Negative ions do not possess Rydberg states, but polar anions may have diffuse dipole-bound states just below the detachment threshold, analogous to Rydberg states of neutral molecules. Excitation to vibrational levels of the dipole-bound state can induce autodetachment via vibronic coupling. The resulting resonant photoelectron spectrum is highly non-Franck-Condon and yields much richer vibrational information than conventional photoelectron spectroscopy. We developed an experimental apparatus integrating an electrospray ionization source with photoelectron spectroscopy [1], which allowed negative ions from solution samples to be studied in the gas phase. Subsequent development of a cryogenically-cooled Paul trap to create cold anions from electrospray [2] has allowed high-resolution photoelectron imaging to be conducted for complex molecular anions [3], opening opportunities to probe dipole-bound excited states using photodetachment spectroscopy and resonant photoelectron imaging [4]. I will present recent advances in our investigation of dipole-bound excited states, including the observation of pi-type dipole-bound states [5], electron correlation induced by the electric field of the diffuse dipole-bound electron [6], the observation of polarization-assisted dipole-bound states. References: [1] L. S. Wang, C. F. Ding, X. B. Wang, and S. E. Barlow, Rev. Sci. Instrum. 70, 1957-1966 (1999). [2] X. B. Wang and L. S. Wang, Rev. Sci. Instrum. 79, 073108 (2008). [3] L. S. Wang, J. Chem. Phys. 143, 040901 (2015). [4] G. Z. Zhu and L. S. Wang, Chem. Sci. 10, 9409-9423 (2019). [5] D. F. Yuan, Y. Liu, C. H. Qian, Y. R. Zhang, B. M. Rubenstein, and L. S. Wang, Phys. Rev. Lett. 125, 073003 (2020). [6] D. F. Yuan, Y. Liu, C. H. Qian, G. S. Kocheril, Y. R. Zhang, B. M. Rubenstein, and L. S. Wang, J. Phys. Chem. Lett. 11, 7914-7919 (2020).

Vinylidene (H₂CC) has been shown to have a complex vibronic structure, with strong coupling between the ground and excited electronic states as well as vibrational wavefunction mixing between vinylidene and its isomeric counterpart, acetylene (HCCH). Our group has previously used slow photoelectron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) to probe such dynamics. However, the vibrational states of neutral vinylidene that exhibit the strongest coupling between electronic states are nominally inaccessible from the vibrational ground state of the anion, making them difficult or even impossible to observe with the cryo-SEVI technique. Here, we use the recently developed technique of infrared-pump, cryo-SEVI-probe (IR-cryo-SEVI) to further uncover vinylidene’s complexities by vibrationally exciting the anion along the antisymmetric CH2 stretch (ν₅) and CH2 in-plane rocking (ν₆) modes prior to photodetachment. Vibrational pre-excitation along these antisymmetric modes gives access to vibrational states in the neutral that are otherwise inaccessible to cryo-SEVI. Additionally, IR-cryo-SEVI is used as an action technique, able to measure the infrared absorption spectrum of the bare anion without perturbations from tagged atoms or matrices, giving the true vibrational frequencies of these modes.

The vast majority of closed-shell organic molecules follow Hund's rule, namely: their first excited triplet state T₁ lies energetically below their first excited singlet state S₁. Recent computational studies, however, suggest that energetic inversion of these states is possible in a family of closed-shell species known as azaphenalenes. The phenomenon is believed to arise from (i) minimal overlap between highest-occupied and lowest-unoccupied molecular orbitals, and (ii) stabilization of the S₁ state via coupling to higher-excited electronic configurations. Despite experimental efforts to confirm S₁-T₁ inversion in an azaphenalene species using optical techniques (1), quantification of the T₁ energy--and thus the S₁-T₁ gap--is precluded by the spin selection rule. Using photoelectron spectroscopy, which circumvents the spin selection rule, we report a S₁-T₁ gap for 1,3,4,6,9b-pentaazaphenalene of either 374 cm^-1 or -374 cm^-1 (+- 46 meV), pending definitive assignment of the S₁ state. Once confirmed, these results will have important implications in the fields of organic light-emitting diodes, catalysis, and benchmarking for high-level quantum chemical calculations of electronically excited states. (1) Ehrmaier et al. J. Phys. Chem. A 2019, 123 (38), 8099–8108.

Photodetachment spectroscopy is a powerful spectroscopic technique for determining the internal state distribution of a molecular anion. Previously, our group studied the threshold photodetachment spectroscopy of CN⁻ at both 16 Kelvin and 295 Kelvin in a 22-pole ion trap and measured the electron affinity of CN with great precision (EA: 3.864(2) eV)[1]. The dicarbon anion, our current molecule of interest, is a well-studied system due to its stable electronic level structure and potential laser cooling transition[2]. Here we present the threshold photodetachment spectroscopy study of C₂⁻ in a radiofrequency 16-pole ion trap at 8 Kelvin. We investigated the behaviour of the cross section near the threshold for the ground state transition, C₂X¹Σ⁺_g ← C₂⁻X²Σ⁺_g. We see a p-wave behavior for this ground state detachment. And we measured the electron affinity, which is consistent with the previously measured values[3][4].
[1]. M. Simpson et al., J. Chem. Phys. 153, 184309 (2020). [2]. M. Nötzold et al., Phys. Rev. A 106, 023111 (2022). [3]. K. M. Ervin, et al., J. Phys. Chem. 95, 2244 (1991). [4]. B. A. Laws et al., Nat. Commun. 10, 1(2019).

Although the photoelectron spectrum of copper trioxide was first reported in the late 1990’s, it has remained unassigned to date. Several calculations have predicted the presence of multiple isomers, but the complexity of the system has prevented any assignment of the experimental spectrum. We have revisited the photoelectron spectrum of copper trioxide, now with high-resolution photoelectron imaging and a cryogenically-cooled ion trap. The new temperature dependent photoelectron spectra have resolved the presence of two distinct isomers, allowing for the full assignment of the photoelectron spectrum of copper trioxide for the first time.

Employing size-selective cryogenic photoelectron spectroscopy (cryo-PES) coupled with electrospray ionization (ESI), we have been studying the physical and chemical properties of a wide variety of complex molecular clusters ranging from microsolvated anions, hydrogen bonded complexes to atmospherically relevant pre-nucleation clusters. Our studies on a series of B12X122-(H2O)n (X = H, F, I) clusters established the formation of hydridic-to-protonic dihydrogen bonds (DHBs) and unraveled the predominance of DHB strength by comparing to those conventional B-X···H-O (X = F, I) HBs. The primary gas phase hydration shell of hydroxide has recently been investigated as well.

l0pt Figure Highly unsaturated C_xH_y⁺ clusters are important in combustion and the chemistry of interstellar space. However, interrogating carbonaceous molecules containing more than 10 carbon atoms is complicated by the coexistence of different isomers possessing unique spectroscopic properties. To disentangle this complexity, we have developed an apparatus that allows formation and selection of a particular C_xH_y⁺ isomer population by ion mobility and mass to charge ratio. The selected ions are then incarcerated in a cryogenically cooled ion trap and subjected to tunable radiation. Resonant excitation of an electronic transition leads to cluster fragmentation, which when monitored as a function of wavelength yields an action spectrum. We have also measured electronic spectra for a variety of C_xH_y⁺ ions, which are formed by photoionizing and photodissociating PAHs in the ion source. The C_xH_y⁺ clusters can be linear or cyclic, but can be selected by their ion mobility arrival time and mass to charge ratio. Pyrene photoproducts include C_2n+1H₃⁺ and C_2n+1H⁺ clusters. The C_2n+1H₃⁺ clusters are mostly linear isomers with characteristic vibronic progressions of low frequency modes (see Figure). C_2n+1H⁺ clusters are shown to exist as linear and cyclic isomers that have distinct electronic spectra. These results provide clues on how different C_xH_y⁺ cluster isomers form, dissociate, and isomerise in interstellar space, and provide spectroscopic fingerprints to aid their astronomical detection.

The nitrate anion is the final sink for NOx species and plays a crucial role in the chemical processes of Earth’s atmosphere, hydrosphere, and cryosphere. In particular, the photodissociation of nitrate from Antarctic snow forms radical oxygen which, when reacted with ambient water, forms radical OH, arguably the most important oxidative species in the atmosphere. While it is well-known that actinic radiation ( > 290 nm) can be absorbed by the nitrate ion and photodissociate it, the exact mechanism is unclear. In particular, the absorption mechanism is unclear since the only singlet state in that range is a symmetry forbidden transition. Theoretical studies have shown that the absorption could be allowed via vibronic coupling or by spin-orbit coupling to the nearby triplet state. It has also been shown that the intensity of the forbidden transition is highly sensitive to the presence of water. Here, we perform photodissociation action spectroscopy over the 300 nm range to experimentally determine a gas phase NO₃⁻ and NO₃⁻(H₂O) UV absorption spectrum. Our setup also enables the investigation of the photodissociation products branching ratios as a function of wavelength. This will allow for a more comprehensive understanding of the nitrate photodissociation mechanism.

Diamondoids are a class of aliphatic molecules arranged in cage-like structures and serve as a link between small, cyclic hydrocarbons and bulk nanodiamonds. They are well-known for their interesting optical properties, as they typically exhibit large optical band gaps of around 6 eV or higher.¹ Their radical cations, however, are predicted to display transitions in the visible and, due to their favorable structural and optical properties, may be viable candidates as carriers of the diffuse interstellar bands.² So far, only the optical spectra of adamantane and diamantane cations have been measured, which exhibit broad and mostly vibronically unresolved features.^3,4 To explore the effects of functionalization, we report the first optical spectra of N-substituted adamantane cations, namely 1-cyanoadamantane (C₁₁H₁₅N⁺) and urotropine ((CH)₆N₄⁺) between 260 and 1200 nm in the gas phase. Measurements were taken in a tandem mass spectrometer by photodissociation of mass-selected ions cooled in a cryogenic 22-pole ion trap held at 5 K. The experimental results are compared to photoelectron spectra and time-dependent DFT calculations for interpretation.

Literature:
¹ L. Landt et al., PRL, 103, 047402 (2009).
² M. Steglich et al., ApJ, 729, 91 (2011).
³ P. Crandall et al., ApJL, 900, L20 (2020).
⁴ P. Crandall et al., ApJ, 940, 104 (2022).

The impact of atmospheric aerosols on climate is one of the largest uncertainties in global climate models so far. New particle formation (NPF) is an all-important initial step that brings together trace gases in the atmosphere to form aerosols. Identifying the structural motifs and intermolecular interactions stabilizing newly-formed clusters is key to establishing their growth mechanisms and developing quantitative models for their growth. Sulfuric acid and bases such as ammonia have been heavily studied as prototypical new particles, but recent evidence suggests that organic vapors (such as dicarboxylic acids, amino acids, etc.) are likely to play a significant role in NPF. In this work we explore the structural motifs in cationic clusters composed of glycine, ammonia, and sulfuric acid. We find that glycine stabilizes these clusters, with collision induced dissociation (CID) mass spectra showing that ammonia is preferentially lost from these clusters. Gas phase vibrational spectra and quantum chemical calculations show that the largest of these clusters have strong hydrogen bonds between bisulfate groups, hinting at potential phase separation in pre-nucleation clusters containing both organic and inorganic compounds. Lastly, variable temperature mass spectrometry (VTMS) experiments show that the glycine-containing clusters uptake water at atmospherically relevant temperatures. Combined, these results imply that in the atmosphere, glycine will likely replace ammonia in ammonium bisulfate clusters, forming stable clusters that can potentially increase NPF efficiency.