In recent years, optical microresonators with exquisite sensitivity have grown into powerful platforms for label-free sensing and imaging, including reaching the single-molecule detection limit. Combining optical microresonators with spectroscopic measurements on nanoscale objects adds chemical identification to label-free detection schemes, offering deeper insights into their fundamental chemical, biological, material, and photonic properties. Particularly, single-molecule measurements allow distinct observations of unsynchronized chemical dynamics, properties otherwise obscured in bulk measurements. Simultaneously, optical microresonators are also flexible playgrounds for exploring nanophotonic phenomena and quantum optics. I will tell two stories focusing on microbubble resonators. In the first, I will describe how microbubble resonators can be used to watch chemical dynamics of single nanoparticles. In the second, I will describe how microbubble resonators allow solvent control of photonic-plasmonic hybridization.

Polar molecules with sufficiently large dipole moments can form highly diffuse dipole-bound anions. Dipole-bound anions possess noncovalent dipole-bound states (DBSs) just below the detachment threshold by the long-range electron-dipole interaction. The diffuse electron in a DBS is spatially well separated from the valence electrons and is known to have negligible effects on the DBS’s molecular structure. Electron correlation effects between the distant dipole-bound electron and the valence electrons of the neutral cores are known to be important for the accurate calculation of the binding energies of the dipole-bound electron. However, how the oriented intramolecular electric field of the dipole-bound electron influences the valence electrons has not been examined. We present the observation of a DBS in deprotonated 4-(2-phenylethynyl)-phenoxide anions. The photodetachment of the dipole-bound electron is observed to accompany a simultaneous shakeup process in valence orbitals in this aromatic molecular anion. This shakeup process is due to configuration mixing as a result of valence orbital polarization by the intramolecular electric field of the dipole-bound electron.

The interplay between chemical functionality and structure is a key factor in the photophysics and photochemistry of complex, flexible molecules, often giving rise to multiple potential energy surfaces. Adequate description of this relationship to understand the outcomes and properties of polyatomic molecules is made even more difficult as the number of isomers and conformations increase substantially with the size of the system. The inclusion of water-mediated interactions is often needed due to dramatic effects on the conformational preferences and photophysics. Therefore, the synergy between spectroscopy and chemical dynamics methods is required to obtain a molecular-level view of such complex chemical systems. To address these opportunities, we will illustrate our efforts to investigate the intermolecular interactions of molecular complexes using single-conformation spectroscopy and dynamics techniques to probe the photo-initiated outcomes on multiple potential energy surfaces. Thus, the photophysical, photochemical and structural details of the target conformational isomers and complexes enable multifaceted comparisons to several theoretical predictions.

The importance of bare and hydrogenated carbon clusters in combustion and in the chemistry of interstellar space has motivated numerous spectroscopic studies, most of which have focused on smaller neutral and charged clusters with fewer than 10 carbon atoms and on the and fullerenes. Recently, we have obtained electronic spectra of bare and hydrogenated carbon cation clusters containing between up to 36 carbon atoms. Spectroscopically interrogating carbonaceous molecules containing more than 10 carbon atoms is complicated by the coexistence of different isomers possessing unique spectroscopic properties. To address this issue, we have developed an apparatus that allows formation and selection by ion mobility of a particular isomer population, which is 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 used this approach to obtain electronic spectra for monocyclic clusters with 12 ≤ n ≤ 36, which exhibit sharp transitions that progressively shift to longer wavelength with increasing cluster size. We have also probed clusters, which are shown to exist as linear and cyclic isomers with distinct electronic spectra. Linear isomers (7 ≤ n ≤ 17), feature sharp, intense absorptions across the UV and visible range, whereas cyclic isomers (n ≥ 15) have much weaker, and broader absorptions. Addition of more hydrogen atoms precipitates formation of bi-cyclic structures that may be precursors of polycyclic aromatic hydrocarbons.

l0pt Figure The bottom-up formation of polycyclic aromatic hydrocarbons (PAHs) in combustion and interstellar gas-phase environments is still subject of extensive debate. As a result, accounting for PAH abundance in soot and the interstellar medium is far from trivial. Over the past years, a number of new reaction mechanisms have been proposed, spectroscopically identified and added as possible growth routes for larger PAHs.Kaiser, R. I., Hansen, N. (2021), JPC A, 125(18), 3826-3840hese include reactions with radical phenyl rings and several small hydrocarbon radicals. We show here that a combination of these barrierless reactions is necessary to fully describe the chemistry leading to PAH growth. To this purpose we follow and characterize in our experiments the formation of PAHs in an electrical discharge. The fragments, products and reactive intermediates that are produced in this discharge are entrained in a molecular beam and structurally identified by mass selective IR-UV spectroscopy using an IR Free Electron Laser. Comparison of the mass-selected IR absorption spectra with IR spectra calculated for potential species associated with each of these spectra enables us to identify products including larger PAHs, radicals, and intermediates. The assigned structures serve as promising candidates for radio astronomical searches and highlight the necessity of describing PAH growth by an interconnected network of pathways.Lemmens, A. K., Rap, D. B., Thunnissen, J. M., Willemsen, B., Rijs, A. M. (2020), Nat. Comm., 11(1), 1-7html:<hr /><h3>Footnotes: Kaiser, R. I., Hansen, N. (2021), JPC A, 125(18), 3826-3840T Lemmens, A. K., Rap, D. B., Thunnissen, J. M., Willemsen, B., Rijs, A. M. (2020), Nat. Comm., 11(1), 1-7

The photochemistry of flexible molecular complexes, such as a nitric oxide:methane (NO:CH₄), are defined by potential energy surfaces that depend on the chemical functionality and the relative orientation of conformational isomers. Experimental results are required to test and improve modern theoretical methods for accurate prediction of photoinitiated processes and chemical mechanisms. A thorough understanding of the intermolecular interactions and reaction mechanism outcomes can be obtained by elucidating the conformations adopted by NO:CH₄ molecular complexes. Moreover, by investigating the specific vibrational modes inherent to NO:CH₄ conformational isomers, we can assess their impact on energy transfer following fragmentation of the molecular complex isomers. We will leverage a synergy of laser-induced spectroscopy and chemical dynamics techniques, in particular conformation-specific infrared spectroscopy and velocity map imaging, to understand these fundamental mechanisms and dynamics at play within NO:CH₄ and NO:alkane complexes more broadly. Ultimately, we will gain insights into the mode-specific energy transfer pathways following fragmentation of NO:CH₄ molecular complex isomers. Furthermore, our experimental results will be compared to several theoretical approaches in order to reveal the multifaceted signatures of dynamical events using spectroscopy probes.

This research attempts to electronic and vibrational properties of metal complexes of chromone Schiff base ligands were calculated using the gauge independent atomic orbital (GIAO) method. Using time-dependent density functional theory, the theoretical electronic absorption spectra were determined. To obtain information about the ability of the molecule to react with chemicals, Frontier Molecular Orbital properties, energies, descriptors, and total/partial state density diagram were obtained. The charge distribution and chemical reactivity sites were visualized monitored by mapping electron density isosurface with electrostatic potential surfaces (ESP). To learn nonlinear optical properties (NLO), the polarizability and hyper polarizability tensors of the complexes were computed using density functional (DFT) theory at mPW1PW91 6-311+G(d,p) and LanL2DZ level. The study's second section focused on vibrational spectroscopic analysis. To fit the calculated harmonic wavenumbers with the observed Fourier Transform Infrared (FT-IR) and Raman spectra in the solid phase of the complexes, the calculated harmonic force constants were refined using the Scaled Quantum Mechanical Force Field (SQM-FF) procedure. When combined with the results of the SQM approach, it is possible to create a comprehensive assignment of the observed spectra.

The indenyl and phenylpropargyl radicals are the most stable isomers of C₉H₇ and have emerged as ubiquitous products in flames and hydrocarbon discharges. The o-, m-, and p- ethynylbenzyl isomers lie ca. 150-160 kJ/mol and 20-30 kJ/mol above the 1-indenyl and 1-phenylpropargyl minima, respectively, and can presumably form by barrierless addition of CH radical to phenylacetylene, but they are thoroughly unexamined spectroscopically. We have recently observed the D₀−D₁ optical transitions of the para and meta variants by resonant two-color two-photon ionization and laser-induced fluorescence / dispersed fluorescence spectroscopy. For the para form, extension of the benzyl chromophore by a C₂ unit engenders a relatively (w.r.t. benzyl) strong transition that can in large part be rationalized on Franck-Condon premises; while most of the vibronic structure in the much weaker transition of the meta isomer arises from intensity-borrowing among totally symmetric modes that are only weakly FC-active. Modes of a₁ symmetry of para-ethynylbenzyl are subject to pervasive Fermi resonances, as is established by DF spectroscopy. In the jet-cooled discharge, both para and meta forms are found in coexistence with 1-phenylpropargyl (which one might call α-ethynylbenzyl) at levels that cannot be explained by putative sample impurities unless computed oscillator strengths are too small by several orders of magnitude, suggesting rearrangement of all three radicals via an intermediate that remains unobserved. The para form has not been detected at the time of writing, perhaps because it cyclizes to indenyl.

Since gas-phase hydrogen-bonded clusters are treated as a microscopic model of hydrogen bond networks, a huge number of spectroscopic studies have been performed, so far. Structural fluctuations are one of features of hydrogen bond network. Such fluctuations correspond to isomerizations among isomers having distinct hydrogen bond structures in the cases of clusters. To elucidate dynamical aspects of microscopic hydrogen-bond networks, we have been investigating an IR-induced isomerization of phenol-methanol cluster cations, [PhOH(MeOH₃)]⁺, trapped in a cold ion trap, and its backward reaction. In our experiment, we recorded UV-photodissociation (PD) spectra of [PhOH(MeOH₃)]⁺ with and without IR excitation. An isomerization from the ring- to chain-type isomers can be observed as a decrease in the band intensity of the ring-type isomer and also an increase in that of the chain-type isomer in the UV-PD spectra. In the last symposium, we reported a clear evidence of the isomerization process and also its backward reaction.M. Ozeki, et al. RM14, International Symposium on Molecular Spectroscopy, (2021).n the present experiment, we carefully examined time propagations of the spectra. As a result, we observed a rapid generation of the chain-type isomers and a change in the UV-PD spectral profiles of the chain-type isomer in the order of 10 μs. This is due to the re-cooling and the backward reaction of the chain-type isomers within a cold trap. We estimated the temperatures of the chain-type isomers during the re-cooling process by referring to the UV-PD spectra of [PhOH(MeOH₃)]⁺ measured at various temperatures.M. Orito, et al. RM13, International Symposium on Molecular Spectroscopy, (2021).etails of the observations are presented in the paper.

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Footnotes:

M. Ozeki, et al. RM14, International Symposium on Molecular Spectroscopy, (2021).I M. Orito, et al. RM13, International Symposium on Molecular Spectroscopy, (2021).D