Spectroscopy of astrophysically relevant ions in traps

Infrared Spectroscopy of Neutral and Cationic Sumanene (C21H12 & C21H12 +) in the Gas Phase: Implications for Interstellar Aromatic Infrared Bands (AIBs)

Polycyclic aromatic hydrocarbons (PAHs) are known to be omnipresent in various astronomical sources. Ever since the discovery of C60 and C70 fullerenes in a young planetary nebula in 2010, uncovering the reaction pathways between PAHs and fullerenes has been one of the primary goals in astrochemistry. Several laboratory studies have attempted to elucidate these pathways through experiments simulating top-down and bottom-up chemistry. Recently, indene (c-C9H8, a fused pentagon and hexagonal ring) has been detected in the TMC-1 molecular cloud. This is a significant finding since pentagon-bearing PAHs could be key intermediates in the formation of fullerenes in space. Spectroscopic studies of pentagon-bearing PAHs are thus essential for their detection in molecular clouds, which would eventually lead to unraveling the intermediate steps in PAH's chemistry. This work reports the infrared (IR) spectra of both neutral and cationic sumanene (C21H12 and C21H12 +): a bowl-shaped PAH containing three pentagon rings. Apart from its relevance for furthering our understanding of the chemistry of PAHs in an astronomical context, the presence of three sp3 hybridized carbons makes the vibrational spectroscopy of this molecule highly interesting also from a spectroscopic point of view, especially in the CH stretching region. The experimental IR spectra of both species are compared with quantum chemically calculated IR spectra as well as with the aromatic infrared bands (AIBs) of the photodissociation regions of the Orion Bar obtained using the James Webb Space Telescope (JWST).

State selective preparation and nondestructive detection of trapped O2

The ability to prepare molecular ions in selected quantum states enables studies in areas such as chemistry, metrology, spectroscopy, quantum information, and precision measurements. Here, we demonstrate (2 + 1) resonance-enhanced multiphoton ionization (REMPI) of oxygen, both in a molecular beam and in an ion trap. The two-photon transition in the REMPI spectrum is rotationally resolved, allowing ionization from a selected rovibrational state of O2. Fits to this spectrum determine spectroscopic parameters of the O2d1Πg state and resolve a discrepancy in the literature regarding its band origin. The trapped molecular ions are cooled by co-trapped atomic ions. Fluorescence mass spectrometry nondestructively demonstrates the presence of the photoionized O2+. We discuss strategies for maximizing the fraction of ions produced in the ground rovibrational state. For (2 + 1) REMPI through the d1Πg state, we show that the Q(1) transition is preferred for neutral O2 at rotational temperatures below 50 K, while the O(3) transition is more suitable at higher temperatures. The combination of state-selective loading and nondestructive detection of trapped molecular ions has applications in optical clocks, tests of fundamental physics, and control of chemical reactions.

Photofragmentation of corannulene (C20H10) and sumanene (C21H12) cations in the gas phase and their astrophysical implications

Aromatic infrared bands (AIBs) dominate the mid-infrared spectra of many galactic and extragalactic sources. These AIBs are generally attributed to fluorescence emission from aromatic molecules. Unified efforts from experimentalists and theoreticians to assign these AIB features have recently received additional impetus with the launch of the James Webb Space Telescope (JWST) as the Mid-InfraRed Instrument (MIRI) delivers a mid-IR spectrum with greatly increased sensitivity and spectral resolution. PAHs in space can exist in either neutral or ionic form, absorb UV photons and undergo fragmentation, becoming a rich source of small hydrocarbons. This top-down mechanism of larger PAHs fragmenting into smaller species is of utmost importance in photo-dissociation regions (PDR) in space. In this work, we experimentally and theoretically investigate the photo-fragmentation pathways of two astronomically significant PAH cations - corannulene (C20H10) and sumanene (C21H12), which are structural motifs of fullerene C60, to understand their sequential fragmentation pathways. The photo-fragmentation experiments exhibit channels that are significantly different from planar PAHs. The breakdown of the carbon skeleton is found to follow different pathways for C20H10 and C21H12 because of the number and positioning of pentagon rings, yet the most abundant low mass cations produced by these two species are found to be similar. The low mass cations showcased in this work could be of interest due to their possible astronomical detections. For completeness, the qualitative photofragmentation behaviour of dicationic corannulene and sumanene has also been investigated, but the potential energy surface of these dications is beyond the scope of this paper.

Metallofullerenes as potential candidates for the explanation of astrophysical phenomena

Detection of complex organic species in space has been one of the biggest challenges of the astrophysical community since the beginning of space exploration, with C60-fullerene representing one of the largest molecules so far detected. The presence of small metal-containing organic molecules, like MgNC or CaCN, in space, promoted the idea that C60 may also interact with metals and form metallofullerenes based on the fact that in certain circumstellar and interstellar environments, the ingredients for the formation of metallofullerenes, i.e., metal and fullerenes, are abundant. In this perspective, we summarized the current effort to explore the presence of metallofullerenes in space, which started soon after the discovery of fullerenes about 40 years ago. Several implications of astrophysical phenomena were briefly discussed and shown to be addressable as the possible consequence of metallofullerenes' presence. We highlighted the spectral fingerprints that might be followed to achieve the future detection of cosmic metallofullerenes from a combined effort of laboratory and quantum chemical calculations. These results are expected to gain great importance with the James Webb Space Telescope (JWST), whose capability of unprecedented high sensitivity and high spectral resolution in the far- to mid-infrared range could aid the unequivocal detection of metallofullerenes in space.

Electronic Spectroscopy of Monocyclic Carbon Ring Cations for Astrochemical Consideration

Gas phase electronic spectra of pure carbon cations generated by laser vaporization of graphite in a supersonic jet and cooled to below 10 K and tagged with helium atoms in a cryogenic trap are presented. The measured C_2n⁺–He with n from 6 to 14, are believed to be monocyclic ring structures and possess an origin band wavelength that shifts linearly with the number of carbon atoms, as recently demonstrated through N₂ tagging by Buntine et al. (J. Chem. Phys.2021, 155, 21430234879679). The set of data presented here further constrains the spectral characteristics inferred for the bare C_2n⁺ ions to facilitate astronomical searches for them in diffuse clouds by absorption spectroscopy.