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 nonresonant, 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.

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. For these applications the aligning laser field should ideally be turned-off, to avoid undesired perturbations, and the strong alignment should last long enough that reactions and dynamics can be mapped out. Presently, this is only possible for small, linear molecules and for times less than 1 picosecond.

Femtosecond laser induced Coulomb explosion imaging of aligned OCS oligomers inside helium nanodroplets.

Dimers and trimers of carbonyl sulfide (OCS) molecules embedded in helium nanodroplets are aligned by a linearly polarized 160 ps long moderately intense laser pulse and Coulomb exploded with an intense 40 fs long probe pulse in order to determine their structures. For the dimer, recording of 2D images of OCS and S ions and covariance analysis of the emission directions of the ions allow us to conclude that the structure is a slipped-parallel shape similar to the structure found for gas phase dimers. For the trimer, the OCS ion images and the corresponding covariance maps reveal the presence of a barrel-shaped structure (as in the gas phase) but also other structures not present in the gas phase, most notably a linear chain structure.

Hyperfine-Structure-Induced Depolarization of Impulsively Aligned I_{2} Molecules.

A moderately intense 450 fs laser pulse is used to create rotational wave packets in gas phase I_{2} 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 nonperiodic 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.

Alignment and Imaging of the CS_{2} Dimer Inside Helium Nanodroplets.

The carbon disulphide (CS_{2}) dimer is formed inside He nanodroplets and identified using fs laser-induced Coulomb explosion, by observing the CS_{2}^{+} ion recoil velocity. It is then shown that a 160 ps moderately intense laser pulse can align the dimer in advantageous spatial orientations which allow us to determine the cross-shaped structure of the dimer by analysis of the correlations between the emission angles of the nascent CS_{2}^{+} and S^{+} ions, following the explosion process. Our method will enable fs time-resolved structural imaging of weakly bound molecular complexes during conformational isomerization, including formation of exciplexes.

Three-Dimensional Molecular Alignment Inside Helium Nanodroplets.

We demonstrate 3D spatial alignment of 3,5-dichloroiodobenzene molecules embedded in helium nanodroplets using nonresonant elliptically polarized 160 ps laser pulses at a 1 kHz repetition rate. Through Coulomb explosion imaging and ion-ion covariance mapping, the 3D alignment is characterized and found to be stronger than that of isolated molecules. The 3D alignment follows the intensity profile of the alignment laser pulse almost adiabatically, except for a delayed response in the helium droplets, which could be exploited for field-free 3D alignment.

Nonadiabatic laser-induced alignment of molecules: Reconstructing ⟨𝖼𝗈𝗌 θ⟩ directly from ⟨𝖼𝗈𝗌 θ⟩ by Fourier analysis.

We present an efficient, noise-robust method based on Fourier analysis for reconstructing the three-dimensional measure of the alignment degree, ⟨cosθ⟩, directly from its two-dimensional counterpart, ⟨cosθ⟩. The method applies to nonadiabatic alignment of linear molecules induced by a linearly polarized, nonresonant laser pulse. Our theoretical analysis shows that the Fourier transform of the time-dependent ⟨cosθ⟩ trace over one molecular rotational period contains additional frequency components compared to the Fourier transform of ⟨cosθ⟩.

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 ⟨cos𝜃⟩, is measured as a function of the laser intensity. The results are well described by ⟨cos𝜃⟩ calculated for a gas of isolated molecules each with an effective rotational constant of 0.

Laser-Induced Rotation of Iodine Molecules in Helium Nanodroplets: Revivals and Breaking Free.

Rotation of molecules embedded in helium 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.

Dynamic stark control of torsional motion by a pair of laser pulses.

The torsional motion of a molecule composed of two substituted benzene rings, linked by a single bond, is coherently controlled by a pair of strong (3×10^{13}  W cm^{-2}), nonresonant (800 nm) 200-fs-long laser pulses-both linearly polarized perpendicular to the single-bond axis. If the second pulse is sent at the time when the two benzene rings rotate toward (away from) each other the amplitude of the torsion is strongly enhanced (reduced). The torsional motion persists for more than 150 ps corresponding to approximately 120 torsional oscillations.

Electronic spectroscopy of toluene in helium nanodroplets: evidence for a long-lived excited state.

Optical excitation of toluene to the S1 electronic state in helium nanodroplets is found to alter the rate of production of the fragment ions C7H7(+) and C5H5(+) when the droplets are subjected to subsequent electron ionization. The optical excitation process reduces the abundance of C7H7(+) ions delivered into the gas phase, whereas C5H5(+) ions become more abundant beyond a minimum droplet size. This process contrasts with normal optical depletion spectroscopy, where the optical absorption of a molecular dopant in a helium nanodroplet shrinks the helium droplet, and thus, the electron impact cross-sections because of dissipation of the absorbed energy by evaporative loss of helium atoms.

Communication: Electron impact ionization of binary H2O∕X clusters in helium nanodroplets: an ab initio perspective.

In a recent experiment (H(2)O)(n)∕X(m) binary clusters (where X = Ar, N(2), CO, CO(2), and several other molecules) were formed in superfluid helium nanodroplets and investigated by electron impact mass spectrometry [Liu et al., Phys. Chem.

Communication: the formation of helium cluster cations following the ionization of helium nanodroplets: influence of droplet size and dopant.

The He(n)(+)/He(2)(+) (n ≥ 3) signal ratios in the mass spectra derived from electron impact ionization of pure helium nanodroplets are shown to increase with droplet size, reaching an asymptotic limit at an average droplet size of approximately 50,000 helium atoms. This is explained in terms of a charge hopping model, where on average the positive charge is able to penetrate more deeply into the liquid helium as the droplet size increases. The deeper the point where the charge localizes to form He(2)(+), the greater the likelihood of collisions with the surrounding helium as the ion begins to leave the droplet, thus increasing the probability that helium will be ejected in the form of He(n)(+) (n ≥ 3) cluster ions rather than He(2)(+).

Core-shell effects in the ionization of doped helium nanodroplets.

Core-shell particles with water clusters as the core and surrounded by an atomic or molecular shell have been synthesized for the first time by adding water and a co-dopant sequentially to helium nanodroplets. The co-dopants chosen for investigation were Ar, O(2), N(2), CO, CO(2), NO and C(6)D(6). These co-dopants have been used to investigate the effect of an outer shell on the ionization of the core material by charge transfer in helium nanodroplets.

Ionization of doped helium nanodroplets: residual helium attached to diatomic cations and their clusters.

Electron impact ionization of helium nanodroplets containing a dopant, M, can lead to the detection of both M(+) and helium-solvated cations of the type M(+)·He(n) in the gas phase. The observation of helium-doped ions, He(n)M(+), has the potential to provide information on the aftermath of the charge transfer process that leads to ion production from the helium droplet. Here we report on helium attachment to the ions from four common diatomic dopants, M = N(2), O(2), CO, and NO.