An experimental and computational view of the photoionization of diol-water clusters.

In the interstellar medium, diols and other prebiotic molecules adsorb onto icy mantles surrounding dust grains. Water in the ice may affect the reactivity and photoionization of these diols. Ethylene glycol (EG), 1,2-propylene glycol, and 1,3-propylene glycol clusters with water clusters were used as a proxy to study these interactions. The diol-water clusters were generated in a continuous supersonic molecular beam, photoionized by synchrotron-based vacuum ultraviolet light from the Advanced Light Source, and subsequently detected by reflectron time-of-flight mass spectrometry. The appearance energies for the detected clusters were determined from the mass spectra, collected at increasing photon energy. Clusters of both diol fragments and unfragmented diols with water were detected. The lowest energy geometry optimized conformers for the observed EG-water clusters and EG fragment-water clusters have been visualized using density functional theory (DFT), providing insight into hydrogen bonding networks and how these affect fragmentation and appearance energy. As the number of water molecules clustered around EG fragments (m/z 31 and 32) increased, the appearance energy for the cluster decreased, indicating a stabilization by water. This trend was supported by DFT calculations. Fragment clusters from 1,2-propylene glycol exhibited a similar trend, but with a smaller energy decrease, and no trend was observed from 1,3-propylene glycol. We discuss and suggest that the reactivity and photoionization of diols in the presence of water depend on the size of the diol, the location of the hydroxyl group, and the number of waters clustered around the diol.

Cold collisions of hot molecules.

In this Perspective, we review our recent work on rotationally inelastic collisions of highly vibrationally excited NO molecules prepared in single rotational and parity levels at v = 10 using stimulated emission pumping (SEP). This state preparation is employed in a recently developed crossed molecular beam apparatus where two nearly copropagating molecular beams achieve an intersection angle of 4° at the interaction region. This near-copropagating beam geometry of the molecular beams permits very wide tuning of the collision energy, from far above room temperature down to 2 K where we test the theoretical treatment of the attractive part of the potentials and the difference potential for the first time. We have obtained differential cross sections for state-to-state collisions of NO (v = 10) with Ar and Ne in both spin-orbit manifolds using velocity map imaging. Overall good agreement of the experimental results was seen with quantum mechanical close-coupling calculations done on both coupled-cluster and multi-reference configuration interaction potential energy surfaces. Probing cold collisions of NO carrying ∼2 eV of vibrational excitation allows us to test state-of-the-art theory in this extreme nonequilibrium regime.

A Direct Probe of the Hydrogen Bond Network in Aqueous Glycerol Aerosols.

The properties of aerosols are of paramount importance in atmospheric chemistry and human health. The hydrogen bond network of glycerol-water aerosols generated from an aqueous solution with different mixing ratios is probed directly with X-ray photoelectron spectroscopy. The carbon and oxygen X-ray spectra reveal contributions from gas and condensed phase components of the aerosol. It is shown that water suppresses glycerol evaporation up to a critical mixing ratio. A dielectric analysis using terahertz spectroscopy coupled with infrared spectroscopy of the bulk solutions provides a picture of the microscopic heterogeneity prevalent in the hydrogen bond network when combined with the photoelectron spectroscopy analysis. The hydrogen bond network is composed of three intertwined regions. At low concentrations, glycerol molecules are surrounded by water forming a solvated water network. Adding more glycerol leads to a confined water network, maximizing at 22 mol %, beyond which the aerosol resembles bulk glycerol. This microscopic view of hydrogen bonding networks holds promise in probing evaporation, diffusion dynamics, and reactivity in aqueous aerosols.

Probing Self-Assembly in Arginine-Oleic Acid Solutions with Terahertz Spectroscopy and X-ray Scattering.

A study of the formation of microstructures in the reaction of oleic acid with arginine elucidates dynamical self-assembly processes at the molecular level. Terahertz spectroscopy combined with density functional calculations reveals the initial hydrogen-bonding motifs in the assembly process, leading to the formation of micelles and vesicles. Small-angle X-ray scattering measurements allow for kinetic analysis of the growth processes of these nanostructures, revealing a prenucleation pathway of vesicles and micelles which lead to spongelike structures. This final stage of the assembly into spongelike aggregates is investigated with optical microscopy. The formed structures only occur at pH > 8 and are resistant to extreme acidic and basic conditions. A mechanistic pathway to the formation of the spongelike aggregates is described.

A versatile molecular beam apparatus for cold/ultracold collisions.

We have developed an apparatus capable of performing intrabeam and near-copropagating beam scattering experiments at collision energies from room temperature to below 1 K where interesting quantum phenomena can be observed. A detailed description of the major components of the apparatus, single and dual molecular beam valves, high speed chopper, and the discharge source, is presented. With the intrabeam scattering setup, a novel dual-slit chopper permits collision energies down to millikelvins with a collision energy spread of 20%. With the near-copropagating beam configuration, state-to-state differential cross sections for rotationally inelastic collisions of highly vibrationally excited NO molecules with Ar have been measured at broadly tunable energies documenting the versatility of the instrument. Future applications in stereodynamics and cold state-to-state collisions of vibrationally excited polyatomic molecules are briefly discussed.

State-to-state scattering of highly vibrationally excited NO at broadly tunable energies.

Experimental developments continue to challenge the theoretical description of molecular interactions. One key arena in which these advances have taken place is in rotationally inelastic scattering. Electric fields have been used with great success to select the initial quantum state and slow molecules for scattering studies, revealing novel stereodynamics, diffraction oscillations and scattering resonances. These have enjoyed excellent agreement with quantum scattering calculations performed on state-of-the-art coupled-cluster potential energy surfaces. To date these studies have largely employed reactants in the ground vibrational state (v = 0) and the lowest low-field-seeking quantum state. Here we describe the use of stimulated emission pumping to prepare NO molecules in arbitrary single rotational and parity states of v = 10 for inelastic scattering studies. These are employed in a near-copropagating molecular beam geometry that permits the collision energy to be tuned from above room temperature to 1 K or below, with product differential cross-sections obtained by velocity map imaging. This extremely nonequilibrium condition, not found in nature, tests current theoretical methods in a new regime.

Ethylene Intersystem Crossing Caught in the Act by Photofragment Sulfur Atoms.

Ethylene, C2H4, the simplest π-bonded molecule, is of enormous fundamental and commercial importance. Its lowest triplet state, in which the CH2 moieties occupy perpendicular planes, is well known from theory, but there has been no definitive experimental observation of this species. Here, velocity map imaging of the sulfur atoms in ethylene sulfide (c-C2H4S) photodissociation at 217 nm is used to reveal the internal state distribution of co-product ethylene. While both S (1D) and S (3P) translational energy distributions display three distinct regions that find their origins in singlet and triplet excited states of c-C2H4S, respectively, the S (3P) distribution is dominated by a fourth, low-recoil region. In this region, the distribution is fully isotropic at a recoil of 9 ± 1 kcal/mol, corresponding to the opening of the triplet ethylene channel. Multireference calculations suggest that this photodissociation pathway is mediated by a hot, transient biradical CH2CH2S that strongly favors CH2-hindered rotations in the predissociated complex. This photochemical ring-opening mechanism is invoked to account for the vibrational features observed in this low-recoil region, which are attributed to triplet ethylene relaxing to the torsional saddle point on the ground-state singlet surface. This study thereby gives for the first time the experimental confirmation of an adiabatic singlet-triplet splitting of 66 ± 1 kcal/mol and a torsional barrier height of 64 ± 1 kcal/mol in ethylene.

Differential Cross Sections for State-to-State Collisions of NO( v = 10) in Near-Copropagating Beams.

State-to-state differential cross sections for rotationally inelastic collisions of vibrationally excited NO with Ar have been measured in a near-copropagating crossed beam experiment at collision energies of 530 and 30 cm-1. Stimulated emission pumping (SEP) to prepare NO in specific rovibrational levels is coupled with direct-current slice velocity map imaging to obtain a direct measurement of the differential cross sections. The use of nearly copropagating beams to achieve low NO-Ar collision energies and broad collision energy tuning capability are also demonstrated. The experimental differential cross sections (DCSs) for NO in v = 10 in specific rotational and parity states are compared with the corresponding DCSs predicted for NO in v = 0 obtained from quantum mechanical close coupling calculations to highlight the differences between the NO( v = 10)-Ar and NO( v = 0)-Ar interaction potentials.

Demonstration of multi-hit and multi-mass capability of 3D imaging in a conventional velocity map imaging experiment.

Coincidence and three-dimensional (3D) imaging offer unique capability in photodissociation and scattering experiments, and a variety of methods have been developed. The basic concept behind all these approaches is to register both the position (x, y) at which the particle hits the detector and the arrival time (t). A novel advance to the time and position sensitive detection was introduced recently by Li and co-workers [Rev. Sci. Instrum. 85(12), 123303 (2014)]. This method utilizes a high-speed digitizer and a computer algorithm along with the camera and detector usually employed in a conventional velocity map imaging apparatus. Due to the normal intensity variations of the ion spots, a correlation can be made between ion intensity recorded by the camera and peak intensity in the digitizer. This makes it possible to associate each ion spot's position with its respective arrival time, thereby constructing a 3D distribution. The technique was primarily introduced for ultrafast ion and electron imaging experiments at high repetition rate with single or few events per image frame. We have recently succeeded in adapting this approach at low repetition rate. Modifications were done to the initial setup to enhance the acquisition efficiency to obtain and correlate multiple hits per laser shot rather than single-hit events. The results are demonstrated in two experiments, dimethyl amine dissociative ionization at 205 nm and carbonyl sulfide photodissociation at 217 nm, with up to 27 events correlated in a single frame. Temporal and spatial slicing capabilities were achieved with good resolution, giving the photofragment velocity and angular distribution for multiple masses simultaneously.

Intrabeam Scattering for Ultracold Collisions.

We demonstrate a method to probe cold and ultracold chemistry in a single molecular beam. The approach exploits beam slippage, the velocity difference of different species in the same beam, to establish the relative velocity. Average collision energies of 2.5 mK are achieved but with a spread of 100% or more. However, by implementing a dual-slit chopper that can separately fix the velocities of the two species at the interaction region, we achieve precise control over the relative velocity and narrow its spread. Relative velocities of 7-10 ± 1.1 m/s are achieved with an angular divergence less than 0.25°. In the present study, we observe l-changing collisions occurring between Xe Rydberg atoms and Xe ground state atoms at subKelvin temperatures. We show that in this case the collision energies are tunable between 200 to 450 mK with a root-mean-square deviation of ∼18%. Application of the method to other species and access to much lower energies is straightforward.

Imaging the Photodissociation Dynamics of Nitrous Acid (HONO): The Role of Torsion.

We present the findings of the first imaging study of trans-HONO and cis-HONO photodissociation through the [Formula: see text] band and [Formula: see text] band of the Ã1A″-X̃1A' transition. The NO photofragment was probed by (1 + 1) resonance-enhanced multiphoton ionization and studied using the direct-current slice imaging technique with our finite slice reconstruction method. The NO state-specific translational energy distributions show some rotational structure corresponding to the internal state distribution in the OH cofragment. All images showed a perpendicular angular distribution with a recoil anisotropy parameter from ca. -0.6 to -0.8. In both bands, cis-HONO showed greater anisotropy than trans-HONO. Deviation from the limiting value expected for a pure perpendicular dissociation is ascribed to deviation of the transition moment from normal to the heavy atom plane owing to OH torsion, rather than lifetime effects assumed in the large body of previous work.