Submicrometer Particle Impact Dynamics and Chemistry.

Experimental studies of the collision phenomena of submicrometer particles is a developing field. This review examines the range of phenomena that can be observed with new experimental approaches. The primary focus is on single-particle impact studies enabled by charge detection mass spectrometry (CDMS) implemented using the Aerosol Impact Spectrometer (AIS) at the University of California, San Diego.

Detection of intact amino acids with a hypervelocity ice grain impact mass spectrometer.

Astrobiology studies are a top priority in answering one of the most fundamental questions in planetary science: Is there life beyond Earth? Saturn's icy moon Enceladus is a prime target in the search for life in our solar system, identified by NASA as the second-highest priority site for a flagship mission in the next decade. The orbital sampling technique of impact ionization mass spectrometry indicated the presence of complex organics in the small icy plume particles ejected by Enceladus encountered previously by Cassini. However, high interaction velocities caused ambiguity as to the origin and identity of the organics.

Photoinduced Reactions of Nitrate in Aqueous Microdroplets by Triplet Energy Transfer.

In-situ Raman spectroscopy of single levitated charged aqueous microdroplets irradiated by dual-beam (266 and 532 nm) lasers demonstrates that the nitrate anion (NO) can be depleted in the droplet through an energy transfer mechanism following excitation of sulfanilic acid (SA), a UV-absorbing aromatic organic compound. Upon 266 nm irradiation, a fast decrease of the NO concentration was observed when SA is present in the droplet. This photoinduced reaction occurs without the direct photolysis of NO.

Size-Dependent Sigmoidal Reaction Kinetics for Pyruvic Acid Condensation at the Air-Water Interface in Aqueous Microdroplets.

The chemistry of pyruvic acid (PA) under thermal dark conditions is limited in bulk solutions, but in microdroplets it is shown to readily occur. Utilizing in situ micro-Raman spectroscopy as a probe, we investigated the chemistry of PA within aqueous microdroplets in a relative humidity- and temperature-controlled environmental cell. We found that PA undergoes a condensation reaction to yield mostly zymonic acid.

Probing the Exit Channel of the OH + CHOH → HO + CHO Reaction by Photodetachment of CHO(HO).

Transition state dynamics of bimolecular reactions can be probed by photodetachment of a precursor anion when the Franck-Condon region of the corresponding neutral potential energy surface is near a saddle point. In this study, photodetachment of anions at / = 49 enabled investigation of the exit channel of the OH + CHOH → HO + CHO reaction using photoelectron-photofragment coincidence spectroscopy. High-level coupled-cluster calculations of the stationary points on the anion surface show that the methoxide-water cluster CHO(HO) is the stable minimum on the anion surface.

Dissociative Photodetachment Dynamics of the OH(CH) Anion Complex.

Photoelectron-photofragment coincidence (PPC) measurements on OH(CH) anions at a photon energy of 3.20 eV revealed stable and dissociative photodetachment product channels, OH-CH + e and OH + CH + e, respectively. The main product channel observed was dissociation to the reactants (>67%), OH + CH ( = 0, 1, 2) + e, where vibrational excitation in the C-H stretching modes of the CH photofragments corresponds to a minor channel.

Sampling Accelerated Micron Scale Ice Particles with a Quadrupole Ion Trap Mass Spectrometer.

The Enceladus plume is a target of astrobiological interest in planetary science since it may carry signs of extraterrestrial life entrapped in ice grains formed from the subsurface ocean of this moon of Saturn. Fly-by mission concepts have been proposed to perform close investigations of the plume, including detailed in situ measurements of chemical composition with a new generation of mass spectrometer instrumentation. Such a scenario involves high-velocity collisions (typically around 5 km/s or higher) of the instrument with the encountered ice grains.

Correction: Dissociative detachment of the fluoroformate anion.

Correction for 'Dissociative detachment of the fluoroformate anion' by Eugene Shirman et al., Phys. Chem.

Dissociative detachment of the fluoroformate anion.

Dissociative photodetachment of the FCO2- fluoroformate complex by intense laser pulses is studied using 3D coincidence fragment imaging. The main channels are found to be CO2 + F and FCO + O. Cleavage of the C-F bond is attributed to dissociation on the B[combining tilde]2A1 excited state of the neutral FCO2 radical with significant internal excitation of the molecular fragment, while reductive dissociation of the CO2 moiety is assigned to higher lying states.

Evolution of Hydrogen-Bond Interactions within Single Levitated Metastable Aerosols Studied by In Situ Raman Spectroscopy.

Atmospheric aerosols can exist as supersaturated (metastable) liquid or glassy states, with physical and chemical properties that are distinct from the solid or liquid phases. These unique properties of aerosols have substantial implications on climate and health effects. Direct investigations on metastable aerosols remain a challenge because any interfacial contact can cause heterogeneous nucleation.

Tapered image charge detector for measuring velocity distributions of submicrometer particle scattering.

A novel detector for measuring the post-impact velocities (trajectory and speed) of charged submicrometer particles is presented. A stack of tapered cylindrically symmetric electrodes connected to a set of image charge detection circuits is used in conjunction with an image-charge-sensitive target to measure the incident velocity and scattered trajectories of charged particles following impact with the target. This particle detector is used in conjunction with a mass, charge, and energy-selected source of collimated charged particles.

Water diffusion measurements of single charged aerosols using HO/DO isotope exchange and Raman spectroscopy in an electrodynamic balance.

Sea spray aerosols contain a large array of organic compounds that contribute to high viscosities at low relative humidity and temperature thereby slowing translational diffusion of water. The Stokes-Einstein equation describes how viscosity is inversely correlated with the translational diffusion coefficient of the diffusing species. However, recent studies indicate that the Stokes-Einstein equation breaks down at high viscosities achieved in the particle phase (>1012 Pa s), underestimating the predicted water diffusion coefficient by orders of magnitude and revealing the need for directly studying the diffusion of water in single aerosols.

Photoelectron-Photofragment Coincidence Studies on the Dissociation Dynamics of the OH-CH Complex.

Photoelectron-photofragment coincidence (PPC) spectroscopy was used to characterize the energetics and dynamics of the OH + CH → HO + CH reaction initiated by photodetachment of the OH(CH) anion complex. PPC measurements at a photon energy of 3.20 eV yielded stable (OH-CH + e) and dissociative (OH + CH (ν or ν, v = 0, 1) + e) channels.

Photoelectron-Photofragment Coincidence Spectroscopy With Ions Prepared in a Cryogenic Octopole Accumulation Trap: Collisional Excitation and Buffer Gas Cooling.

A cryogenic octopole accumulation trap (COAT) has been coupled to a photoelectron-photofragment coincidence (PPC) spectrometer allowing for improved control over anion vibrational excitation. The anions are heated and cooled via collisions with buffer gas <17 K. Shorter trapping times (500 μs) prevent thermalization and result in anions with high internal excitation while longer trapping times (80 ms) at cryogenic temperatures thermalize the ions to the temperature of the buffer gas.

Resonance-Mediated Below-Threshold Delayed Photoemission and Non-Franck-Condon Photodissociation of Cold Oxyallyl Anions.

The photoexcitation of cold oxyallyl anions was studied below the adiabatic detachment threshold at a photon energy of 1.60 eV. Photodetachment was observed through two product channels, delayed electron emission from a long-lived anionic state and dissociative photodetachment via absorption of a second photon.

Double Photodetachment of F·HO: Experimental and Theoretical Studies of [F·HO].

Double photodetachment of the cluster F·HO in a strong laser field is explored in a combined experimental-theoretical study. Products are observed experimentally by coincidence photofragment imaging following double ionization by intense laser pulses. Theoretically, equation of motion coupled cluster calculations (EOM-CC), suitable for modeling strong correlation effects in the electronic wave function, shed light on the Franck-Condon region, and ab initio molecular dynamics simulations also performed using EOM-CC methods reveal the fragmentation dynamics in time on the lowest-lying singlet and triplet states of [F·HO].

Spectroscopy of Ethylenedione and Ethynediolide: A Reinvestigation.

In an effort to characterize the electronic states of ethylenedione, OCCO, photoelectron-photofragment coincidence (PPC) spectroscopy was applied to measure anions at m/z 56 and 57 using a pulsed discharge of glyoxal vapor and N O. PPC measurements at a photon energy of 3.20 eV yield photoelectron spectra in coincidence with either neutral photofragments or stable neutral products.

Dynamics of transient species via anion photodetachment.

The dynamics of chemical reactions are often governed by transient species, including the transition state for activated bimolecular reactions. Such transient species are difficult to study experimentally, but it has proven valuable to prepare and probe transition-state dynamics by the photodetachment of anions with an equilibrium geometry similar to the neutral transition state. In this review, recent experimental advances in photoelectron and photoelectron-photofragment coincidence spectroscopy are discussed, as well as the latest progress in the calculation of multidimensional potential energy surfaces and quantum dynamics calculations that have enabled an extension of studies of transition-state dynamics to increasingly multidimensional polyatomic systems.

Effects of vibrational excitation on the F + HO → HF + OH reaction: dissociative photodetachment of overtone-excited [F-H-OH].

The reaction F + HO → HF + OH is a four-atom system that provides an important benchmark for reaction dynamics. Hydrogen atom transfer at the transition state for this reaction is expected to exhibit a strong dependence on reactant vibrational excitation. In the present study, the vibrational effects are examined by photodetachment of vibrationally excited F(HO) precursor anions using photoelectron-photofragment coincidence (PPC) spectroscopy and compared with full six-dimensional quantum dynamical calculations on potential energy surfaces.

Internal energy dependence of the photodissociation dynamics of O using cryogenic photoelectron-photofragment coincidence spectroscopy.

Photoelectron-photofragment coincidence (PPC) spectra of ozonide, O, were measured at 388 nm (E = 3.20 eV) using a newly constructed cryogenic octopole accumulation trap coupled to a PPC spectrometer. The photoelectron spectra reveal three processes consisting of a stable photodetachment channel, and two distinct photodissociation pathways yielding (1) O + O or (2) O + O.

Energetics and transition-state dynamics of the F + HOCH → HF + OCH reaction.

The F + HOCH → HF + OCH reaction is a system with 15 internal degrees of freedom that can provide a benchmark for the development of theory for increasingly complex chemical reactions. The dynamics of this reaction were studied by photoelectron-photofragment coincidence (PPC) spectroscopy carried out on the F(HOCH) anion, aided by a computational study of both the anion and neutral potential energy surfaces, with energies extrapolated to the CCSDT(Q)/CBS level of theory. Photodetachment at 4.

Imaging a multidimensional multichannel potential energy surface: Photodetachment of H(-)(NH3) and NH4 (.).

Probes of the Born-Oppenheimer potential energy surfaces governing polyatomic molecules often rely on spectroscopy for the bound regions or collision experiments in the continuum. A combined spectroscopic and half-collision approach to image nuclear dynamics in a multidimensional and multichannel system is reported here. The Rydberg radical NH4 and the double Rydberg anion NH4 (-) represent a polyatomic system for benchmarking electronic structure and nine-dimensional quantum dynamics calculations.

Photoelectron-photofragment coincidence studies of the tert-butoxide anion (CH3)3CO((-)), the carbanion isomer (CH3)2CH2COH((-)), and corresponding radicals.

A study of the photodetachment and dissociative photodetachment (DPD) of the C(4)H(9)O(-) isomers tert-butoxide, (CH(3))(3)CO(-), and the α-hydroxy carbanion (CH(3))(2)C(CH(2))OH(-) is reported. Photoelectron-photofragment coincidence spectroscopy was used to study these anions at 387, 537, and 600 nm. Supported by CBS-QB3 ab initio calculations, the product mass and translational energy distributions were found to be consistent with dissociation of either highly excited (CH3)(3)CO radicals or (CH(3))(2)C(CH2)OH alkylhydroxy radicals.