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 \approx 10 ^ { -14 } mbar and temperature of \approx 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 \frac { 1 } { e } cooling lifetimes of 3.1 \pm 0.1 s ( C _ { 3 } ^ { - } ) , 6.8 \pm 0.5 s ( C _ { 4 } ^ { - } ) and 24 \pm 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 .