are isomeric polycyclic aromatic hydrocarbons (PAHs) and are topical in the context of astrochemistry due to the recent discovery of substituted naphthalenes in the Taurus Molecular Cloud-1 (TMC-1). Here, the thermal- and photo-induced isomerization, dissociation, and radiative cooling dynamics of energized (vibrationally hot) naphthalene (Np+) and azulene (Az+) radical cations, occurring over the microsecond to seconds timescale, are investigated using a cryogenic electrostatic ion storage ring, affording "molecular cloud in a box" conditions. Measurement of the cooling dynamics and kinetic energy release distributions for neutrals formed through dissociation, until several seconds after hot ion formation, are consistent with the establishment of a rapid (sub-microsecond) Np+ ⇌ Az+ quasi-equilibrium. Consequently, dissociation by C2H2-elimination proceeds predominantly through common Az+ decomposition pathways. Simulation of the isomerization, dissociation, recurrent fluorescence, and infrared cooling dynamics using a coupled master equation combined with high-level potential energy surface calculations [CCSD(T)/cc-pVTZ], reproduce the trends in the measurements. The data show that radiative cooling via recurrent fluorescence, predominately through the Np+ D0 ← D2 transition, efficiently quenches dissociation for vibrational energies up to ≈1 eV above dissociation thresholds. Our measurements support the suggestion that small cations, such as naphthalene, may be more abundant in space than previously thought. The strategy presented in this work could be extended to fingerprint the cooling dynamics of other PAH ions for which isomerization is predicted to precede dissociation.

Efficient radiative cooling of tetracene cations C18H12+: absolute recurrent fluorescence rates as a function of internal energy.

We have measured recurrent fluorescence (RF) cooling rates of internally hot tetracene cations, C18H12+, as functions of their storage times and internal energies in two different electrostatic ion-beam storage rings - the cryogenic ring DESIREE with a circumference of 8.6 meters in Stockholm and the much smaller room temperature ring Mini-Ring in Lyon, which has a circumference of 0.71 meters. The RF rates were measured to be as high as 150 to 1000 s-1 for internal energies in the 7 to 9.4 eV energy range, where we have probed the time evolution of the internal energy distribution with nanosecond laser pulses with a 1 kHz repetition rate. These RF rates are found to be significantly higher than those of previously investigated smaller PAHs such as e.g. anthracene and naphthalene, for which the lowest non-forbidden electronic excited state, the D2 state, is populated with a smaller probability by inverse internal conversion. Furthermore, the D2-D0 transition rate is smaller for these smaller molecules than for tetracene. The complementary features of the two storage rings allow for RF rate measurements in a broader internal energy range than has been possible before. The smaller sampling period of about 6 µs in Mini-Ring is ideal to study the cooling dynamics of the hotter ions that decay fast, whereas DESIREE with a sampling period of about 60 µs is better suited to study the colder ions that decay on longer timescales ranging up to hundreds of milliseconds. The excellent agreement between the two series of measurements in the region where they overlap demonstrates the complementarity of the two electrostatic ion-beam storage rings.

Erratum: State-Resolved Mutual Neutralization of Mg

This corrects the article DOI: 10.1103/PhysRevLett.128.033401.

Efficient stabilization of cyanonaphthalene by fast radiative cooling and implications for the resilience of small PAHs in interstellar clouds.

After decades of searching, astronomers have recently identified specific Polycyclic Aromatic Hydrocarbons (PAHs) in space. Remarkably, the observed abundance of cyanonaphthalene (CNN, C10H7CN) in the Taurus Molecular Cloud (TMC-1) is six orders of magnitude higher than expected from astrophysical modeling. Here, we report unimolecular dissociation and radiative cooling rate coefficients of the 1-CNN isomer in its cationic form. These results are based on measurements of the time-dependent neutral product emission rate and kinetic energy release distributions produced from an ensemble of internally excited 1-CNN+ studied in an environment similar to that in interstellar clouds. We find that Recurrent Fluorescence - radiative relaxation via thermally populated electronic excited states - efficiently stabilizes 1-CNN+, owing to a large enhancement of the electronic transition probability by vibronic coupling. Our results help explain the anomalous abundance of CNN in TMC-1 and challenge the widely accepted picture of rapid destruction of small PAHs in space.

Radiative cooling rates of substituted PAH ions.

The unimolecular dissociation and infrared radiative cooling rates of cationic 1-hydroxypyrene (OHPyr+, C16H10O+) and 1-bromopyrene (BrPyr+, C16H9Br+) are measured using a cryogenic electrostatic ion beam storage ring. A novel numerical approach is developed to analyze the time dependence of the dissociation rate and to determine the absolute scaling of the radiative cooling rate coefficient. The model results show that radiative cooling competes with dissociation below the critical total vibrational energies Ec = 5.39(1) eV for OHPyr+ and 5.90(1) eV for BrPyr+. These critical energies and implications for radiative cooling dynamics are important for astrochemical models concerned with energy dissipation and molecular lifecycles. The methods presented extend the utility of storage ring experiments on astrophysically relevant ions.

Spontaneous and photo-induced decay processes of WF5 - and HfF5 - molecular anions in a cryogenic storage ring.

Spontaneous and photo-induced decay processes of HfF5 - and WF5 - molecular anions were investigated in the Double ElectroStatic Ion Ring ExpEriment (DESIREE). The observation of these reactions over long time scales (several tens of ms) was possible due to the cryogenic temperatures (13 K) and the extremely low residual gas pressure (∼10-14 mbar) of DESIREE. For photo-induced reactions, laser wavelengths in the range 240 to 450 nm were employed. Both anion species were found to undergo spontaneous decay via electron detachment or fragmentation. After some ms, radiative cooling processes were observed to lower the probability for further decay through these processes. Photo-induced reactions indicate the existence of an energy threshold for WF5 - anions at about 3.5 eV, above which the neutralization yield increases strongly. By contrast, HfF5 - ions exhibit essentially no enhanced production of neutrals upon photon interaction, even for the highest photon energy used in this experiment (∼5.2 eV). This suppression will be highly beneficial for the efficient detection, in accelerator mass spectrometry, of the extremely rare isotope 182Hf using the 182HfF5 - anion while effectively reducing the interfering stable isobar 182W in the analyte ion 182WF5 -. The radionuclide 182Hf is of great relevance in astrophysical environments as it constitutes a potential candidate to study the events of nucleosynthesis that may have taken place in the vicinity of the solar system several million years ago.

State-Resolved Mutual Neutralization of Mg

We present experimental final-state distributions for Mg atoms formed in Mg^{+}+D^{-} mutual neutralization reactions at center-of-mass collision energies of 59±12 meV by using the merged-beams method. Comparisons with available full-quantum results reveal large discrepancies and a previously underestimated total rate coefficient by up to a factor of 2 in the 0-1 eV (<10^{4} K) regime. Asymptotic model calculations are shown to describe the process much better and we recommend applying this method to more complex iron group systems; data that is of urgent need in stellar spectral modeling.

Survival of polycyclic aromatic hydrocarbon knockout fragments in the interstellar medium.

Laboratory studies play a crucial role in understanding the chemical nature of the interstellar medium (ISM), but the disconnect between experimental timescales and the timescales of reactions in space can make a direct comparison between observations, laboratory, and model results difficult. Here we study the survival of reactive fragments of the polycyclic aromatic hydrocarbon (PAH) coronene, where individual C atoms have been knocked out of the molecules in hard collisions with He atoms at stellar wind and supernova shockwave velocities. Ionic fragments are stored in the DESIREE cryogenic ion-beam storage ring where we investigate their decay for up to one second. After 10 ms the initially hot stored ions have cooled enough so that spontaneous dissociation no longer takes place at a measurable rate; a majority of the fragments remain intact and will continue to do so indefinitely in isolation. Our findings show that defective PAHs formed in energetic collisions with heavy particles may survive at thermal equilibrium in the interstellar medium indefinitely, and could play an important role in the chemistry in there, due to their increased reactivity compared to intact or photo-fragmented PAHs.

Mutual neutralisation of O+ with O-: investigation of the role of metastable ions in a combined experimental and theoretical study.

The mutual neutralisation of O+ with O- has been studied in a double ion-beam storage ring with combined merged-beams, imaging and timing techniques. Branching ratios were measured at the collision energies of 55, 75 and 170 (± 15) meV, and found to be in good agreement with previous single-pass merged-beams experimental results at 7 meV collision energy. Several previously unidentified spectral features were found to correspond to mutual neutralisation channels of the first metastable state of the cation (O+(2Do), τ ≈ 3.6 hours), while no contributions from the second metastable state (O+(2Po), τ ≈ 5 seconds) were observed. Theoretical calculations were performed using the multi-channel Landau-Zener model combined with the anion centered asymptotic method, and gave good agreement with several experimentally observed channels, but could not describe well observed contributions from the O+(2Do) metastable state as well as channels involving the O(3s 5So) state.

Unimolecular fragmentation and radiative cooling of isolated PAH ions: A quantitative study.

Time-resolved spontaneous and laser-induced unimolecular fragmentation of perylene cations (C20H12 +) has been measured on timescales up to 2 s in a cryogenic electrostatic ion beam storage ring. We elaborate a quantitative model, which includes fragmentation in competition with radiative cooling via both vibrational and electronic (recurrent fluorescence) de-excitation. Excellent agreement with experimental results is found when sequential fragmentation of daughter ions co-stored with the parent perylene ions is included in the model. Based on the comparison of the model to experiment, we constrain the oscillator strength of the D1 → D0 emissive electronic transition in perylene (fRF = 0.055 ± 0.011), as well as the absolute absorption cross section of the D5 ← D0 excitation transition (σabs > 670 Mb). The former transition is responsible for the laser-induced and recurrent fluorescence of perylene, and the latter is the most prominent in the absorption spectrum. The vibrational cooling rate is found to be consistent with the simple harmonic cascade approximation. Quantitative experimental benchmarks of unimolecular processes in polycyclic aromatic hydrocarbon ions like perylene are important for refining astrochemical models.

Thermal evaporation of pyrene clusters.

This work presents a study of the thermal evaporation and stability of pyrene (C16H10)n clusters. Thermal evaporation rates of positively charged mass-selected clusters are measured for sizes in the range n = 3-40 pyrene units. The experimental setup consists of a gas aggregation source, a thermalization chamber, and a time of flight mass spectrometer. A microcanonical Phase Space Theory (PST) simulation is used to determine the dissociation energies of pyrene clusters by fitting the experimental breakdown curves. Calculations using the Density Functional based Tight Binding combined with a Configuration Interaction (CI-DFTB) model and a hierarchical optimization scheme are also performed in the range n = 2-7 to determine the harmonic frequencies and a theoretical estimation of the dissociation energies. The frequencies are used in the calculations of the density of states needed in the PST simulations, assuming an extrapolation scheme for clusters larger than 7 units. Using the PST model with a minimal set of adjustable parameters, we obtain good fits of the experimental breakdown curves over the full studied size range. The approximations inherent to the PST simulation and the influence of the used parameters are carefully estimated. The derived dissociation energies show significant variations over the studied size range. Compared with neutral clusters, significantly higher values of the dissociation energies are obtained for the smaller sizes and attributed to charge resonance in line with CI-DFTB calculations.

Photoprocessing of large PAH cations.

In cosmic environments, polycyclic aromatic hydrocarbons (PAHs) strongly interact with vacuum ultraviolet (VUV) photons emitted by young stars. Trapped PAH cations ranging in size from 30 to 48 carbon atoms were irradiated by tunable synchrotron light (DESIRS beamline at SOLEIL). Their ionization and dissociation cross sections were determined and compared with TD-DFT computed photoabsorption cross sections. Evidence for radiative cooling is reported.