r0pt Figure Quantum dot - metal oxide heterostructures (QDHs) have been heavily studied in the past 15 years for their use in harvesting solar energy. Great strides have been made in understanding their limitations, what is still unclear however is how much the two materials interact after charge injection. So far it's been presumed that QDHs share behavior with dye sensitized solar cells after charge injection, but this is not necessarily true. This work applies UV probe transient absorption spectroscopy to measure spectrally distinct signals of ZnO and CdSe, and uses global target analysis to distinguish models of interfacial dynamics. Four samples are measured and analyzed in this way to distinguish effects that can be attributed to quantum dots from effects that can be attributed to ZnO surface chemistry, three with different sizes of CdSe and one with a post-synthesis annealed ZnO sample. All samples showed evidence of split population dynamics that indicates two energetic regimes at play. It's found that an interfacial excitonic state, which is currently a favored explanation, is likely not formed due to the environmental screening of the coulombic interaction. Instead, carriers may localize near the surface due to band bending induced by adsorbed species. This explanation is consistent with a holistic view of the involved materials and previous terahertz conductivity results. Annealing the ZnO increases the overall yield and shifts charge injection towards longer timescales, which can only be explained in the context of band bending.

Small polaron formation in transition metal oxides is believed to be a reaction bottleneck in many solar energy conversion applications. While polaron formation has been previously confirmed, the microscopic interaction between a small polaron and its host material is largely unexplored. Here, using femtosecond XUV reflection spectroscopy, we report the evidence of electronic and structural interaction between the small polaron and its host material in CuFeO₂, a photoelectrode material for CO₂ reduction. Initial small polaron formation is observed as a spectral blue shift occurring within the first 100 fs. After polaron formation, we observe an increased coherent oscillation signal around the polaron sites, which is attributed to polaron-induced optical phonons. This observation suggests that the polaron-associated local lattice distortion can launch optical phonons in neighboring unit cells. In addition to structural coupling, the electronic states in the host materials can also be modified during polaron formation. As an example, we report an increase of Fe oxidation states after photoexcitation. The population of these highly oxidized Fe atoms strongly correlates with polaron dynamics, suggesting that a polaron can alter its surrounding electronic states in host materials.

Formamide, NH₂CHO, is invoked in the prebiotic chemistry on Earth and outside of our planet because it is the simplest molecule containing a peptide linkage [−NH−C(=O)−] that has a relatively high boiling point of 210 ^°C. There are few laboratory studies on its photodissociation possibly because the formamide vapor pressure is too low for supersonic jet experiments. In this work, we use a low-pressure (1 μbar) flow-tube reactor and in situ chirped-pulse Fourier transform millimeter-wave (CP-FTmmW) spectroscopy in the 260-290 GHz region to study the post-photolysis kinetics of the HCN, HNC, HNCO, and HCO products of the 193 nm photodissociation of formamide. The time evolution of the HCN and HNC CP-FTmmW signals is analyzed alongside complementary ab initio quasiclassical trajectory and transition state theory calculations to understand the dynamics on the NH₂CHO potential energy surface following absorption of a UV photon. The HCN/HNC branching is deduced, with good agreement demonstrated for the experimental and theoretical results.

Tunable laser photodissociation spectroscopy and photofragment imaging experiments are employed to investigate the dissociation energies of the Fe⁺(acetylene), Fe⁺(benzene), and Fe⁺(benzene)₂ ion-molecule complexes. In the spectroscopy experiment, continuous dissociation is observed above a certain energy threshold throughout the visible wavelength region for all three complexes. Photofragment imaging of the Fe⁺ photoproduct in the cases of Fe⁺(acetylene) and Fe⁺(benzene), and imaging of the benzene⁺ charge transfer photoproduct of Fe⁺(benzene), provide upper limits on the dissociation energies of these two complexes. The dissociation energies measured from this two-pronged approach agree nicely with values determined previously by collision-induced dissociation. However, these values are inconsistent with those produced from computational chemistry at the DFT level, despite the implementation of functionals recommended for transition metal chemistry.

We have investigated the photochemistry of molecular dyads composed of a light absorbing Cu(I) diimine species covalently linked to an anthraquinone moiety (CuAnQ). These donor-acceptor molecules serve as Earth-abundant prototypes for studying charge accumulation mechanisms in donor-acceptor-donor triads. We have utilized ultrafast optical and NIR spectroscopy to study the kinetics of the electron transfer, taking advantage of the optical and NIR signatures of the AnQ⁻ radical anion. l0pt Figure After excitation into a ¹MLCT band, we observe electron transfer from Cu to AnQ in 10's of picoseconds, forming a charge-separated state (CSS). The assignment of the CSS to Cu²⁺AnQ⁻ was confirmed by spectroelectrochemical study of the Cu⁺AnQ⁻ species. The CSS relaxes back to the ground state in 3 nanoseconds. In the presence of a protic solvent, the charge-separated state further transforms into a new species. The spectral changes suggest this product results from protonation of the AnQ⁻ into the semiquinone radical (HAnQ^•). Accompanying this protonation is an extension of the charge-separated state lifetime from 3 ns to 15 ns. These results provide promise for future studies on the CuAnQCu triad, which can potentially form the double reduced Cu²⁺AnQ²⁻Cu²⁺ and Cu²⁺H₂AnQCu²⁺ CSSs. Preliminary experiments which indicate CO₂ binding to the Cu²⁺AnQ⁻ and Cu²⁺HAnQ^• CSSs are still underway.

We have experimentally demonstrated conclusive evidence of solvent-to-chromophore excited state proton transfer (ESPT) as a deactivation mechanism in a binary complex isolated in the gas phase. The above was achieved by quantitatively determining the energy barrier of the ESPT processes, qualitatively analyzing the quantum tunneling rates, and evaluating the kinetic isotope effect. The 1:1 complexes of 2,2'-pyridylbenzimidazole (PBI) with H₂O, D₂O, and NH₃, produced in a supersonic jet-cooled molecular beam, were characterized spectroscopically. The vibrational frequencies of the complexes in the S₁ electronic state were recorded using a resonant two-color two-photon ionization method coupled to a Time-of-Flight mass spectrometer set-up. In the PBI-H₂O, the ESPT energy barrier of 431±10 cm⁻¹ was measured using UV-UV hole-burning spectroscopy. The exact reaction pathway was experimentally determined by isotopic substitution of the tunneling-proton (in PBI-D₂O) and increasing the width of the proton transfer barrier (in PBI-NH₃). In both cases, the energy barrier was significantly increased to > 1030 cm⁻¹ in the PBI-D₂O and to > 868 cm⁻¹ in PBI-NH₃. The heavy atom in PBI-D₂O decreased the zero-point energy in the S₁ state significantly, resulting in the elevation of the energy barrier. Secondly, the solvent-to-chromophore proton tunneling was found to decrease drastically upon deuterium substitution. In the PBI-NH₃ complex, the solvent molecule formed a preferential hydrogen bonding with the acidic (PBI)N-H group. This led to the formation of a weak hydrogen bonding between the ammonia and the pyridyl-N atom, thus, increasing the proton transfer barrier width (H₂N-H...N_pyridyl(PBI)). The above resulted in both an increase in barrier height and a decrease in the quantum tunneling rate in the excited state. The experiment, aided by computational investigations, demonstrated the variation observed for both the energy barrier and the quantum tunneling rate by substituting NH₃ in place of H₂O can be directly correlated to the drastically different photochemical and photo-physical reactions of biomolecules under various microenvironments. To validate the above methods in different chemical environments, such as PBI-CH₃OH and PBI-CH₃OD complexes show a similar mechanism.

Ozone photodissociation in the Hartley (200-300 nm) and Huggins (310-370 nm) bands has been the focus of numerous studies due to its critical role in atmospheric chemistry. While Hartley band dissociation occurs primarily through two spin-allowed dissociation channels, three additional spin-forbidden channels have previously been observed following dissociation in the Huggins band. We report 1-D and 2-D REMPI spectra and velocity-mapped ion images of O₂(a¹∆_g) and O₂(b¹Σ_g⁺) fragments following spin-forbidden dissociation in the Huggins band. We have previously observed a preference for the formation of even rotational states of O₂(a¹∆_g) arising from Hartley band dissociation due to a Λ-doublet propensity. In the Huggins band, however, odd rotational states are enhanced in the REMPI spectrum, which we attribute to greater coupling between the initial excited state of O₃ and ³A" states producing odd rotational states of O₂(a¹∆_g), than between the initial excited state and ³A’ states producing even rotational states. Ion image angular distributions of the O₂(a¹∆_g) fragment in odd and even rotational states showed significant differences following Hartley band dissociation, supporting the Λ-doublet propensity model, but are indistinguishable following Huggins band dissociations. This supports a preference for the A’ Λ-doublet and even rotational states following O₃ transitions from the B state to ³A’ states and a preference for the A" Λ-doublet and odd rotational states following O₃ transitions from the B state to the ³A" states, but indicates that in the Huggins band, the A" Λ-doublet does not originate from a warmer distribution of parent molecules as seen in the Hartley band. 2D-REMPI allows simultaneous measurements of the rotational distributions for v=0-2 of the b¹Σ_g⁺ state as well as v=0 of the a¹∆_g state. The relative signal in v=0-2 of the b¹Σ_g⁺ can provide information about the vibrational distribution, and rotational state distributions of each vibrational state can be fit individually. Spectra indicate a broad rotational distribution of the O₂(a¹∆_g) fragment and a narrow distribution of the O₂(b¹Σ_g⁺) fragment. While determination of the O₂(a¹∆_g) rotational distribution is limited due to the highly perturbed resonant state accessed in the REMPI scheme, a broad distribution is additionally supported by the multimodality of the radial distributions in the ion images.

NO is an important reactive intermediate in combustion and atmospheric chemistry. The experimental detection of NO commonly utilizes laser-induced fluorescence (LIF) on the A²Σ⁺← X²Π transition band. However, the electronic quenching of NO (A²Σ⁺) with other molecular species provides alternative photochemical pathways that compete with fluorescence. Prior experimental studies have demonstrated that collisions with C₂H₂ are effective at driving the non-radiative relaxation of NO (A²Σ⁺). Moreover, H-atom production has been observed in this electronic quenching. However, no detailed experimental or theoretical studies have been performed on this system, and the specific photochemical pathways of NO (A²Σ⁺)+C₂H₂ remain unexplored. Here, we describe the development of high-quality potential energy surfaces (PESs) that provide new physical insights into the long-range interactions and conical intersections that facilitate the electronic quenching of NO (A²Σ⁺) by C₂H₂. The PESs are calculated at the EOM-EA-CCSD/d-aug-cc-pVTZ//EOM-EA-CCSD/aug-cc-pVDZ level of theory, an approach that ensures a balanced treatment of the valence and Rydberg electronic states as well as an accurate description of the open-shell character of NO. We demonstrate that intermolecular interactions between NO (A²Σ⁺) and C₂H₂ cause C₂H₂ to isomerize into its trans-bent confirmation. We further identify a downhill pathway for internal conversion. Finally, we are beginning to explore the role that low-lying electronic excited states of C₂H₂ play in the electronic quenching of NO (A²Σ⁺) by C₂H₂. Our work informs future velocity-map imaging experiments and non-adiabatic dynamics simulations on this system.

Biogenic alkenes, such as isoprene and α-pinene are the prominent source of volatile organic compounds (VOCs) emitted into the atmosphere. Atmospheric processing of alkene molecules via reaction with ozone leads to formation of zwitterionic reactive intermediates with a carbonyl oxide functional group, known as Criegee intermediates (CIs).D. Johnson; G. Marston, Chem. Soc. Rev 37, 699(2008).Is exhibit strong absorption π^*←π in the near ultraviolet and visible (UV-vis) region due to the carbonyl oxide moiety. Previously, this laboratory reported the electronic spectra of the four-carbon CIs with unsaturated substituents derived from isoprene ozonolysis, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide), on the first π^*←π to transition under jet-cooled conditions.M. F. Vansco; B. Marchetti, M.I. Lester, J. Chem. Phys. 149, 244309(2018).^, M. F. Vansco; B. Marchetti; N. Trongsiriwat; T. Bhagde, G. Wang, P. J. Walsh; S.J. Klippenstein; M: I. Lester, J. Am. Chem. Soc. 141, 15058(2019).I

Polycyclic aromatic hydrocarbons (PAHs) are an important reservoir of carbon in the interstellar medium, and central to proposed formation mechanisms for identified fullerenes. The process of PAH photofragmentation is not fully understood, and depends on molecular identity and light source characteristics. In this work, protonated coronene and per-deuterated coronene were photofragmented in a modified commercial linear ion-trap mass spectrometer via focused continuous-wave 532 nm laser light. The laser power and ion trapping time were scanned to discover trends in the photodissociation process. A sequential photodissociation process was discovered: first the creation of the coronene cation, then hydrogen loss through molecular hydrogen removal, and finally carbon cluster fragmentation producing a series of shrinking carbon cations. The fragment distribution could qualitatively change under constant total photodissociation energy, being second-order with respect to laser power but first-order with respect to irradiation time. Lastly, several unexpected spectroscopic states and potential isomerization were discovered in the photodissociation process.