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

Abstract

Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, C − n (n = 3 − 5), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of ≈10 −14 mbar and temperature of ≈13 K, allowing storage of mass-selected ions for hours and providing conditions coined a "molecular cloud in a box". Here, we construct two-dimensional (2D) photodetachment spectra for the target anions by recording photodetachment signal as a function of irradiation wavelength and ion storage time (seconds to minute timescale). Ion cooling lifetimes, which are associated with infrared radiative emission, are extracted from the 2D photodetachment spectrum for each ion by tracking the disappearance of vibrational hot-band signal with ion storage time, giving 1 e cooling lifetimes of 3.1±0.1 s (C − 3 ), 6.8±0.5 s (C − 4 ) and 24±5 s (C − 5 ). Fits of the photodetachment spectra for cold ions, i.e. those stored for at least 30 s, provides adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy. Ion cooling lifetimes are simulated using a Simple Harmonic Cascade model, finding good agreement with experiment and providing a mode-by-mode understanding of the radiative cooling properties. The 2D photodetachment strategy and radiative cooling modeling developed in this study could be applied to investigate the ultraslow cooling dynamics of wide range of molecular anions. arXiv:1909.07087v1 [physics.atm-clus]

Radiative cooling of carbon cluster anions C2n+1− (n = 3–5)

“…The action spectrum of C − 7 is attributed predominantly to direct photodetachment, and shows a clear Wigner-type threshold behavior above the adiabatic detachment energy [51], similar to the results of Bull et al for C − n (n = 3−5) [28]. However, unlike in the study of Bull et al [28], no significant time-dependent contribution due to depletion of hot-bands was observed for C − 7 (see Fig. 2 and compare with Fig.…”

“…less than a second) with time constant τ 1 ≈ 0.5 s. It is likely that this increase in the photo-action signal is due to recovery of the ground and low-lying vibrational states of the anion, which in turn increases the probability for resonant excitation. A similar effect was previously characterised for C − 5 [28]. The decay in photoaction signal at longer times (τ 2 > 100 s) in Figure 5 is probably associated with the ion beam storage lifetime (≈300 s for C − 11 ), but may have some contribution from further cooling of photo-activated ions that have insufficient vibrational energy to produce neutrals.…”

“…The nascent ions were accelerated to 10 keV (8 keV for C − 7 action spectroscopy measurements), selected according to their mass-to-charge ratio using a bending magnet, and injected into the storage ring [30]. Transport from the source to the ring takes ≈80−100 µs and the 1/e beam storage lifetimes are typically >100 s, limited by loss of ions through collisions with background gas of ∼10 4 H 2 molecules per cm 3 [28,31]. The ions initially stored in the ring have a broad distribution of internal excitations, similar to thermal distributions with temperatures exceeding 1000 K [26].…”

“…Infrared radiative cooling was modelled using a SHC model [28]. Briefly, the model utilizes the vibrational density of states ρ computed with the Beyer-Swinehart algorithm and scaled harmonic vibrational mode frequencies ν s calculated at the ωB97X-D//aug-cc-pVTZ level of Density Functional Theory (DFT) in Gaussian 16 [36][37][38].…”

“…In a recent study we developed a strategy for probing ultraslow radiative cooling of carbon cluster anions, C − n (n = 3−5) [28]. The strategy involves construction of twodimensional (i.e.…”