Stilbene is a prototype molecule for studying photoisomerization and photocyclization mechanisms which are pivotal to converting light into chemical and mechanical energy in nature. Despite multiple decades of research, the exact details of the photoisomerization of cis-stilbene into trans-stilbene and/or its cyclization into 4a,4b-dihydrophenanthrene (DHP) have remained a topic of debate. We have used mega-electronvolt ultrafast electron diffraction (UED) to capture and spatially resolve the photoexcitation dynamics with sub-angstrom resolution. At the SLAC MeV-UED beamline, cis-stilbene was optically pumped with 267 nm ultraviolet light with different pulse energy and probed with 3.7 MeV electrons. We compare our experimental difference-diffraction signals with Ab initio multiple spawning simulations (AIMS) for single-photon excitation dynamics and molecular dynamics simulation for two-photon excitation leading to ionization. We found that with 80uJ pulse energy, the single and two-photon excitation channels are comparable, while with 130uJ pulse energy the two-photon channel dominates. Our data and simulations revealed very different dynamics and end products in the one-photon and two-photon channels.

We present photodissociation reaction dynamics utilizing rotationally-resolved Chirped-Pulse Fourier Transform Millimeter-Wave (CP-FTmmW) spectroscopy. The state-specific vibrational population distribution (VPD) of the photofragments contains mechanistic details of the reaction. VPD obtained from our experiments is complementary to dynamics experiments that measure the kinetic energy distributions of the photofragments. This complementarity to previous classic experiments yields tantalizing insights. We study photodissociation dynamics of 1,3,5-Triazine (symmetric-Triazine) to form 3 HCN molecules in a seeded supersonic jet using 266 nm radiation. The vibrational cooling inefficiency in the jet preserves the VPD of the photofragments, while rotational cooling enhances the signal of low-J pure-rotational transitions. The multiplexed nature of the spectrometer enables simultaneous sampling of several ‘vibrational satellite transitions’ that belong to different vibrational states but the same (J+1)-J rotational transition, in this case the J:1-0 transition of HCN. With at least 3.2% vibrational excitation among the photofragments, we observe a higher excitation in the bending vibrational mode of HCN (up to ν₂=6) than in the CN stretching mode (up to ν₃=2); no C-H stretching (ν₁) excitation was observed, nor was any HNC isomer population detected. The observed VPD along the even-v states of ν₂ vibrational progression is at least bimodal, implying an asymmetric partitioning of vibrational energy among the HCN photofragments. This signature in the HCN VPD indicates a sequential dissociation mechanism of symmetric-Triazine initiated by 266 nm radiation. Since VPDs are mode-specific, the adiabatic Franck-Condon projection onto the vibrational modes is capable of providing local structural information regarding the photolysis transition state geometry. The reaction mechanism, encoded in the VPD recorded as vibrational satellites by CP-FTmmW spectroscopy, could be deduced for any chemical reaction in which the products (i) are produced in the gas-phase, (ii) have a permanent dipole moment, and (iii) are small-sized and undergo inefficient vibrational cooling in the time period between the initiation of the reaction and the spectroscopic detection.

Our group is interested in characterizing the diffusion of chemical impurities in quantum solids such as solid parahydrogen (pH₂). In this work we report FTIR studies of the 193 nm photodissociation of diborane (B₂H₆) isolated in solid pH₂ in the 1.5 K to 4.3 K temperature range. In the gas phase, diborane photodissociation at 193 nm has been shown to produce BH₃ with a quantum yield of 2.00(25).M. P. Irion and K.-L. Kompa, J. Photochem. 32, 139 (1986).n our studies, we deposit B₂H₆ in solid pH₂ and fully resolve vibrational peaks for the ¹¹B₂H₆, ¹¹B¹⁰BH₆, and ¹⁰B₂H₆ isotopologs of diborane in natural abundance. We then photolyze the B₂H₆/pH₂ sample for a short period of time (30 min, 100 mW/cm²) and observe the B₂H₆ peaks decrease in intensity as new peaks grow in. We suspect that we are producing BH₃ upon photolysis which forms a complex with the pH₂ host to form BH₃-pH₂. What we found surprising is that after we stop the 193 nm laser, we observe regrowth of the diborane peaks indicating that the photoproducts can readily diffuse through solid pH₂ even at the lowest temperatures studied (T = 1.52 K). We can track the regrowth in the concentration of each diborane isotopolog separately, which means we can extract rate constants for the kinetics of recombination for each isotopolog. Assuming the recombination process is diffusion-controlled, we can use the fitted rate constants to look for kinetic isotope effects in the recombination reaction. We use these measurements to distinguish between two potential diffusion mechanisms, activated thermal hopping and quantum tunneling. This project is still ongoing, and the most recent results and analysis will be presented.

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Footnotes:

M. P. Irion and K.-L. Kompa, J. Photochem. 32, 139 (1986).I

The C₅H₅N potential energy surface is experimentally and computationally investigated through matrix isolation photolysis of 1-cyano-2-methylenecyclopropane, a pyridine isomer containing nitrile and methylenecyclopropane functional groups. Synthesis of the novel isomer 1-cyano-2-methylenecyclopropane, presented here for the first time, involves dehydration of the corresponding amide. 1-Cyano-2-methylenecyclopropane is also generated upon photoisomerization (λ > 200 nm) of (cyanomethylene)cyclopropane in argon at 19 K. Under these irradiation conditions, the photoisomerization favors the formation of 1-cyano-2-methylenecyclopropane, for which the UV absorption is blue-shifted, relative to (cyanomethylene)cyclopropane. The solution-phase UV-vis spectra and matrix-isolation IR spectra of both pure species are presented and compared to predicted spectra obtained using TD-DFT and anharmonic vibrational frequency calculations, respectively. Photoisomerization to pyridine was not observed.

In this work, the photodissociation of bromocyclopropane (BCP) was captured using gas phase ultrafast electron diffraction at the SLAC MeV-UED facility. BCP is a prototypical molecule for the study of organobromides, a class of molecules that have a significant impact on the Earth’s atmospheric ozone through their photochemistry. Thus, studying the dynamics of BCP offers a deeper understanding of organobromide photoreactivity. Previous studiesOrr-Ewing et al., J. Chem. Phys. 144, 244312 (2016).ave revealed two possible reaction pathways for the dissociation of BCP; either the bromine dissociates from the BCP molecule leaving behind a cyclopropyl ring, or there is a concerted opening of the cyclopropyl ring along with the dissociation of the bromine. Experimental results are compared to simulation for the first few picoseconds of the reaction, showing good agreement for the first picosecond. The results of this study indicate that the majority of BCP molecules follow the first reaction pathway in which the cyclopropyl ring remains closed during the bromine dissociation.

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Footnotes:

Orr-Ewing et al., J. Chem. Phys. 144, 244312 (2016).h

PhotoElectron Circular Dichroism (PECD) is a forward/backward asymmetry in the angular photoemission of an electron from a non-racemic sample, upon irradiation by circularly polarized light. In comparison to other chiroptical effects (e.g. absorptive circular dichroism), this effect is not reliant on weak interactions with the molecule’s magnetic moment, which leads to a significant increase in the sensitivity to the molecule’s chirality. This characteristic holds promise for analytical techniques, which target the study of dilute chiral samples. Additionally, the use of anions for this technique would allow for mass-selectivity and enable simple experimental schemes that employ table-top light sources, leading to a potentially robust analytical tool for chiral discrimination of multicomponent gas-phase samples. Support for PECD in anion photodetachment is limited, and knowledge of the forces that govern PECD electron dynamics in this photoemission process is missing. By coupling pre-photodetachment mass selection, tunable detachment, and velocity-map imaging-anion photoelectron spectroscopy, we provide an energy-resolved PECD signal for mass-selected anions, for the first time: In the study of the deprotonated 1-indanol anion we observed a PECD effect for many detachment channels, and a maximum PECD effect of 11%, which is similar to what has been measured for neutral species.¹ This work marks the first steps to understanding this chiral effect in this new photoemission regime.

¹Triptow, J., Fielicke, A., Meijer, G., Green, M. (2023). Imaging Photoelectron Circular Dichroism in the Detachment of Mass Selected Chiral Anions. Angew. Chem. 62, e2022120.

Recent JWST observations of icy grains in molecular clouds have highlighted the complexity of organic chemistry developing in water ice at the early stages of star formation. Following the chemical evolution of icy grain mantles as stars form can provide clues about the observed compositions of comets and icy planetary bodies. We deposited ice mixtures of water and methanol in varying ratios under cryogenic, ultrahigh vacuum conditions. We monitored the chemical composition of the ice mixtures during UV photolysis and their sublimated products during subsequent controlled warmup using infrared spectroscopy in transmission, quadrupole mass spectrometry, and rotational spectroscopy in the mm/submm-wave regime. Here we highlight the impact of the water:methanol ratio on the production of various carbon-containing volatiles (carbon dioxide, carbon monoxide, methane, formaldehyde, and the formyl radical). We find that higher proportions of water favor the production of more highly oxidized carbon-containing volatiles over their reduced counterparts.

Ultraviolet radiation facilitates chemical reactions in interstellar ice mantle during star and planet formation. These chemical reactions produce complex organic molecules (COMs) that may be essential in the production of prebiotic molecules. We are studying UV-driven chemistry in ices with the SubLIME technique, a laboratory set up that works at cryogenic conditions and ultra-high vacuum. Experiments were conducted to study the abundance ratios of organics sublimated from UV-photolyzed methanol/water ice mixtures with varying methanol concentrations. Submillimeter rotational spectroscopy, quadrupole mass spectrometry, and Fourier-transform infrared transmission spectroscopy were used to monitor the products in both the solid and gas phases. COMs were seen to form at all concentrations of methanol. However, at higher concentrations of methanol, COMs containing oxygen atoms formed quickly. In contrast, we detected COMs with higher hydrogen content at higher concentrations of water. This talk will present the experimental design and results for the ice chemistry. A second talk will discuss the impact of the water:methanol ratio on the formation of carbon-containing gas phase volatiles under the same experimental conditions.

Interstellar dust is an essential constituent in the evolution of solar systems, stars, and planets - like our Earth - but its origin and processes are still insufficiently understood. Key components of such interstellar dust are μm-sized silicates, which are mainly silicon-containing compounds, with oxygen, among others.¹ Furthermore, silicon and oxygen are the two most abundant elements in our Earth's crust. However, only molecular SiO has been identified as a possible precursor in space and circumstellar disks yet.² Experimental data and quantum chemical calculations on larger Si_nO_m⁺ cations are expected to provide information on further intermediate steps of the dust grain formation. Therefore, we focus on these and present the first results on the Si₃O₂⁺ molecular ion. The optical spectra are among the first spectroscopic information for Si_nO_m⁺ cations larger than SiO⁺ and Si₂⁺.²⁻⁵ The electronic photodissociation spectra are obtained by photodissociation spectroscopy of mass-selected ions in a tandem mass spectrometer coupled to a laser vaporization source.⁶ The data generated by action spectroscopy are compared and interpreted with TD-DFT calculations.
Literature:
¹ K. Nagashima et al., Nat. 428, 6986 (2004)
² R. W. Wilson et al., Astrophys. J. 167 (1971)
³ R. R. Reddy et al., Astrophys. Space Sci. 281, 4 (2002)
⁴ S. D. Rosner et al., J. Mol. Spectrosc. 189, 1 (1998)
⁵ T. Studemund et al., J. Phys. Chem. Lett. 13, 33 (2022)
⁶ M. Förstel et al., Rev. Sci. Instrum. 88, 12 (2017)

As a reactive radical species, NO has the potential to interact with other atmospheric molecules in unique ways. The method of experimental quantification, laser-induced fluorescence (LIF), measures NO along its A²Σ⁺← X²Π transition band. The electronic quenching of NO (A²Σ⁺) through interactions with other molecules provides alternate photochemical pathways that compete with fluorescence. Previous experimental studies demonstrated that the room temperature electronic quenching cross section of the NO (A²Σ⁺)+CO₂ system is quite large at 64 Ų. Recent experimental work by Paci et al. demonstrated that NO (A²Σ⁺)+CO₂ electronic quenching is accompanied by the formation of vibrationally excited CO₂ in its asymmetric stretching mode. However, the specific photochemical pathways responsible for this have not been identified. In this work, we develop potential energy surfaces (PESs) to identify the photochemical pathways for the electronic quenching of NO (A²Σ⁺) by CO₂. The PESs are calculated at the EOM-EA-CCSD/d-aug-cc-pVTZ//EOM-EA-CCSD/aug-cc-pVDZ level of theory. This method is well-suited for open-shell systems and provides a balanced treatment of valence and Rydberg states. Long-range interactions between the N of NO (A²Σ⁺) and the O of CO₂ are attractive, causing the two molecules to move closer together. As the intermolecular distance decreases, electronic density shifts from the 3sσ orbital of NO (A²Σ⁺) to a 2pπ^* orbital of CO₂. This phenomenon, known as the harpoon mechanism, causes increased intermolecular attractions, distortion of CO₂ into a bent conformation, and a downhill pathway to a conical intersection. Overall, our work gives new insights into the chemical physics of an atmospherically relevant system that will inform future velocity map imaging experiments.

Photocatalysts have been paid great attention owing to their excellent performance in the degradation of dangerous organic pollutants. A facile green route has been employed for the synthesis of un-doped ZnO, Ag-doped, Cu-doped, and Co-doped ZnO using curcuma (turmeric) root and moringa oleifera (drumstick) leaf extract as the reducing and stabilizing agent. These nanoparticles have been employed as photocatalytic agents to degrade the organic dyes present in the industrial wastewater under visible and ultraviolet irradiation. Herein, we compared the photocatalytic degradation activity of these nanoparticles using three industrial wastewater samples. We found that Ag-doped ZnO nanoparticles under UV-irradiation performed the best in comparison to the un-doped ZnO, Co-doped and Cu-doped ZnO nanoparticles for these samples. In addition, the antibacterial activities of the nanoparticles with moringa oleifera leaf extract were analyzed against different human pathogenic bacterial strains, E. coli C and E. coli K-12 by the optical density method.