The Chemical Makeup of the Interstellar Medium

At the American Chemical Society Fall 2026 Meeting, Pavithraa “Pavi” Sundararajan, who is a postdoctoral fellow at the National Aeronautics and Space Administration (NASA) Ames Research Center, presented her research into polycyclic aromatic hydrocarbons (PAHs).^1,2

Sundararajan discussed the chemical makeup of the interstellar medium and how it connects to infrared (IR) astronomy. Hydrogen is the most abundant element in space, but carbon, which is the fourth-most abundant, plays the key role in building complex organic molecules.² Carbon appears in several forms throughout the interstellar medium, ranging from small diatomic molecules like carbon monoxide, to simple hydrocarbons such as methane, up to large, complex structures called polycyclic aromatic hydrocarbons (PAHs).^1,2

Sundararajan then discussed a century-old astronomical puzzle: distinctive IR signatures detected from various sources in space, now known as aromatic infrared bands (AIBs), recognized for their rich spectral features in the mid-infrared (MIR) range.² Large, compact PAHs are considered the most likely carriers of these bands, since their molecular structure allows them to survive the intense ultraviolet radiation present in interstellar environments.² This link between PAHs and AIBs helps explain both the chemistry of space and the IR signals astronomers observe.²

Spectroscopy: At the ACS Fall 2026 Meeting, you are set to deliver a talk titled, “Infrared Spectroscopy of Carbon Grains Formed from Nitrogen substituted Polycyclic Aromatic Hydrocarbons (PAHs) Precursors Under Astrophysical Conditions: Implications for Space Return Samples.” Can you provide our audience with a brief overview of what you will be discussing in your talk?

Pavi Sundararajan: So hydrogen, as we know, is the most abundant element in space, and carbon is the fourth-most abundant in space, and it is also the fundamental to the formation of complex organic molecules in space. In the interstellar medium, carbon can be present in the form of small diatomic molecules like carbon monoxide, or small hydrocarbons like methane, or large and complex molecules like the polycyclic aromatic hydrocarbons, or the PAHs, that I will be talking about. So, the infrared (IR) signatures observed from various astronomical sources have puzzled astronomers for over a century. And these are called the aromatic infrared bands (AIBs) because of their rich aromatic features in the mid-IR region, especially. The large and compact PAHs, the polycyclic aromatic hydrocarbons, which I call the PAHs, can withstand harsh UV radiation in space, and they are thought to be the carriers of the AIBs.

You can watch the full clip with Sundararajan here: https://www.spectroscopyonline.com/view/studying-polycyclic-aromatic-hydrocarbons-under-astrophysical-conditions

References

1. Sundararajan, P.; Wetzel, W. Understanding Nitrogen-containing Polycyclic Aromatic Hydrocarbons (PAHs). Spectroscopy Online, 2026. https://www.spectroscopyonline.com/view/understanding-nitrogen-containing-polycyclic-aromatic-hydrocarbons-pahs- (accessed August 19, 2026).
2. Sundararajan, P.; Wetzel, W. Studying Polycyclic Aromatic Hydrocarbons Under Astrophysical Conditions. Spectroscopy Online, 2026. https://www.spectroscopyonline.com/view/studying-polycyclic-aromatic-hydrocarbons-under-astrophysical-conditions (accessed August 19, 2026).