Radiative cooling rates of substituted PAH ions

Abstract

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.

Cooling dynamics of energized naphthalene and azulene radical cations

“…3b are fits of EQN 7 to the R(t) data for t > 5 × 10 −4 s. Extracted values of k c are ≈460 s −1 (Np + ) and ≈410 s −1 (Az + ) and are comparable with those for 1-CNN + (k c = 300 ± 20 s −1 ) 15,62 and other larger PAH cations. 58,59,63 Significantly, the k c values for precursor Np + and Az + (as well as the cooling profiles) are similar, indicating similar dissociation and cooling dynamics, i.e., a common mechanism, over the fitted timescale. Application of the alternative fitting model…”

“…These data are averaged over a large number of injection and storage cycles, and have a similar shape to those reported for other PAH cations. 58,59 Intriguingly, the measured storage time dependence of the neutralization rates for the two precursor ions are near identical. In terms of the underlying physics of the neutralization rate curves, during the first ≈10 −3 s after ion injection, the neutralization rate follows a power law trend, R(t) ∝ t −1 , due to the broad distribution of internal energies, [g(E, t)], of the ions and the rapid variation of the dissociation rate coefficient, k d (E), with E. 60 After a critical time (k −1 c ), the dissociation rate is quenched by competition with radiative cooling (RF and IR), 61 resulting in a dissociation rate with approximate time dependence…”

“…While experiments and supporting theoretical studies have demonstrated that C 2 H 2 -and H-elimination are the lowest energy dissociation pathways for the Np + -Az + couple, 30,34,36 with approximately equal branching ratio at low excess internal energies (e.g., 1 eV above threshold), the imaging detector in DESIREE is insensitive to H atoms, 59 so the KER distributions should reflect C 2 H 2 formation. H-loss, if present, would be spread widely over the detector due to the low mass of H, and would be indistinguishable from background in the KER image.…”

Experimental radiative cooling rates of a polycyclic aromatic hydrocarbon cation

“…22,23 Electrostatic ion storage devices feature sampling times exceeding 1 s, orders of magnitude longer than conventional mass spectrometers, enabling time-resolved observations of delayed processes such as unimolecular dissociation 16 and thermionic electron emission. 24 DESIREE has also been used for vibrational energy dependent action spectroscopy of astrophyscially relevant ions including PAHs, 25,26 substituted PAHs thought to form in interstellar ices, 27,28 and carbon cluster anions. 29,30 The methods used here have largely been described previously.…”

Gas-phase electronic action absorption spectra of protonated oxygen-functionalized polycyclic aromatic hydrocarbons (OPAHs)

“…In particular, RF occurs for molecular ions that possess strongly coupled low-lying electronic states that facilitate a redistribution of the absorbed energy through inverse internal conversion. The electronic transition to the ground state results in fluorescence and has been demonstrated to be a relevant process in the radiative cooling of hot PAH cations in dedicated electrostatic ion-storage ring experiments such as the Mini-Ring (Martin et al 2013) and DESIREE (Zhu et al 2022).…”