Session D03: Spectroscopy in Space and Time II; Modern Microwave Spectroscopy & Astrochemistry Investigations

Exploiting the Intensity Stability of Broadband Rotational Spectroscopy to Analyze Chemical Mixtures

The introduction of chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy improved the ability to analyze complex chemical mixtures by making it possible to acquire a broadband spectrum in each measurement acquisition. The availability of a broadband spectrum makes it possible to identify components of the mixture through their rotational spectroscopy signature. The ability to measure the full spectrum on each data acquisition combats the challenges caused by drift in the sample sources that can cause time variability in the sample composition. Early applications of broadband rotational spectroscopy to complex mixtures included analyzing reaction products in electric discharge reactions related to astrochemistry and identifying the isomers of water clusters in the size range 2-15. Over time, an appreciation grew for the long-term stability in the relative intensities in spectra from CP-FTMW spectrometers. It is common to observe 0.1% reproducibility in the relative intensities across the full frequency range of the spectrometer. This feature of CP-FTMW instruments has enabled a new set of experimental approaches to the analysis of complex mixtures. The common measurement concept is the variation of the sample composition through control of an external parameter. For traditional chemical mixtures, like volatiles in essential plant oils, the composition of the vapor entrained by the inert carrier gas can be achieved by varying the temperature of the liquid. Analysis of the time variation in the intensity of each transition makes it possible to cluster the peaks associated with each chemically distinct species. In a more difficult analysis challenge, the relative composition of two gases can be varied to help identify the set of mixed clusters that can form in a pulsed jet expansion. Isotope dilution can also be used in the analysis of clusters to provide information about the number of monomers in each cluster – an approach recently used to identify a family of water-14 isomers. Finally, variation of the enantiomer distribution of a sample can be used to quantitatively measure the enantiomeric excess of chiral sample without the need for any spectroscopic analysis.   

Noble gas atom as a quantum oscillator in the fullerene molecular cage: THz spectroscopy study.

Nobel gas endofullerene provides a unique opportunity to study the non-covalent interaction between atoms and fullerene cages. Here, we use THz spectroscopy to study the interaction between noble gas atoms (³He, ⁴He, Ne, Ar, Kr) and the fullerene cage. The temperature dependence of the THz absorption spectra of powdered samples were measured between 5 to 300K.  The translational motion of the atom quantized by the confining potential of C₆₀ defines the THz spectrum.  The spectra of the He isotopes exhibit clearly resolved THz peaks at high temperatures, indicating that the potential energy function V(r) for the encapsulated atom is anharmonic [1,2]. Other studied atoms display one broad peak at elevated temperatures which is also an indication of the anharmonicity of the potential. We fitted the THz absorption spectra with a three-dimensional anharmonic oscillator model including translation-induced dipole moment. The fit results show that the harmonic potential term increases with the size of the noble gas atom. Our results are a valuable input to the quantum calculations of non-bonded intermolecular interactions.

    

Unbiased Molecular Discovery: Laboratory and Machine Learning Approaches to Expanding Our View of Interstellar Chemistry

As the sensitivity and spectral grasp of our radio telescopes continues to expand, so too does our window on the shocking degree of chemical complexity shaping and being shaped by the process of forming stars and planets. Last year we reported the first detections of individual polycyclic aromatic hydrocarbon molecules in space. These species are thought to be the reservoir of a vast fraction of interstellar organic carbon, but our understanding of the chemistry leading to their formation is shockingly limited, largely by a dearth of detections of both these kinds of molecules and their probably reaction intermediates. Here, I will describe how our collaboration -- GOTHAM -- is tackling this challenge on two fronts. In the laboratory, our team members are using Microwave Spectral Taxonomy to perform unbiased reaction screening studies to identify new molecules of potential interest and provide the spectra necessary to study them observationally. As well, our team has developed novel machine learning approaches to reproducing and, critically, predicting, the chemical inventories of molecularly rich interstellar sources with astonishing accuracy.   

SubLIME: Sublimation Laboratory Ice Millimeter/submillimeter Experiment

To test possible chemical mechanisms in interstellar ices, we have built a novel laboratory experiment that couples the traditional tools of ice studies – FTIR spectroscopy and mass spectrometry – with the structure specificity of rotational spectroscopy.  Such measurements can provide the “ground truth” to guide observations of star- and planet-forming zones.  This novel experimental approach has been benchmarked with pure ice studies of water and methanol, and initial photoprocessing results for a pure methanol ice have been obtained.  A complex network of organic chemistry was observed and offers fascinating hints at the processes driving interstellar prebiotic chemistry.  Our current studies investigate mixed ices of varying ratios of water and methanol to better simulate interstellar chemistry.  In this talk I will present the experimental design and the results of our laboratory studies, and compare these results to observations.  I will then discuss these results and our future work in the context of prebiotic astrochemistry.   

Two-dimensional terahertz spectroscopy of vibrational modes in a molecular solid.

Molecular vibration modes are a basic excitation in molecular solids. It’s coupling with other dynamic process influences the relaxation channel of atoms, electrons and spins.

Two-dimension (2D) THz is a useful method to study molecular  vibrations, rotations, dielectric relaxations, electron and spin excitations, and particularly to reveal the coupling between different degrees of freedom. By using high field THz pulses, we have measured the 2D nonlinear transmission spectrum on a single molecular magnet powder, and analyzed the different contributions to the nonlinear signal.  The 2D nonlinear signal indicates a dynamic coupling between the vibrational modes and other degrees of freedom.   

Tuning quantum coherence time scales in molecules via light-matter hybridization

Protecting quantum coherences in matter from the detrimental effects introduced by its environment is essential to employ molecules and materials in quantum technologies and develop enhanced spectroscopies. Here, we show how dressing molecular chromophores with quantum light in the context of optical cavities can be used to generate quantum superposition states with  tunable coherence time scales that are longer than those of the bare molecule, even at room temperature and for molecules immersed in solvent. 

For this, we develop a theory of decoherence rates for molecular polaritonic states and demonstrate that quantum superpositions that involve such hybrid light-matter states can survive for times that are orders of magnitude longer than those of the bare molecule while remaining optically controllable. Further, by studying these tunable coherence enhancements in the presence of lossy cavities we demonstrate that they can be enacted using present-day optical cavities. The analysis offers a viable strategy to engineer and increase quantum coherence lifetimes in molecules. 

    

Transient Raman studies of hemibonded thiopyrimidine dimer radicals in water

Structures of dimeric thiopyrimidine intermediates were examined by means of transient Raman spectroscopy and DFT calculations. Neutral and anionic dimer radicals were prepared by pulse radiolysis of aqueous 2-thiopyrimidine (TP) solutions via one electron oxidation at pH 4 and 10, respectively. Raman spectra of these radicals were obtained in resonance with their pronounced electronic transitions using 415 and 500 nm as excitation wavelengths. Fifteen Raman bands of neutral dimer radical (TP)₂^• and twelve Raman bands of dimer radical anion (TP)₂^•- were observed in the 80-1800cm^-1 region and interpreted in conjunction with our previous studies of thiourea dimeric intermediate models as well as theoretical calculations. The highest enhancement of bands at 212 or ~214 cm^-1 in (TP)₂^• or (TP)₂^•-, respectively, indicates that S-S hemibond stretch undergoes greatest geometrical change upon electronic excitation. This confirms supposition that excited states of these intermediates have mostly SS antibonding character. The pattern of relative intensities of higher frequency bands to SS stretch frequency provides insight into coupling of other molecular vibrations to SS displacement coordinate, by which we gain information upon change in relative structural arrangement of pyrimidine rings in solution upon deprotonation at higher pH.   

Optoelectronic Characterization of tunable Luminescent Organic Molecules

Luminescent organic molecules (LOMs) combine the photo-induced charge carrier generation and recombination of inorganic semiconductors with the lightweight and easy fabrication techniques common to organic materials. We present results of a novel class of LOMs, Benzoyl Pyraziniums, whose optical properties can be controlled by tuning the electronic and chemical structure, excitation energy, and their concentration and interaction with surrounding materials. Optical characterization of different salts of the LOMs showed increase in PL intensity, blue shift in emission spectra, and increase in recombination lifetimes with decreasing concentration. As pyridiniums are capable of intramolecular charge transfer, assuming that the primary concentration dependent non-radiative decay is through the same, this behavior can be explained by variation in the number of non-radiative decay pathways with concentration. Cumulative emission studies in solid state, to study their behavior as composites in the mesoscale regime, showed spectral blue shift in the order of just a few nanometers over the entire range of 0.1 – 100 mM but with significantly narrowed emission. Further investigations of the electrochemistry of these molecules offer unique perspectives on the charge transfer properties within them, which is used to optimize their optical emission. 

This work was supported with funding from: NSF-CREST: Center for Cellular and Biomolecular Machines at UC Merced (HRD-1547848 and 2112675)     

Perturbative model of transient Circular Dichroism of molecular aggregates

Microscopic, quantum-mechanical models of spectroscopic techniques allow researchers to interpret the information contained within a measured spectrum.  Theoretical models of transient circular dichroism (TCD) spectroscopy—an analogue to transient absorption (TA) spectroscopy—are relatively understudied and would aid in the interpretation of signals arising from laboratory TCD measurements. Steady-state CD spectra can report on the chirality induced by electronic coupling between chromophores in molecular aggregates, and it is therefore anticipated that TCD can report on the electronic and orientational dynamics of excitons in molecular aggregates. In this work [1], we derived a perturbative theoretical model of femtosecond TCD spectroscopy, with some similarities and differences to prior works [2-4]. We study the signals that arise from small molecular aggregates including dimers, trimers, and tetramers. The results indicate that TCD can produce new insights into the electronic and structural orientation properties of molecular aggregates. 

[1] Arpin and Turner, J. Chem. Phys. 157, 154101 (2022)

[2] Cho, J. Chem. Phys. 119, 7003 (2003).

[3] Abramavicius et al., J. Chem. Phys. 124, 034113 (2006).

[4] Holdaway et al., J. Chem. Phys. 144, 194112 (2016).