Cooling of isolated anthracene cations probed with photons of different wavelengths in the Mini-Ring

Abstract

We report on a direct measurement of the Internal Energy Distribution (IED) shift rate of an initially hot polycyclic aromatic hydrocarbon (PAH) molecular ensemble, anthracene cations (CH). The ions were produced in an electron cyclotron resonance (ECR) ion source and stored in an electrostatic ion storage ring, the Mini-Ring. Laser pulses of two wavelengths were sent successively to merge the stored ion bunch at different storage times to enhance the neutral fragment yield due to fast laser induced dissociation. Using this technique, we have been able to determine directly the energy shift rate of the IED, without involving any theoretical simulation or any assumption on dissociation rates, cooling rates, or the initial IED. Theoretical energy shift rates have been estimated from the evolution of simulated IEDs by taking into account the effects of the unimolecular dissociation and two radiative decay mechanisms: the Poincaré fluorescence and the infrared vibrational emission. The comparison between the experimental results and the model provides new evidence of the important role of the Poincaré fluorescence in the overall cooling process of anthracene cations. Although in the short time range the commonly accepted intuition says that the cooling would result mostly from the dissociation of the hottest ions (depletion cooling), we demonstrate that the Poincaré fluorescence is the dominant contribution (about 85%) to the net cooling effect.

Radiative cooling of size-selected gas phase clusters

“…Gas phase experiments performed in a storage ring device, mimicking the conditions encountered by this molecule in the outer space, have shown that a beam of (C 14 H 10 ) + cools down radiatively at rates of around 200 s −1 upon laser excitation. These rates are two orders of magnitude higher than those expected from vibrational transitions, thus suggesting transitions of electronic nature via the RF mechanism [102][103][104][105].…”

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

“…Since the turn of the century, radiative cooling rates of clusters have been inferred from experiments probing time-dependent spontaneous neutralization rates of ensembles of hot ions stored in electrostatic ion-beam storage devices (see e.g. [12][13][14][15][16][17][18][19][20][21]). Such devices have been used to isolate and probe the cooling of cluster ions for durations of up to tens of milliseconds.…”

Decay Dynamics in Molecular Beams

“…The derivation of the power law decay is fairly general, and it has been observed in a number of experimental studies, most often at relatively short times (Hansen & Echt, 1997;Andersen et al, 2001;Hansen et al, 2001;Andersen et al, 2003a, b;Tomita et al, 2003;Andersen, Heber, & Zajfman, 2004;Fedor et al, 2005;Toker et al, 2007;Sundén et al, 2009;Froese et al, 2011;Lange et al, 2012;Goto et al, 2013;Martin et al, 2013;Najafian et al, 2014;Martin et al, 2015;Breitenfeldt et al, 2016;Hansen et al, 2017b;Ji et al, 2017;Martin et al, 2017;Anderson et al, 2018;Bernard et al, 2019). However, corrections may arise to these results, both in the power of −1 and in the long term behavior, caused by factors both intrinsic and external to the decaying system.…”

“…These aspects are by now well established experimentally, primarily by work at ion storage rings (Andersen et al, 1996(Andersen et al, , 2001Hansen et al, 2001;Tomita et al, 2001;Andersen et al, 2003a;Andersen, Heber, & Zajfman, 2004;Fedor et al, 2005;Toker et al, 2007;Froese et al, 2011;Lange et al, 2012;Toker et al, 2012;Martin et al, 2013;Menk et al, 2014;Martin et al, 2015;Breitenfeldt et al, 2016;Hansen et al, 2017b;Ji et al, 2017;Martin et al, 2017;Anderson et al, 2018;Bernard et al, 2019;Martin et al, 2019), occasionally in a form where other channels are present, but also at single pass devices such as time-offlight mass spectrometers (Hansen & Campbell, 1996;Hansen & Echt, 1997;Ferrari et al, 2015;Hansen et al, 2017a;Ferrari et al, 2018a, b), and here the focus will be on the theoretical aspects.…”