How Organic Molecules Survive in Space
Organic molecules called polycyclic aromatic hydrocarbons (PAHs) populate interstellar space and represent a major reservoir of carbon, an essential element for life. The smallest of these molecules mysteriously survive the harsh environment of space, and a research team has now explained how they do it [1]. In experiments in space-like conditions, the team showed that the molecules can use a process called recurrent fluorescence to shed some of the potentially destructive vibrational energy they receive from ultraviolet photons and molecular collisions. The results will help theorists model the dissemination of the building blocks of life throughout the cosmos.
PAHs form in dying stars and get ejected via supernovae into the interstellar medium. In 2021, they were detected in cold interstellar clouds (molecular clouds), and the JWST observatory has since confirmed widespread evidence for small PAHs at higher abundance than models predict. Small PAHs somehow survive ultraviolet radiation, molecular collisions, and other processes that trigger internal vibrations that can tear them apart.
“One of the biggest puzzles right now in astrochemistry is how these PAHs can possibly exist” in interstellar space, says Ilsa Cooke, an astrochemist at the University of British Columbia, Canada. Somehow, small PAHs shed their extra vibrational energy, or “cool,” and avoid dissociation. The astronomical community generally believes that only large PAHs—ones with 50 or more carbon atoms—can survive, according to James Bull of the University of East Anglia in the UK. Most models assume that such large molecules cool by emitting infrared radiation, but this so-called radiative cooling process doesn’t work efficiently for smaller molecules.
Laboratory experiments have shown another way that some small PAHs can shed energy; the process is called recurrent fluorescence: A vibrationally excited molecule can boost itself into an electronically excited state and then emit a photon that takes away much of its vibrational energy. This process can take milliseconds, whereas ordinary fluorescence—in which a photon is absorbed and then rapidly emitted—takes nanoseconds.
But lab studies have only investigated open-shelled PAH ions, which have unpaired valence electrons. In contrast, the JWST’s recent interstellar gas cloud observations indicate small, neutral PAHs with closed-shell electronic structures. Consistent with the JWST, radio astronomers have identified a PAH called indene ( ) in Taurus Molecular Cloud 1. It’s uncertain whether the cooling process observed in the lab for open-shell PAH ions also applies to the closed-shell neutral molecules observed in space, such as indene.
To measure the radiative cooling rate and determine the importance of recurrent fluorescence in a closed-shell PAH, Bull designed an experiment at the DESIREE cryogenic ion storage ring at Stockholm University. The facility allows ions to circulate for up to an hour under conditions that mimic those found in some regions of space—a temperature of 13 K and a gas density of 104 particles/cm3. “We commonly say that the infrastructure is providing a molecular cloud in a box,” Bull says. The idea was that the molecules, excited by collisions in a plasma before entering the ring, would then cool as they circulated around the ring.
He and colleagues used an ionized form of indene called indenyl ( ), which is thought to be equally abundant in space. The team sent vibrationally excited indenyl molecules circulating hundreds of thousands of times per second in the 8.6-m-circumfrence storage ring. As these molecules broke apart, the neutral fragments lost their stable orbits and left the ring, and some were collected by detectors. The number of indenyl fragments dropped over time, and the researchers found that this stabilization process occurred 5 times faster than is typical for systems cooled by infrared emission alone.
To pinpoint a mechanism, the researchers turned to molecular dynamics simulations. For initial vibrational energies in the range between 5 and 8 eV, indenyl is subject to three competing processes: energy loss via infrared emission, energy loss via recurrent fluorescence, and dissociation. A model combining these processes matched the observed stabilization rate of indenyl. In contrast, models excluding recurrent fluorescence substantially overestimated the dissociation rate. Unlike previous modeling, which assumed that PAHs have a single rigid structure, this new model included internal molecular vibrations that can enhance the rate of recurrent fluorescence, Bull says.
Cooke says that Bull’s work has “provided critical insights into the survival of small PAHs” in the interstellar medium. It indicates that recurrent fluorescence plays a key role in stabilizing isolated PAH molecules. “The next step is for astrochemists to establish the best way to incorporate these findings into their models to better probe the interstellar lifecycle of PAHs,” she says.
–Rachel Berkowitz
Rachel Berkowitz is a Corresponding Editor for Physics Magazine based in Vancouver, Canada.
References
- J. N. Bull et al., “Radiative stabilization of the indenyl cation: Recurrent fluorescence in a closed-shell polycyclic aromatic hydrocarbon,” Phys. Rev. Lett. 134, 228002 (2025).