New Approaches to Measuring Sticky Molecules: Improvement of Instrumental Response Times Using Active Passivation.

A novel method has been developed to improve sampling system response times for nominally "sticky" molecules such as HNO3 and NH3. The method reported here makes use of active, continuous passivation, where the instrument interfaces are continuously exposed to 0.01-1 ppm of fluorinated acidic or basic surfactants. To reduce HNO3 response times, perfluoroheptanoic acid and perfluorobutanesulfonic acid vapors are evaluated as passivation species. 1H,1H-perfluorooctylamine is used to improve NH3 response times. The resulting time responses using the perfluoroalkanoic acids are on the order of 0.4-0.7 s for a 75% quantitative recovery of HNO3, and 1-5 s for 90% recovery. Similar response time improvements are seen in detection of NH3 using perfluorooctylamine (<1 s for a 75% recovery, ∼ 2 s for 90% recovery). This generally applicable methodology significantly improves the capability of eddy covariance flux and real-time plume-based measurements of highly polar molecules that have historically been hampered by slow response times due to adsorption on sampling system surfaces. The utility of this approach is demonstrated by field measurements of HNO3 eddy covariance fluxes in a central U.S. prairie.

State-resolved velocity map imaging of surface-scattered molecular flux.

This work describes a novel surface-scattering technique which combines resonance enhanced multiphoton ionization (REMPI) with velocity-map imaging (VMI) to yield quantum-state and 2D velocity component resolved distributions in the scattered molecular flux. As an initial test system, we explore hyperthermal scattering (E(inc) = 21(5) kcal mol(-1)) of jet cooled HCl from Au(111) on atomically flat mica surfaces at 500 K. The resulting images reveal 2D (v(in-plane) and v(out-of-plane)) velocity distributions dominated by two primary features: trapping/thermal-desorption (TD) and a hyperthermal, impulsively scattering (IS) distribution. In particular, the IS component is strongly forward scattered and largely resolved in the velocity map images, which allows us to probe correlations between rotational and translational degrees of freedom in the IS flux without any model dependent deconvolution from the TD fraction. These correlations reveal that HCl molecules which have undergone a large decrease in velocity parallel to scattering plane have actually gained the most rotational energy, reminiscent of a dynamical energy constraint between these two degrees of freedom. The data are reduced to a rotational energy map that correlates with velocity along and normal to the scattering plane, revealing that exchange occurs primarily between rotation and the in-plane kinetic energy component, with v(out-of-plane) playing a relatively minor role.

Quantum structure of the intermolecular proton bond.

A proton shared between two closed-shell molecules, [A.H+.B], constitutes a ubiquitous soft binding motif in biological processes. The vibrational transitions associated with the shared proton, which provide a direct probe of this interaction, have been extensively studied in the condensed phase but have yielded only limited detailed information because of their diffuse character. We exploited recent advances in gas-phase ion spectroscopy to identify sharp spectral features that can be assigned to both the shared proton and the two tethered molecules in a survey of 18 cold, isolated [A.H+.B] ions. These data yield a picture of the intermolecular proton bond at a microscopic scale, facilitating analysis of its properties within the context of a floppy polyatomic molecule.

Isomer-specific spectroscopy of the (H2O)8- cluster anion in the intramolecular bending region by selective photodepletion of the more weakly electron binding species (isomer II).

The vibrational predissociation spectra of the two more strongly electron binding forms of the (H2O)8- anion are obtained in the HOH intramolecular bending region. This is accomplished by deconvoluting the overlapping spectra obtained from a mixed ensemble using a population modulation scheme in which the low electron binding isomer (II) is removed from the ion packet prior to spectroscopic analysis. By choosing the energy of the photodepletion laser to lie between the vertical detachment energies of the two isomers, the contribution from isomer II can be quantitatively eliminated, leaving the population of I largely unaffected. The low binding energies involved in the application of the method to the water cluster anions necessitate that this should be carried out in the midinfrared, thus requiring two tunable ir laser systems for implementation. The isolated spectrum of isomer 1 displays a strong, redshifted feature associated with a double H-bond acceptor (AA) water molecule in direct contact with the excess electron and a large gap before higher energy features appear that are typically associated with (acceptor/donor) AD and ADD binding sites in the network. The more weakly binding isomer II does not display the AA feature and instead contributes broad structure at intermediate redshifts that merges with the region associated with neutral water cluster networks.

Infrared spectrum and structural assignment of the water trimer anion.

The bending vibrational spectrum of the perdeutero isotopomer of the water trimer anion has been measured and compared with spectra calculated using the MP2, CCSD, and Becke3LYP electronic structure methods. Due to its low electron binding energy (approximately 150 meV), only the OD bending region of the IR spectrum of (D2O)3(-) is accessible experimentally, with electron ejection dominating at higher photon energies. The calculated spectrum of the isomer having three water molecules arranged in a chain agrees best with the experimental spectrum. In the chain isomer, the excess electron is bound to the terminal water monomer with two dangling OH groups. This is consistent with the electron binding mechanism established previously for the (H2O)n(-) (n = 2, 4-6) anions.