arXiv subjects

Adam S. Chatterley

Publications and source records attributed to Adam S. Chatterley.

10 recordsLinked to original sources

arXiv2023

Nonadiabatic Laser-Induced Alignment Dynamics of Molecules on a Surface

We demonstrate that a sodium dimer, Na$_{2}$($1^3\Sigma_{u}^+$), residing on the surface of a helium nanodroplet, can be set into rotation by a nonresonant 1.0 ps infrared laser pulse. The time-dependent degree of alignment measured, exhibits a periodic, gradually decreasing structure that deviates qualitatively from that expected for gas phase dimers. Comparison to alignment dynamics calculated from the time-dependent rotational Schr\"{o}dinger equation shows that the deviation is due to the alignment dependent interaction between the dimer and the droplet surface. This interaction confines the dimer to the tangential plane of the droplet surface at the point where it resides and is the reason that the observed alignment dynamics is also well-described by a 2D quantum rotor model.

physics.atm-clus↗

arXiv2023

Generation of Large Vortex-Free Superfluid Helium Nanodroplets

Superfluid helium nanodroplets are an ideal environment for the formation of metastable, self-organized dopant nanostructures. However, the presence of vortices often hinders their formation. Here, we demonstrate the generation of vortex-free helium nanodroplets and explore the size range in which they can be produced. From x-ray diffraction images of xenon-doped droplets, we identify that single compact structures, assigned to vortex-free aggregation, prevail up to $10^8$ atoms per droplet. This finding builds the basis for exploring the assembly of far-from-equilibrium nanostructures at low temperatures.

cond-mat.mes-hall↗

arXiv2022

Can vibrational sum frequency scattering spectra be measured from the surface of 40-100 nm aerosols in a cloud containing 10$^{6}$ particles/mL?

Understanding the interfacial properties of aerosol particles is important for science and medicine, crucial for air quality, human health, and environmental chemistry. Qian et al. presented vibrational sum frequency scattering (SFS) measurements of organic molecules on aerosol particles.$^{1}$ Relating an aerosol sample with a 40 nm average size and 10 - 300 nm particle size distribution at a density of 10$^{6}$ particle/mL to vibrational sum frequency scattering spectra recorded in a different apparatus, it was concluded that the vibrational spectra reported on the surface structure of the particles in the aerosol. Here, we show that the SF scattering power of such small particles with a density of 10$^{6}$ particle/mL is $\sim$ 10$^{7}$ smaller than the detection limit of the presented SFS experiment. We determine the detectable number density of particles, both theoretically and experimentally, to demonstrate the limits of the SFS method. We also propose possible reasons for the 10$^{7}$ order of magnitude discrepancy.

physics.chem-ph↗

arXiv2022

A simple model for high rotational excitations of molecules in a superfluid

We present a simple quantum mechanical model describing excited rotational states of molecules in superfluid helium nanodroplets, as recently studied in non-adiabatic molecular alignment experiments [Cherepanov et al., Phys. Rev. A 104, L061303 (2021)]. We show that a linear molecule immersed in a superfluid can be seen as an effective symmetric top, similar to the rotational structure of radicals, such as OH or NO, but with the angular momentum of the superfluid playing the role of the electronic angular momentum in free molecules. The model allows to evaluate the effective rotational and centrifugal distortion constants for a broad range of species and to explain the crossover between light and heavy molecules in superfluid $^4$He in terms of the many-body wavefunction structure. Most important, the simple theory allows to answer the question as to what happens when the rotational angular momentum of the molecule increases beyond the lowest excited states accessible by infrared spectroscopy. Some of the above mentioned insights can be acquired by analyzing a simple 2x2 matrix.

physics.chem-ph↗

arXiv2021

Excited rotational states of molecules in a superfluid

We combine experimental and theoretical approaches to explore excited rotational states of molecules embedded in helium nanodroplets using CS$_2$ and I$_2$ as examples. Laser-induced nonadiabatic molecular alignment is employed to measure spectral lines for rotational states extending beyond those initially populated at the 0.37 K droplet temperature. We construct a simple quantum mechanical model, based on a linear rotor coupled to a single-mode bosonic bath, to determine the rotational energy structure in its entirety. The calculated and measured spectral lines are in good agreement. We show that the effect of the surrounding superfluid on molecular rotation can be rationalized by a single quantity -- the angular momentum, transferred from the molecule to the droplet.

physics.chem-ph↗

arXiv2021

Laser-induced Coulomb explosion imaging of \ce{(C6H5Br)2} and \ce{C6H5Br-I2} dimers in helium nanodroplets using the Timepix3

We have deduced the structure of the \ce{bromobenzene}--\ce{I2} heterodimer and the \ce{(bromobenzene)2} homodimer inside helium droplets using a combination of laser-induced alignment, Coulomb explosion imaging, and three-dimensional ion imaging. The complexes were fixed in a variety of orientations in the laboratory frame, then in each case multiply ionized by an intense laser pulse. A three dimensional ion imaging detector, including a Timepix3 detector allowed us to measure the correlations between velocity vectors of different fragments and, in conjunction with classical simulations, work backward to the initial structure of the complex prior to explosion. For the heterodimer, we find that the \ce{I2} molecular axis intersects the phenyl ring of the bromobenzene approximately perpendicularly. The homodimer has a stacked parallel structure, with the two bromine atoms pointing in opposite directions. These results illustrate the ability of Coulomb explosion imaging to determine the structure of large complexes, and point the way toward real-time measurements of bimolecular reactions inside helium droplets.

physics.atm-clus↗

arXiv2020

Rotational coherence spectroscopy of molecules in helium nanodroplets: Reconciling the time and the frequency domains

Alignment of OCS, CS$_2$ and I$_2$ molecules embedded in helium nanodroplets is measured as a function of time following rotational excitation by a non-resonant, comparatively weak ps laser pulse. The distinct peaks in the power spectra, obtained by Fourier analysis, are used to determine the rotational, B, and centrifugal distortion, D, constants. For OCS, B and D match the values known from IR spectroscopy. For CS$_2$ and I$_2$, they are the first experimental results reported. The alignment dynamics calculated from the gas-phase rotational Schr\"{o}dinger equation, using the experimental in-droplet B and D values, agree in detail with the measurement for all three molecules. The rotational spectroscopy technique for molecules in helium droplets introduced here should apply to a range of molecules and complexes.

physics.atm-clus↗

arXiv2019

Structure determination of the tetracene dimer in helium nanodroplets using femtosecond strong-field ionization

Dimers of tetracene molecules are formed inside helium nanodroplets and identified through covariance analysis of the emission directions of kinetic tetracene cations stemming from femtosecond laser-induced Coulomb explosion. Next, the dimers are aligned in either one or three dimensions under field-free conditions by a nonresonant, moderately intense laser pulse. The experimental angular covariance maps of the tetracene ions are compared to calculated covariance maps for seven different dimer conformations and found to be consistent with four of these. Additional measurements of the alignment-dependent strong-field ionization yield of the dimer narrows the possible conformations down to either a slipped-parallel or parallel-slightly-rotated structure. According to our quantum chemistry calculations, these are the two most stable gas-phase conformations of the dimer and one of them is favorable for singlet fission.

physics.atm-clus↗

arXiv2018

Long-lasting field-free alignment of large molecules inside helium nanodroplets

Molecules with their axes sharply confined in space, available through laser-induced alignment methods, are essential for many current experiments, including ultrafast molecular imaging. Most of these applications require both that the aligning laser field is turned-off, to avoid undesired perturbations, and that the molecules remain aligned sufficiently long that reactions and dynamics can be mapped out. Presently, this is only possible for small, linear molecules and for times less than 1 picosecond. Here, we demonstrate strong, field-free alignment of large molecules inside helium nanodroplets, lasting tens of picoseconds. Molecular alignment in either one or three dimensions is created by a slowly switched-on laser pulse, made field-free through rapid pulse truncation, and retained thanks to the impeding effect of the helium environment on molecular rotation. We illustrate the opportunities that field-free aligned molecules open by measuring the alignment-dependent strong-field ionization yield of a thiophene oligomer. Our technique will enable molecular-frame experiments, including ultrafast excited state dynamics, on a variety of large molecules and complexes.

physics.chem-ph↗

arXiv2017

Strongly aligned molecules inside helium droplets in the near-adiabatic regime

Iodine (I$_2$) molecules embedded in He nanodroplets are aligned by a 160 ps long laser pulse. The highest degree of alignment, occurring at the peak of the pulse and quantified by $\langle \cos^2 \theta_{2D} \rangle$, is measured as a function of the laser intensity. The results are well described by $\langle \cos^2 \theta_{2D} \rangle$ calculated for a gas of isolated molecules each with an effective rotational constant of 0.6 times the gas-phase value, and at a temperature of 0.4 K. Theoretical analysis using the angulon quasiparticle to describe rotating molecules in superfluid helium rationalizes why the alignment mechanism is similar to that of isolated molecules with an effective rotational constant. A major advantage of molecules in He droplets is that their 0.4 K temperature leads to stronger alignment than what can generally be achieved for gas phase molecules -- here demonstrated by a direct comparison of the droplet results to measurements on a $\sim$ 1 K supersonic beam of isolated molecules. This point is further illustrated for more complex system by measurements on 1,4-diiodobenzene and 1,4-dibromobenzene. For all three molecular species studied the highest values of $\langle \cos^2 \theta_{2D} \rangle$ achieved in He droplets exceed 0.96.

physics.chem-ph↗