arXiv subjects

Melanie Schnell

Publications and source records attributed to Melanie Schnell.

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

Direct observation of time-dependent coherent chiral tunneling dynamics

Superpositions of handed molecular states give rise to achiral eigenstates, delocalized across a double-well potential via tunneling. A coherent superposition of these energy eigenstates could dynamically relocalize the molecules into chiral states, which has only been addressed theoretically. Here, we present a microwave six-wave mixing pump-probe study to create and probe coherent chiral tunneling dynamics in a rotational state. Through a time-resolved scheme, we uncover the periodic time evolution of the induced chiral wavepacket under field-free conditions. Moreover, we demonstrate precise phase control of this coherence via phase modulation during pump excitation.

quant-ph↗

arXiv2023

Towards detection of molecular parity violation by microwave spectroscopy of CpRe(CH$_{3}$)(CO)(NO)

Parity-violating differences in rotational constants of a chiral 5d transition metal complex, that was previously experimentally well-characterised by broad-band microwave spectroscopy, are predicted with a recently established efficient analytical derivative technique. Relative differences $ΔX/X$ between rotational constants $X=A,B,C$ of enantiomers of the title compound are found to be on the order of $10^{-14}$, which is a favourably large effect. The quality of the theoretical estimates is carefully assessed by computing nuclear electric quadrupole coupling constants that agree well with experiment.

physics.chem-ph↗

arXiv2022

The Kinetic Energy of PAH Dication and Trication Dissociation Determined by Recoil-Frame Covariance Map Imaging

We investigated the dissociation of dications and trications of three polycyclic aromatic hydrocarbons (PAHs), fluorene, phenanthrene, and pyrene. PAHs are a family of molecules ubiquitous in space and involved in much of the chemistry of the interstellar medium. In our experiments, ions are formed by interaction with 30.3 nm extreme ultraviolet (XUV) photons, and their velocity map images are recorded using a PImMS2 multi-mass imaging sensor. Application of recoil-frame covariance analysis allows the total kinetic energy release (TKER) associated with multiple fragmentation channels to be determined to high precision, ranging 1.94-2.60 eV and 2.95-5.29 eV for the dications and trications, respectively. Experimental measurements are supported by Born-Oppenheimer molecular dynamics (BOMD) simulations.

physics.chem-ph↗

arXiv2021

State-specific Enrichment of Chiral Conformers with Microwave Spectroscopy

An interesting class of molecules is that in which the molecules do not possess a stereogenic center but can become chiral due to their spatial arrangement. These molecules can be seen as chiral conformers, whose two non-superimposable forms can interconvert from one another by rotations about single bonds. Here, we show that an initially racemic mixture of chiral conformers, such as a sample of cyclohexylmethanol, C$_{7}$H$_{14}$O (CHM), can be enantiomerically enriched by performing the enantio-selective process of coherent population transfer between rotational levels. By first performing a population transfer cycle, followed by a three-wave mixing experiment, we show that an enantiomeric excess in a rotational level of choice can be achieved. This represents the first experimental demonstration of such an effect in a chiral pair of conformers, and it showcases the broad applicability of three-wave mixing not only for analytical applications but also to a wide scope of experiments of fundamental interest.

physics.chem-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↗

arXiv2020

Complete Controllability Despite Degeneracy: Quantum Control of Enantiomer-Specific State Transfer in Chiral Molecules

We prove complete controllability for rotational states of an asymmetric top molecule belonging to degenerate values of the orientational quantum number M. Based on this insight, we construct a pulse sequence that energetically separates population initially distributed over degenerate M-states, as a precursor for orientational purification. Introducing the concept of enantio-selective controllability, we determine the conditions for complete enantiomer-specific population transfer in chiral molecules and construct pulse sequences realizing this transfer for population initially distributed over degenerate M-states. This degeneracy presently limits enantiomer-selectivity for any initial state except the rotational ground state. Our work thus shows how to overcome an important obstacle towards separating, with electric fields only, left-handed from right-handed molecules in a racemic mixture.

quant-ph↗

arXiv2012

Decelerating molecules with microwave fields

We here report on the experimental realization of a microwave decelerator for neutral polar molecules, suitable for decelerating and focusing molecules in high-field-seeking states. The multi-stage decelerator consists of a cylindrical microwave cavity oscillating on the TE 11n mode, with n=12 electric field maxima along the symmetry axis. By switching the microwave field on and off at the appropriate times, a beam of state-selected ammonia molecules with an incident mean velocity of 25 m/s is guided while being spatially focussed in the transverse direction and bunched in the forward direction. Deceleration from 20.0 m/s to 16.9 m/s and acceleration from 20.0 m/s to 22.7 m/s is demonstrated.

physics.chem-ph↗

arXiv2010

Microwave Lens for Polar Molecules

We here report on the implementation of a microwave lens for neutral polar molecules suitable to focus molecules both in low-field-seeking and in high-field-seeking states. By using the TE_11m modes of a 12 cm long cylindrically symmetric microwave resonator, Stark-decelerated ammonia molecules are transversally confined. We investigate the focusing properties of this microwave lens as a function of the molecules' velocity, the detuning of the microwave frequency from the molecular resonance frequency, and the microwave power. Such a microwave lens can be seen as a first important step towards further microwave devices, such as decelerators and traps.

physics.chem-ph↗

arXiv2009

Nonadiabatic transitions in electrostatically trapped ammonia molecules

Nonadiabatic transitions are known to be major loss channels for atoms in magnetic traps, but have thus far not been experimentally reported upon for trapped molecules. We have observed and quantified losses due to nonadiabatic transitions for three isotopologues of ammonia in electrostatic traps, by comparing the trapping times in traps with a zero and a non-zero electric field at the center. Nonadiabatic transitions are seen to dominate the overall loss rate even for samples at relatively high temperatures of 10-50 mK.

physics.atom-ph↗

arXiv2008

Precise dipole moments and quadrupole coupling constants of the cis and trans conformers of 3-aminophenol: Determination of the absolute conformation

The rotational constants and the nitrogen nuclear quadrupole coupling constants of cis-3-aminophenol and trans-3-aminophenol are determined using Fourier-transform microwave spectroscopy. We examine several $J=2\leftarrow{}1$ and $1\leftarrow{}0$ hyperfine-resolved rotational transitions for both conformers. The transitions are fit to a rigid rotor Hamiltonian including nuclear quadrupole coupling to account for the nitrogen nucleus. For cis-3-aminophenol we obtain rotational constants of A=3734.930 MHz, B=1823.2095 MHz, and C=1226.493 MHz, for trans-3-aminophenol of A=3730.1676 MHz, B=1828.25774 MHz, and C=1228.1948 MHz. The dipole moments are precisely determined using Stark effect measurements for several hyperfine transitions to $μ_a=1.7735$ D, $μ_b=1.5195$ D for cis-3-aminophenol and $μ_a=0.5563$ D, $μ_b=0.5376$ D for trans-3-aminophenol. Whereas the rotational constants and quadrupole coupling constants do not allow to determinate the absolute configuration of the two conformers, this assignment is straight-forward based on the dipole moments. High-level \emph{ab initio} calculations (B3LYP/6-31G^* to MP2/aug-cc-pVTZ) are performed providing error estimates of rotational constants and dipole moments obtained for large molecules by these theoretical methods.

physics.chem-ph↗

arXiv2007

Precise dipole moment and quadrupole coupling constants of benzonitrile

We have performed Fourier transform microwave spectroscopy of benzonitrile, without and with applied electric fields. From the field-free hyperfine-resolved microwave transitions we simultaneously derive accurate values for the rotational constants, centrifugal distortion constants, and nitrogen nuclear quadrupole coupling constants of benzonitrile. By measuring the Stark shift of selected hyperfine transitions the electric dipole moment of benzonitrile is determined to $μ=μ_a=4.5152 (68)$ D.

physics.chem-ph↗

arXiv2006

A versatile electrostatic trap

A four electrode electrostatic trap geometry is demonstrated that can be used to combine a dipole, quadrupole and hexapole field. A cold packet of 15ND3 molecules is confined in both a purely quadrupolar and hexapolar trapping field and additionally, a dipole field is added to a hexapole field to create either a double-well or a donut-shaped trapping field. The profile of the 15ND3 packet in each of these four trapping potentials is measured, and the dependence of the well-separation and barrier height of the double-well and donut potential on the hexapole and dipole term are discussed.

physics.atm-clus↗