OVERTONE VIBRATIONAL SPECTROSCOPY AND DYNAMICS IN H2_2-H2_2O COMPLEXES: A COMBINED THEORETICAL AND EXPERIMENTAL STUDY

Author Institution: JILA, University of Colorado and National Institute of; Standards and Technology, Boulder, Colorado; CNRS-Universite de Bourgogne, Dijon, France; CNRS, Institut de Planetologie et d'Astrophysique de Grenoble, France; Radboud University, 6525 AJ Nijmegen, The NetherlandsH$_2$ is the most abundant molecule in the universe and also H$_2$O occurs in relatively high concentrations in various interstellar environments. Processes that occur through the interaction of these molecules may, for example, play a role in the mechanism producing the observed H$_2$O maser activity. Spectroscopic studies of the H$_2$-H$_2$O complex in different stable and metastable states will be reported in the accompanying talk; theoretical studies will be presented here. The latter involve calculations of the bound rovibrational levels of the complex with both monomers in their vibrational ground state, as well as of the metastable levels with H$_2$O in its OH stretch overtone state, on the appropriate \textit{ab initio} five-dimensional intermolecular potential surfaces. Also the line strengths of all the allowed transitions between these levels that may occur in combination with the $v_{\rm OH} = 2 \leftarrow 0$ overtone transition were computed, for all four ortho/para H$_2$ and ortho/para H$_2$O variants of the complex. The spectrum simulated with these data agrees very well with the measured spectrum and was used to assign this spectrum. In addition, the information obtained from the theory was useful to understand the observed predissociation dynamics of the complex

Molecular and electronic dynamics in van der Waals cluster spectroscopy, hydrogen abstraction reactions, and inelastic collisions at liquid surfaces

Quantum mechanical measurements are essential for an understanding of collision and reaction dynamics on the molecular scale. To this end, laser induced fluorescence (LIF) is used to probe rotational, vibrational, and electronic product state distributions following various chemical events. For example, LIF on the hydroxyl radical is employed to examine the propensity to populate different levels of OH following photolysis of H2O molecules using a technique known as vibrationally mediated dissociation (VMD). VMD is also used as an indirect method for obtaining infrared spectra of water clusters (Ar-H2O, H2O-H2O, and H2-H 2O), weakly bound species which are produced in the cold (∼ 5 K) environment of a slit supersonic expansion. Peaks are then assigned with the aid of high level theoretical calculations. LIF is also performed to study systems where reactive precursors produce OH/OD radicals (F + D2O → DF + OD and F + H2O → HF + OH) as well as for nonreactive processes where ground state NO inelastically is scattered from liquid Ga metal or room temperature ionic liquid (RTIL) surfaces. In the reactive scattering experiments, careful examination of OH product spin-orbit branching provides an opportunity to quantify the degree of multiple surface behavior in these systems. Rotational-state-resolved scattering of nitric oxide from a molten metal provides an opportunity to directly observe thermal roughening of the liquid due to capillary wave excitations. Scattered NO electronic distributions, which are out of thermal equilibrium with rotation, are quite sensitive to surface temperature, a possible consequence of interactions with electron-hole pairs during the collision. Finally, NO is scattered from room temperature ionic liquid (RTIL) samples where branching between the two possible scattered spin orbit states (2Π1/2 and 2Π3/2) is found to be highly sensitive to surface heating and choice of ionic liquid. This may serve as a novel means for characterizing these surfaces, which are of technological interest due to their potential role as advanced solvents

Near infrared overtone (v(OH)=2 <- 0) spectroscopy of Ne-H2O clusters

Vibrationally state selective overtone spectroscopy and dynamics of weakly bound Ne-H2O complexes (D0(para) = 31.67 cm-1, D0(ortho) = 34.66 cm-1) are reported for the first time, based on near infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02-⟩←|00+⟩ and |02+⟩←|00+⟩) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level ab initio potential energy surface (CCSD(T)/VnZ-f12 (n = 3,4), corrected for basis set superposition error and extrapolated to the complete basis set limit) are employed for assignment of Σ←Σ,Π←Σ, and Σ←Π infrared bands in the overtone spectra, where Σ(K = 0) and Π (K = 1) represent approximate projections (K) of the body angular momentum along the Ne-H2O internuclear axis. End-over-end tumbling of the ortho Ne-H2O cluster is evident via rotational band contours observed, with band origins and rotational progressions in excellent agreement with ab initio frequency and intensity predictions. A clear Q branch in the corresponding |02+⟩fΠ(111)←eΣ(000) para Ne-H2O spectrum provides evidence for a novel e/f parity-dependent metastability in these weakly bound clusters, in agreement with ab initio bound state calculations and attributable to the symmetry blocking of an energetically allowed channel for internal rotor predissociation. Finally, Boltzmann analysis of the rotational spectra reveals anomalously low jet temperatures (Trot ≈ 4(1) K), which are attributed to "evaporative cooling" of weakly bound Ne-H2O clusters and provide support for similar cooling dynamics in rare gas-tagging studies.status: publishe

Nuclear spin/parity dependent spectroscopy and predissociation dynamics in v(OH)=2 <- 0 overtone excited Ne-H2O clusters: Theory and experiment

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H2O complexes (D0(ortho) = 34.66 cm-1 and D0(para) = 31.67 cm-1) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02-〉 and |02+〉) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level potential energy surface [CCSD(T)/VnZf12 (n = 3, 4)], corrected for basis set superposition error and extrapolated to the complete basis set (CBS) limit, are employed to successfully predict and assign Π-Σ, Σ-Σ, and Σ-Π infrared bands in the spectra, where Σ or Π represent approximate projections of the body-fixed H2O angular momentum along the Ne-H2O internuclear axis. IR-UV pump-probe experimental capabilities permit real-time measurements of the vibrational predissociation dynamics, which indicate facile intramolecular vibrational energy transfer from the H2O vOH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τprediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H2O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H2O bands are surprisingly absent despite ab initio predictions of Q-branch intensities with S/N > 20-40. Such behavior signals the presence of highly selective nuclear spin ortho-para predissociation dynamics in the upper state, for which we offer a simple mechanism based on Ne-atom mediated intramolecular vibrational relaxation in the H2O subunit (i.e., |02±〉 → {|01±〉; v2 = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π1/2,3/2), and lambda doublet product distributions.status: publishe

Femtosecond time-resolved XUV + UV photoelectron imaging of pure helium nanodroplets.

Liquid helium nanodroplets, consisting of on average 2 × 10(6) atoms, are examined using femtosecond time-resolved photoelectron imaging. The droplets are excited by an extreme ultraviolet light pulse centered at 23.7 eV photon energy, leading to states within a band that is associated with the 1s3p and 1s4p Rydberg levels of free helium atoms. The initially excited states and subsequent relaxation dynamics are probed by photoionizing transient species with a 3.2 eV pulse and using velocity map imaging to measure time-dependent photoelectron kinetic energy distributions. Significant differences are seen compared to previous studies with a lower energy (1.6 eV) probe pulse. Three distinct time-dependent signals are analyzed by global fitting. A broad intense signal, centered at an electron kinetic energy (eKE) of 2.3 eV, grows in faster than the experimental time resolution and decays in ~100 fs. This feature is attributed to the initially excited droplet state. A second broad transient feature, with eKE ranging from 0.5 to 4 eV, appears at a rate similar to the decay of the initially excited state and is attributed to rapid atomic reconfiguration resulting in Franck-Condon overlap with a broader range of cation geometries, possibly involving formation of a Rydberg-excited (He(n))* core within the droplet. An additional relaxation pathway leads to another short-lived feature with vertical binding energies ≳2.4 eV, which is identified as a transient population within the lower-lying 1s2p Rydberg band. Ionization at 3.2 eV shows an enhanced contribution from electronically excited droplet states compared to ejected Rydberg atoms, which dominate at 1.6 eV. This is possibly the result of increased photoelectron generation from the bulk of the droplet by the more energetic probe photons

Near infrared overtone spectroscopy of Ne-H2O clusters

Vibrationally state selective overtone spectroscopy and dynamics of weakly bound Ne-H2O complexes (D0 (para) = 31.67 cm−1, D0 (ortho) = 34.66 cm−1) are reported for the first time, based on near infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Charging and ion ejection dynamics of large helium nanodroplets exposed to intense femtosecond soft X-ray pulses

Ion ejection from charged helium nanodroplets exposed to intense femtosecond soft X-ray pulses is studied by single-pulse ion time-of-flight (TOF) spectroscopy in coincidence with small-angle X-ray scattering. Scattering images encode the droplet size and absolute photon flux incident on each droplet, while ion TOF spectra are used to determine the maximum ion kinetic energy, $$E_{\text {kin}}$$, of $$\hbox {He}_{j}^{+}$$ fragments (j = 1–4). Measurements span $$\hbox {He}_N$$ droplet sizes between $$N\sim 10^{7}$$ and $$\sim 10^{10}$$ (radii $$R_0$$ = 78–578 nm), and droplet charges between $$\sim 9\times 10^{-5}$$ and $$\sim 3\times 10^{-3}$$ e/atom. Conditions encompass a wide range of ionization and expansion regimes, from departure of all photoelectrons from the droplet, leading to pure Coulomb explosion, to substantial electron trapping by the electrostatic potential of the charged droplet, indicating the onset of hydrodynamic expansion. The unique combination of absolute X-ray intensities, droplet sizes, and ion $$E_{\text {kin}}$$ on an event-by-event basis reveals a detailed picture of the correlations between the ionization conditions and the ejection dynamics of the ionic fragments. The maximum $$E_{\text {kin}}$$ of He$$^{+}$$ is found to be governed by Coulomb repulsion from unscreened cations across all expansion regimes. The impact of ion-atom interactions resulting from the relatively low charge densities is increasingly relevant with less electron trapping. The findings are consistent with the emergence of a charged spherical shell around a quasineutral plasma core as the degree of ionization increases. The results demonstrate a complex relationship between measured ion $$E_{\text {kin}}$$ and droplet ionization conditions that can only be disentangled through the use of coincident single-pulse TOF and scattering data