arXiv subjects

Thomas F. Giesen

Publications and source records attributed to Thomas F. Giesen.

14 recordsLinked to original sources

arXiv2025

Heavier chalcogenofenchones for fundamental gas-phase studies of molecular chirality

Monoterpene ketones are frequently studied compounds that enjoy great popularity both in chemistry and in physics due to comparatively high volatility, stability, conformational rigidity and commercial availability. Herein, we explore the heavier chalcogenoketone derivatives of fenchone as promising benchmark systems -- synthetically accessible in enantiomerically pure form -- for systematic studies of nuclear charge ($Z$) dependent properties in chiral compounds. Synthesis, structural characterization, thorough gas-phase rotational and vibrational spectroscopy as well as accompanying quantum chemical studies on the density-functional-theory level reported in this work foreshadow subsequent applications of this compound class for fundamental investigations of molecular chirality under well-defined conditions.

physics.chem-ph↗

arXiv2025

Excited state assignment and state-resolved photoelectron circular dichroism in chalcogen-substituted fenchones

Excited electronic states of fenchone, thiofenchone, and selenofenchone are characterized and assigned with different gas-phase spectroscopic methods and \textit{ab initio} quantum chemical calculations. With an increasing atomic number of the chalcogen, we observe increasing bathochromic (red) shifts, which vary in strength for Rydberg states, valence-excited states, and ionization energies. The spectroscopic insight is used to state-resolve the contributions in multi-photon photoelectron circular dichroism with femtosecond laser pulses. This is shown to be a sensitive observable of molecular chirality in all studied chalcogenofenchones. Our work contributes new spectroscopic information, particularly on thiofenchone and selenofenchone. It may open a perspective for future coherent control experiments exploiting resonances in the visible and or near-ultraviolet spectral regions.

physics.chem-ph↗

arXiv2024

Strong coupling of a chiral molecule with circularly polarised modes inside a cavity

We consider the discriminatory interaction of a chiral molecule with circularly polarised modes inside a cavity. Starting from a generalised Jaynes--Cummings model that includes both electric and magnetic dipole couplings, we derive the Rabi frequency and associated Casimir--Polder potential for a cavity with a single mode of given handedness. One finds that both acquire a discriminatory component whose sign depends on the relative handedness of molecule and cavity mode. We generalise this result to a cavity that supports two modes of different handedness on the basis of superradiant collective field states.

quant-ph↗

arXiv2023

Opportunities for Fundamental Physics Research with Radioactive Molecules

Molecules containing short-lived, radioactive nuclei are uniquely positioned to enable a wide range of scientific discoveries in the areas of fundamental symmetries, astrophysics, nuclear structure, and chemistry. Recent advances in the ability to create, cool, and control complex molecules down to the quantum level, along with recent and upcoming advances in radioactive species production at several facilities around the world, create a compelling opportunity to coordinate and combine these efforts to bring precision measurement and control to molecules containing extreme nuclei. In this manuscript, we review the scientific case for studying radioactive molecules, discuss recent atomic, molecular, nuclear, astrophysical, and chemical advances which provide the foundation for their study, describe the facilities where these species are and will be produced, and provide an outlook for the future of this nascent field.

nucl-ex↗

arXiv2021

Accurate \textit{ab initio} calculations of RaF electronic structure indicate the need for more laser-spectroscopical measurements

Recently a breakthrough has been achieved in laser-spectroscopic studies of short-lived radioactive compounds with the first measurements of the radium monofluoride molecule (RaF) UV/vis spectra. We report results from high accuracy \emph{ab initio} calculations of the RaF electronic structure for ground and low-lying excited electronic states. Two different methods agree excellently with experimental excitation energies from the electronic ground state to the $^2Π_{1/2}$ and $^2Π_{3/2}$ states, but lead consistently and unambiguously to deviations from experimental-based adiabatic transition energy estimates for the $^2Σ_{1/2}$ excited electronic state and show that more measurements are needed to clarify spectroscopic assignment of the $^2Δ$ states.

physics.atom-ph↗

arXiv2021

The Rotationally Resolved Infrared Spectrum of TiO and Its Isotopologues

In this study, we present the ro-vibrationally resolved gas-phase spectrum of the diatomic molecule TiO around 1000\,cm$^{-1}$. Molecules were produced in a laser ablation source by vaporizing a pure titanium sample in the atmosphere of gaseous nitrous oxide. Adiabatically expanded gas, containing TiO, formed a supersonic jet and was probed perpendicularly to its propagation by infrared radiation from quantum cascade lasers. Fundamental bands of $^{46-50}$TiO and vibrational hotbands of $^{48}$TiO are identified and analyzed. In a mass-independent fitting procedure combining the new infrared data with pure rotational and electronic transitions from the literature, a Dunham-like parameterization is obtained. From the present data set, the multi-isotopic analysis allows to determine the spin-rotation coupling constant $γ$ and the Born-Oppenheimer correction coefficient $Δ_{\rm U_{10}}^{\mathrm{Ti}}$ for the first time. The parameter set enables to calculate the Born-Oppenheimer correction coefficients $Δ_{\rm U_{02}}^{\mathrm{Ti}}$ and $Δ_{\rm U_{02}}^{\mathrm{O}}$. In addition, the vibrational transition moments for the observed vibrational transitions are reported.

physics.atom-ph↗

arXiv2021

Deciphering the Rotational Spectrum of the First Excited Torsional State of Propylene Oxide

The first excited torsional state of the chiral molecule propylene oxide, $\mathrm{CH_{3}C_{2}H_{3}O}$, is investigated from millimeter up to sub-millimeter wavelengths (75-950 GHz). The first excited vibrational mode of propylene oxide, $\upsilon_{24}$, is analysed using the programs ERHAM and XIAM. Rotational constants and tunneling parameters are provided, and a description of the A-E splittings due to internal rotation is given. Furthermore, the potential barrier height to internal rotation $V_{3}$ is determined to be $V_{3} = 894.5079(259) \mathrm{cm^{-1}}$ . Our results are compared with quantum chemical calculations and literature values. We present a line list of the dense spectrum of the first excited torsional state of propylene oxide in the (sub-)millimeter range. Our results will be useful for further studies of chiral molecules in vibrationally excited states, and will enable astronomers to search for rotational transitions originating from $\upsilon_{24}$ of propylene oxide in interstellar space.

physics.chem-ph↗

arXiv2021

Laboratory blueprints for interstellar searches of aromatic chiral molecules: rotational signatures of styrene oxide

The tracking of symmetry-breaking events in space is a longlasting goal of astrochemists, aiming at an understanding of homochiral Earth chemistry. One current effort at this frontier aims at the detection of small chiral molecules in the interstellar medium. For that, high-resolution laboratory spectroscopy data is required, providing blueprints for the search and assignment of these molecules using radioastronomy. Here, we used chirped-pulse Fourier transform microwave and millimeter-wave spectroscopy and frequency modulation absorption spectroscopy to record and assign the rotational spectrum of the chiral aromatic molecule styrene oxide, $\mathrm{C_{6}H_{5}C_{2}H_{3}O}$, a relevant candidate for future radioastronomy searches. Using experimental data from the 2-12, 75-110, 170-220, and 260-330 GHz regions, we performed a global spectral analysis, complemented by quantum chemistry calculations. A global fit of the ground state rotational spectrum was obtained, including rotational transitions from all four frequency regions. Primary rotational constants as well as quartic and sextic centrifugal distortion constants were determined. We also investigated vibrationally excited states of styrene oxide, and for the three lowest vibrational states, we determined rotational constants including centrifugal distortion corrections up to the sextic order. In addition, spectroscopic parameters for the singly-substituted $^{13}$C and $^{18}$O isotopologues were retrieved from the spectrum in natural abundance and used to determine the effective ground state structure of styrene oxide in the gas phase. The spectroscopic parameters and line lists of rotational transitions obtained here will assist future astrochemical studies of this class of chiral organic molecules.

astro-ph.IM↗

arXiv2021

Mass-independent analysis of the stable isotopologues of gas-phase titanium monoxide -- TiO

More than 130 pure rotational transitions of $^{46}$TiO, $^{47}$TiO, $^{48}$TiO, $^{49}$TiO, $^{50}$TiO, and $^{48}$Ti$^{18}$O are recorded using a high-resolution mm-wave supersonic jet spectrometer in combination with a laser ablation source. For the first time a mass-independent Dunham-like analysis is performed encompassing rare titanium monoxide isotopologues, and are compared to results from high-accuracy quantum-chemical calculations. The obtained parametrization reveals for titanium monoxide effects due to deviations from the Born-Oppenheimer approximation. Additionally, the dominant titanium properties enable an insight into the electronic structure of TiO by analyzing its hyperfine interactions. Further, based on the mass-independent analysis, the frequency positions of the pure rotational transitions of the short lived rare isotopologue $^{44}$TiO are predicted with high accuracy, i.e., on a sub-MHz uncertainty level. This allows for dedicated radio-astronomical searches of this species in core-collapse environments of supernovae.

physics.chem-ph↗

arXiv2021

The Millimeter-Wave Spectrum of Doubly Deuterated Propylene Oxide CH$_3$CHCD$_2$O

Spectra of doubly deuterated propylene oxide, CH$_3$CHCD$_2$O, were recorded in the millimeter-wave spectral region up to 330 GHz utilizing a 2f frequency modulated Terahertz spectrometer. Rotational and centrifugal distortion constants and tunneling parameters for the description of the internal rotation of the methyl group were retrieved. The software XIAM was used to describe the A-E tunneling splitting with a frequency uncertainty of 82 kHz. Molecular parameters derived from our measurements are complemented by quantum chemical anharmonic frequency calculations on the B3LYP/augcc-pVTZ level of theory. In addition, the barrier height to internal rotation of the methyl group,V3, was derived experimentally and found to slightly differ from the value reported for the main isotopologue. In view of astrophysical observations we provide accurate line lists to search for doubly deuterated propylene oxide in space. Furthermore, we deliver improved molecular parameters to further investigate the properties of a simple chiral molecule possessing internal large amplitude motions.

physics.chem-ph↗

arXiv2020

Determination of accurate rest frequencies and hyperfine structure parameters of cyanobutadiyne, HC$_5$N

Very accurate transition frequencies of HC$_5$N were determined between 5.3 and 21.4 GHz with a Fourier transform microwave spectrometer. The molecules were generated by passing a mixture of HC$_3$N and C$_2$H$_2$ highly diluted in neon through a discharge valve followed by supersonic expansion into the Fabry-Perot cavity of the spectrometer. The accuracies of the data permitted us to improve the experimental $^{14}$N nuclear quadrupole coupling parameter considerably and the first experimental determination of the $^{14}$N nuclear spin-rotation parameter. The transition frequencies are also well suited to determine in astronomical observations the local speed of rest velocities in molecular clouds with high fidelity. The same setup was used to study HC$_7$N, albeit with modest improvement of the experimental $^{14}$N nuclear quadrupole coupling parameter. Quantum chemical calculations were carried out to determine $^{14}$N nuclear quadrupole and spin-rotation coupling parameters of HC$_5$N, HC$_7$N, and related molecules. These calculations included evaluation of vibrational and relativistic corrections to the non-relativistic equilibrium quadrupole coupling parameters; their considerations improved the agreement between calculated and experimental values substantially.

astro-ph.IM↗

arXiv2019

First detection of the carbon chain molecules 13CCC and C13CC towards SgrB2(M)

We report the first detection of the isotopologues 13CCC and C13CC. We used the heterodyne receivers GREAT and upGREAT on board SOFIA to search for the ro-vibrational transitions Q(2) and Q(4) of 13CCC and C13CC at 1.9 THz along the line of sight towards SgrB2(M). For both species the ro-vibrational absorption lines Q(2) and Q(4) have been identified, primarily arising from the warm gas physically associated with the strong continuum source SgrB2(M). In addition, to determine the local excitation temperature we analyzed data from nine ro-vibrational transitions of the main isotopologue CCC in the frequency range between 1.6-1.9 THz which were taken from the Herschel Science Data Archive, and derived a gas excitation temperature of Tex = 44.4(+4.7/-3.9) K and a total column density of N(CCC)=3.88(+0.39/-0.35)x10^15 cm^-2. Assuming the excitation temperatures of C13CC and 13CCC to be the same as for CCC, we obtained column densities of the 13C-isotopologues of N(C13CC) = 2.1(+0.9/-0.6)X10^14 cm^-2 and N(13CCC)=2.4(+1.2/-0.8)x10^14 cm^-2. The derived 12C/13C abundance ratio in the C3 molecules is 20.5(4.2), which is in agreement with the elemental ratio of 20, typically observed in SgrB2(M). However, we find the N(13CCC) / N(C13CC) ratio to be 1.2(0.1), which is shifted from the statistically expected value of 2. We propose that the discrepant abundance ratio arises due to the lower zero-point energy of C13CC which makes position exchange reaction converting 13CCC to C13CC energetically favorable.

astro-ph.GA↗

arXiv2019

Principles of enantio-selective excitation in three-wave mixing spectroscopy of chiral molecules

Three-wave mixing spectroscopy of chiral molecules, which exist in left-handed and right-handed conformations, allows for enantio-selective population transfer despite random orientation of the molecules. This is based on constructive interference of the three-photon pathways for one enantiomer and destructive one for the other. We prove here that three mutually orthogonal polarization directions are required to this end. Two different dynamical regimes exist to realize enantio-selective population transfer, and we show that they correspond to different phase conditions in the three-wave mixing. We find the excitation scheme used in current rotational three-wave mixing experiments of chiral molecules with $C_1$ symmetry to be close to optimal and discuss prospects for ro-vibrational three-wave mixing experiments of axially chiral molecules. Our comprehensive study allows us to clarify earlier misconceptions in the literature.

quant-ph↗

arXiv2018

Astronomical detection of a radioactive molecule 26AlF in a remnant of an ancient explosion

Decades ago, $γ$-ray observatories identified diffuse Galactic emission at 1.809 MeV originating from $β^+$ decays of an isotope of aluminium, $^{26}$Al, that has a mean-life time of 1.04 Myr. Objects responsible for the production of this radioactive isotope have never been directly identified, owing to insufficient angular resolutions and sensitivities of the $γ$-ray observatories. Here, we report observations of millimetre-wave rotational lines of the isotopologue of aluminium monofluoride that contains the radioactive isotope ($^{26}$AlF). The emission is observed toward CK Vul which is thought to be a remnant of a stellar merger. Our constraints on the production of $^{26}$Al combined with the estimates on the merger rate make it unlikely that objects similar to CK Vul are major producers of Galactic $^{26}$Al. However, the observation may be a stepping stone for unambiguous identification of other Galactic sources of $^{26}$Al. Moreover, a high content of $^{26}$Al in the remnant indicates that prior to the merger, the CK Vul system contained at least one solar-mass star that evolved to the red giant branch.

astro-ph.SR↗