Observation of b2_2 symmetry vibrational levels of the SO2_2 \tilde{\mbox{C}} 1^1B2_2 state: Vibrational level staggering, Coriolis interactions, and rotation-vibration constants

The $\mathrm{\tilde{C}}$ $^1$B$_2$ state of SO$_2$ has a double-minimum potential in the antisymmetric stretch coordinate, such that the minimum energy geometry has nonequivalent SO bond lengths. However, low-lying levels with odd quanta of antisymmetric stretch (b$_2$ vibrational symmetry) have not previously been observed because transitions into these levels from the zero-point level of the $\mathrm{\tilde{X}}$ state are vibronically forbidden. We use IR-UV double resonance to observe the b$_2$ vibrational levels of the $\mathrm{\tilde{C}}$ state below 1600 cm$^{-1}$ of vibrational excitation. This enables a direct characterization of the vibrational level staggering that results from the double-minimum potential. In addition, it allows us to deperturb the strong $c$-axis Coriolis interactions between levels of a$_1$ and b$_2$ vibrational symmetry, and to determine accurately the vibrational dependence of the rotational constants in the distorted $\mathrm{\tilde{C}}$ electronic state

OBSERVATION OF LEVEL-SPECIFIC PREDISSOCIATION RATES IN S1 ACETYLENE

A new spectroscopic scheme was used to gain insight into the predissociation mechanisms of the S$_1$ electronic state of acetylene in the 47000-47300 cm$^{-1}$ region. To study this mechanism, H-atom action spectra of predissociative S$_1$ were recorded. Instead of detecting H-atom via REMPI, an H-atom fluorescence scheme was developed, in which the H-atom was excited to 3s and 3d levels and the fluorescence was detected. The signal-to-noise ratio of H-atom fluorescence-detected action spectra is superior to REMPI detected H-atom spectra. By comparing the LIF and H-atom spectra, there is direct evidence of level-dependent predissociation rates. Some of the line-widths observed in the H-atom spectra are broader than in the LIF spectra, confirming the triplet-mediated nature of S$_1$ acetylene

Angular momentum in rotating superfluid droplets

The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated submicrometer superfluid 4He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vortex arrays they host. In capsule-shaped droplets, vortices form a distorted triangular lattice, whereas they arrange along elliptical contours in ellipsoidal droplets. The combined action of vortices and capillary waves results in droplet shapes close to those of classical droplets rotating with the same angular velocity. The findings are corroborated by density functional theory calculations describing the velocity fields and shape deformations of a rotating superfluid cylinder

Dynamics in Helium Nanodroplets Induced via Multiphoton Absorption in the XUV and X-ray Regimes

Upon formation, helium nanodroplets evaporatively cool to 0.37 K and thus are in a superfluid state. The ultracold droplets have a very low binding energy and are optically transparent. In contrast, when exposed to extreme ultraviolet (XUV) and x-ray radiation, a variety of complex relaxation and disintegration dynamics may ensue. This dissertation explores dynamics induced in droplets via multiphoton absorption in the XUV and x-ray regimes. In the XUV regime, helium droplets become electronically excited, with two broad absorption features originating from atomic helium states. The lower absorption feature at 21.6 eV originates from n=2 atomic helium states, while the upper feature centered at 23.7 eV arises from higher-lying atomic Rydberg states. After single photon absorption, a variety of relaxation mechanisms have been observed, such as ejection of Rydberg atoms, interband relaxation and Hen* formation. Multiphoton absorption leads to additional deexcitation pathways as a result of interactions between excited helium atoms. At higher photon energies, in the soft x-ray regime, individual atoms in the droplet are ionized via single photon ionization. With high intensity x-ray free electron laser (X-FEL) light sources, droplets can become highly ionized. The ionized droplet may disintegrate from the Coulomb repulsion between ions. Alternatively, if the droplet is sufficiently ionized, the freed electrons are trapped by the collective Coulomb potential of the parent ions, resulting in nanoplasma formation. The quasineutral nanoplasma can then disintegrate via hydrodynamic expansion. The simple electronic structure of atomic helium and uniform density of liquid helium make helium droplets an excellent system for studying complex energy transfer, relaxation, and charging dynamics common to condensed phase media. Energy transfer and relaxation following multiphoton absorption into the lower, n=2 helium droplet absorption feature is studied by femtosecond time-resolved photoelectron spectroscopy in combination with XUV intensity-dependent ion yield measurements. With many photoexcited helium atoms in the droplet, resonant interatomic Coulombic decay (ICD) emerges as a possible deexcitation mechanism. In ICD, an excited atom relaxes by transferring its energy to a neighboring excited atom, resulting in ionization with freed electron carrying away the excess energy. This process is common in van der Waals clusters and condensed phase media, such as biological systems. Previous experiments have revealed that beyond ICD between two excited atoms, with high excitation densities, ICD can occur between many excited atoms leading to a host of inelastic processes as well. Here, measurements are performed in a lower XUV intensity regime than previous studies, such that ICD is limited to interactions between two excited atoms. A high-order harmonic pulse at 21.6 eV in the XUV, resonant with the lower droplet absorption feature, is used to electronically excite the droplet. Relaxation dynamics are then measured using a 3.1 eV UV probe pulse at various XUV-UV pump-probe delays. Ion yield measurements reveal a quadratic dependence on the XUV intensity in smaller droplets (~104 atoms/droplet) and a linear relationship in larger droplets (~106 atoms/droplet). The ICD lifetime is measured to be ~1 ps and found to be a competitive mechanism by which the droplet relaxes, even at low excitation densities.The charging and disintegration dynamics of helium droplets exposed to intense (~1016 W/cm2), soft x-ray pulses at 838 eV photon energy are explored via single shot coincidence measurements of ion time of flight spectra and small angle x-ray scattering patterns. Experimental conditions encompass an extended range of ionization conditions in droplets, from the pure Coulomb explosion regime to the formation of nanoplasmas. Interpretation of these ionization dynamics is important for better understanding of a host of complex processes initiated by intense x-ray pulse light—matter interactions, both intentionally and as unavoidable byproducts of X-FEL based experiments. Ion time-of-flight spectra are used to determine the maximum ion kinetic energy resulting from the x-ray—droplet interaction, while scattering images encode the droplet size and absolute photon fluence. In correlating the droplet size, x-ray fluence, and maximum ion kinetic energy, a continuous relationship between the degree of ionization and ion kinetic energy is observed across the transition from weakly to strongly ionized droplets. Across all experimental conditions, results indicate that the maximum ion kinetic energy is governed by Coulomb repulsion from unscreened cations. Additionally, the results are consistent with the emergence of a spherical shell of unscreened ions around a quasineutral plasma core with the onset of frustrated ionization by electron trapping. The thickness of this shell is reduced to less than 2% of the droplet radius at the highest degrees of ionization frustration

Angular Momentum in Rotating Superfluid Droplets.

The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated submicrometer superfluid ^{4}He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vortex arrays they host. In capsule-shaped droplets, vortices form a distorted triangular lattice, whereas they arrange along elliptical contours in ellipsoidal droplets. The combined action of vortices and capillary waves results in droplet shapes close to those of classical droplets rotating with the same angular velocity. The findings are corroborated by density functional theory calculations describing the velocity fields and shape deformations of a rotating superfluid cylinder

Angular Momentum in Rotating Superfluid Droplets

The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated submicrometer superfluid He-4 droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vortex arrays they host. In capsule-shaped droplets, vortices form a distorted triangular lattice, whereas they arrange along elliptical contours in ellipsoidal droplets. The combined action of vortices and capillary waves results in droplet shapes close to those of classical droplets rotating with the same angular velocity. The findings are corroborated by density functional theory calculations describing the velocity fields and shape deformations of a rotating superfluid cylinder

Aggregation of solutes in bosonic versus fermionic quantum fluids

Quantum fluid droplets made of helium-3 (He-3) or helium-4 (He-4) isotopes have long been considered as ideal cryogenic nanolabs, enabling unique ultracold chemistry and spectroscopy applications. The droplets were believed to provide a homogeneous environment in which dopant atoms and molecules could move and react almost as in free space but at temperatures close to absolute zero. Here, we report ultrafast x-ray diffraction experiments on xenon-doped He-3 and He-4 nanodroplets, demonstrating that the unavoidable rotational excitation of isolated droplets leads to highly anisotropic and inhomogeneous interactions between the host matrix and enclosed dopants. Superfluid He-4 droplets are laced with quantum vortices that trap the embedded particles, leading to the formation of filament-shaped clusters. In comparison, dopants in He-3 droplets gather in diffuse, ring-shaped structures along the equator. The shapes of droplets carrying filaments or rings are direct evidence that rotational excitation is the root cause for the inhomogeneous dopant distributions