21 research outputs found
Laser-induced rotation of iodine molecules in He-nanodroplets: revivals and breaking-free
Rotation of molecules embedded in He nanodroplets is explored by a
combination of fs laser-induced alignment experiments and angulon quasiparticle
theory. We demonstrate that at low fluence of the fs alignment pulse, the
molecule and its solvation shell can be set into coherent collective rotation
lasting long enough to form revivals. With increasing fluence, however, the
revivals disappear -- instead, rotational dynamics as rapid as for an isolated
molecule is observed during the first few picoseconds. Classical calculations
trace this phenomenon to transient decoupling of the molecule from its He
shell. Our results open novel opportunities for studying non-equilibrium
solute-solvent dynamics and quantum thermalization.Comment: 6+7 pages; 4+1 figures; 1 tabl
Strongly aligned molecules inside helium droplets in the near-adiabatic regime
Iodine (I) 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 , is measured as a
function of the laser intensity. The results are well described by 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 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 achieved in
He droplets exceed 0.96.Comment: 11 pages, 8 figure
Hyperfine-Structure-Induced Depolarization of Impulsively Aligned Molecules
A moderately intense fs laser pulse is used to create rotational wave
packets in gas phase molecules. The ensuing time-dependent
alignment, measured by Coulomb explosion imaging with a delayed probe pulse,
exhibits the characteristic revival structures expected for rotational wave
packets but also a complex non-periodic substructure and decreasing mean
alignment not observed before. A quantum mechanical model attributes the
phenomena to coupling between the rotational angular momenta and the nuclear
spins through the electric quadrupole interaction. The calculated alignment
trace agrees very well with the experimental results.Comment: 6 pages, 4 figures, and Supplementary Information. This article has
been accepted for publication in Physical Review Letter
Rotational coherence spectroscopy of molecules in helium nanodroplets: Reconciling the time and the frequency domains
Alignment of OCS, CS and I 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 and I, 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.Comment: ASC and LC contributed equally. 7 pages, 3 figure
"The extreme penalty of the law": mercy and the death penalty as aspects of state power in colonial Nyasaland, c. 1903-47
Open access article.Capital punishment was the pinnacle of the colonial judicial system and its use of state violence, but has previously been neglected as a topic of historical research in Africa. This article is based on the case files and legal records of over 800 capital trials â predominantly for murder â dating between 1900 and 1947. It outlines the functioning of the legal system in Nyasaland and the tensions between âviolenceâ and âhumanitarianismâ in the use and reform of the death penalty. Capital punishment was a political penalty as much as a judicial punishment, with both didactic and deterrent functions: it operated through mercy and the sparing of condemned lives as well as through executions. Mercy in Nyasaland was consistent with colonial political objectives and cultural values: it was decided not only on the facts of cases, but according to British conceptions of âjusticeâ, âorderâ, âcriminalityâ, and âAfricanâ behaviour. This article analyses the use of mercy in Nyasaland to provide a lens on the nature of colonial governance, and the tensions between African and colonial understandings of violence.Arts and Humanities Research Council (UK) and the Beit Fund, University of Oxfor
Helium Droplets: Unique Nanoreactors for the Investigation of Molecular Dopants
Both pulsed and continuous sources of helium nanodroplets were employed for the investigation of molecular dopants. The doped helium droplets were investigated with the use of a range of techniques including electron impact ionization mass spectrometry, electronic spectroscopy and infrared (vibrational) spectroscopy.
Electron impact mass spectrometry was used to investigate the influence of droplet size ( = 4000 - 80 000 helium atoms) and dopant species on the formation of helium cluster cations. The abundance of larger helium cluster cations, produced from pure helium droplets, was found to increase with droplet size until an asymptotic limit was reached for = 50 000 helium atoms. The effect of a dopant species was shown to alter the He[subscript n][superscript +]/ He[subscript 2][superscript +] (n ⼠3) signal ratio for smaller droplet sizes and was attributed to the potential energy gradient created by the cation-dopant interaction, and its potential to draw the positive charge towards the centre of the droplet. Core-shell particles, consisting of a water core and a co-dopant outer shell, were produced using a sequential pickup technique and were analysed with electron impact ionization. Of the co-dopants used O[subscript 2], N[subscript 2], C[subscript 6]D[subscript 6] and CO[subscript 2] were found to provide a softening effect on the ionization of the water clusters, whilst CO and NO increased the fragmentation of some water cluster sizes. Results from ab initio calculations of [X(H[subscript 2]O)[subscript 2]][superscript +] cluster ions, where X = CO, N[subscript 2], Ar and CO[subscript 2], support the experimental results.
A new method for recording electronic spectra of species that reside in long-lived metastable states inside the helium droplets was demonstrated using the electronic excitation of toluene into its S[subscript 1] state as an example. In another spectroscopic study, infrared depletion spectroscopy was used to record vibrational spectra of water-methane clusters. From a comparison of predicted vibrational frequencies from ab initio calculations with the experimental spectra, possible structures for the CH[subscript 4](H[subscript 2]O)[subscript n], for n = 1-3, were identified
Communication: Electron impact ionization of binary H2O/X clusters in helium nanodroplets: An ab initio perspective
In a recent experiment (H2O)n/Xm binary clusters (where X = Ar, N2, CO, CO2, and several other molecules) were formed in superfluid helium nanodroplets and investigated by electron impact mass spectrometry [Liu et al., Phys. Chem. Chem. Phys. 13, 13920 (2011)10.1039/c1cp20653b]. The addition of dopant X was found to affect the branching ratio between H3O+(H2O)n and (H2O)+n+2 formation. Specifically, the addition of CO increased the proportion of protonated water cluster ions, whereas dopants such as Ar, N2, and CO2, had the opposite effect. In this work ab initio calculations have been performed on [X(H2O)2]+ ions, where X = Ar, N2, CO, and CO2, to try and explain this distinct behavior. CO is found to be unique in that it forms a HOCO-H3O+ unit in the most stable cationic complexes where the binding between HO and CO is stronger than that between H3O+ and OH. Thus, on purely energetic grounds, loss of HOCO rather than CO should be the preferred fragmentation process. No comparable chemistry occurs when X = Ar, N2, or CO2 and so the co-dopant requires less energy to depart than OH. The calculations therefore account for the experimental observations and provide evidence that HOCO formation is induced in helium droplets containing (H2O)n clusters and co-doped with CO when subject to electron impact ionization