Ultraslow radiative cooling of Cn− (n = 3–5)

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.

Cascade Infrared Thermal Photon Emission

The later stages of cooling of molecules and clusters in the interstellar medium are dominated by emission of vibrational infrared radiation. With the development of cryogenic storage it has become possible to experimentally study these processes. Recent storage ring results demonstrate that intramolecular vibrational redistribution takes place within the cooling process, and an harmonic cascade model has been used to interpret the data. Here we analyze this model and show that the energy distributions and the photon emission rates develop into near-universal functions that can be characterized with only a few parameters, irrespective of the precise vibrational spectra and oscillator strengths of the systems. We show that the photon emission rate and emitted power vary linearly with total excitation energy with a small offset. The time developments of ensemble internal energy distributions are calculated with respect to their first two moments. The excitation energy decreases exponentially with a rate constant which is the average of all k_1→0 Einstein coefficients, and the time development of the variance is also calculated.

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.

Thermionic Emission of Negative Ions of Molecules and Small Clusters as a Probe of Low-Energy Attachment

We have been studying the thermionic emission of negatively charged molecules and small clusters for more than a decade. The kinetic energy released distribution (KERD) of mass-selected negative ions has been measured with a velocity map imaging spectrometer. A comparison of the experimental KERD to detailed balance models provided information on the reverse process, namely, the electron attachment to the parent. The electron attachment to neutral systems (reverse process of the electron emission from anions) is usually described in a simplified way as a single electron capture in the framework of the classical Langevin model. Our measurements show that this approach is insufficient and that, in addition to the capture step, an intramolecular vibrational redistribution (IVR) step should be included. As far as multiply charged anions are concerned, the electron attachment to anions (reverse process of the electron emission from dianions) is strongly affected by the repulsive Coulomb barrier (RCB). Previous studies assumed a pure over-the-barrier process, which is in disagreement with our study. Indeed, electron emission is measured below the RCB, revealing significant thermal tunneling. In the present review, we summarize these works on singly and doubly charged anions in an attempt to present a unified view of the involved processes. It is worth noting that the detailed measurements of KERDs in the very low kinetic energy region (typically around 0.1 eV) have been made possible thanks to electron imaging methods, without which all of this work could never have been done, with time-resolution capabilities allowing the disentangling of direct and delayed electron emission.

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.

Radiative cooling rates of substituted PAH ions

The unimolecular dissociation and infrared radiative cooling rates of cationic 1-hydroxypyrene (OHPyr$^+$, \ce{C16H10O+}) and 1-bromopyrene (BrPyr$^+$, \ce{C16H9Br+}) are measured using a cryogenic electrostatic \rev{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 \revv{$E_c=5.39(1)$}~eV for OHPyr$^+$ and \revv{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.

Statistical vibrational autodetachment and radiative cooling rates of para-benzoquinone

We report measurements of the statistical vibrational autodetachment (VAD, also called thermionic emission) and radiative cooling rates of isolated para-benzoquinone (pBQ, C₆H₄O₂) radical anions using the cryogenic electrostatic ion storage ring facility DESIREE. The results are interpreted using master equation simulations with rate coefficients calculated using statistical detailed balance theory. The VAD rate is determined by measuring the time-dependent yield of neutral pBQ due to spontaneous electron emission from a highly-excited ensemble of anions formed in an electron-attachment ion source. Competition with radiative cooling quenches the VAD rate after a critical time of τ_c = 11.00(5) ms. Master equation simulations which reproduce the VAD yield provide an estimate of the initial effective vibrational temperature of the ions of 1100(20) K, and provide insight into the anion formation scenario. A second measurement of the radiative cooling rate of pBQ^- stored for up to 0.5 s was achieved using time-dependent photodetachment action spectroscopy across the ²A_u ← ²B_2g and ²B_2u ← ²B_2g transitions. The rate at which hot-band contributions fade from the action spectrum is quantified by non-negative matrix factorisation. This is found to be commensurate with the average vibrational energy extracted from the simulations, with 1/e lifetimes of 0.16(3) s and 0.1602(7) s, respectively. Implications for astrochemistry are discussed.

Radiative relaxation in isolated large carbon clusters: Vibrational emission versus recurrent fluorescence

Recurrent fluorescence (RF) from isolated carbon clusters containing between 24 and 60 atoms is theoretically investigated as a function of internal energy, cluster size, and structural features. The vibrational relaxation kinetics and the associated IR emission spectra are determined by means of a Monte Carlo approach with vibrational density of states computed in the harmonic approximation. RF is generally found to be highly competitive with vibrational emission. The behaviors predicted for clusters of various sizes and archetypal structures indicate that the IR emission spectra are strongly influenced by RF, an energy gap law being obtained for the evolution of the RF rate constant depending on the electronic excitation state. The present results are relevant to the photophysics of the interstellar medium and could contribute to elucidating the carriers of the extended red emission bands and the continuum emission lying below the aromatic infrared bands believed to originate from mixed aromatic–aliphatic compounds.

Autodetachment dynamics of 2-naphthoxide and implications for astrophysical anion abundance

Astrochemical modelling has proposed that 10% or more of interstellar carbon could be tied up as polycyclic aromatic hydrocarbon (PAH) molecules. Developing reliable models of the interstellar carbon lifecycle requires calibration data obtained through laboratory studies on relevant chemical and physical processes, including on the photo-induced and electron-induced dynamics of potential interstellar PAHs. Here, the excited state dynamics of the S1(ππ*) state of 2-naphthoxide are investigated using frequency-, angle-, and time-resolved photoelectron imaging. Frequency-resolved photoelectron spectra taken over the S1(ππ*) band reveal low electron kinetic energy structure consistent with an indirect, vibrational mode-specific electron detachment mechanism. Time-resolved photoelectron imaging using a pump photon energy tuned to the 0-0 transition of the S1(ππ*) band (hν = 2.70 eV) and a non-resonant probe photon provides the excited state autodetachment lifetime at τ = 130 ± 10 fs. There is no evidence for internal conversion to the ground electronic state or a dipole-bound state. These results imply that 2-naphthoxide has no resilience to photodestruction through the absorption of visible radiation resonant with the S1(ππ*) band, and that electron capture by the S1(ππ*) state, which is formally a shape resonance, is not a doorway state to a stable interstellar anion.

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 (f^RF = 0.055 ± 0.011), as well as the absolute absorption cross section of the D₅ ← D₀ excitation transition (σ_abs > 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.