Metastable CO Research Articles

Electrostatic surface guiding of cold polar molecules with a single charged wire

This paper proposes a scheme to guide cold polar molecules by using a single charged wire half embanked in an insulating substrate and a homogeneous bias electric field, which is generated by a plate capacitor composed of two infinite parallel metal plates. The spatial distributions of the electrostatic field produced by the combination of the charged wire and the plate capacitor and the corresponding Stark potentials (including dipole forces) for metastable CO molecules are calculated, the relationships between the electric field and the parameters of our charged-wire layout are analysed. It also studies the influences of the insulator on the electric field distribution and the discharge effect. This study shows that the proposed scheme can be used to guide cold polar molecules in the weak-field – seeking states, and to form various molecule-optical elements, such as molecular funnel, molecular beam-splitters and molecule interferometer, even to construct a variety of integrated molecule-optical elements and their molecule chips.

Reactions in trifluoroacetic acid (CF3COOH) induced by low energy electron attachment

Abstract Dissociative electron attachment to trifluoroacetic acid (CF 3 COOH) is characterized by an intense low energy shape resonance located near 1 eV and a comparatively weaker core excited resonance located near 7 eV. The shape resonance decomposes into the fragment ions CF 3 COO − , CF 2 COO − , and CF 2 - . The underlying reactions include simple bond cleavage but also more complex sequences involving multiple bond cleavages, rearrangement in the precursor ion and formation of new molecules (HF, CO 2 ). The core excited resonance additionally decomposes into F − , CF 3 - and probably metastable CO 2 - .

Singlet–triplet excitation spectrum of the CO–He complex. I. Potential surfaces and bound–bound CO(a 3Π←X 1Σ+) transitions

The interaction of He with metastable CO(a 3Π) gives rise to two adiabatic potential surfaces of reflection symmetry A′ and A″ which were calculated with the partially spin-restricted open-shell single and double excitation coupled cluster method with perturbative triples, RCCSD(T). Two diabatic potentials were constructed and fitted analytically; the appropriate form of the angular expansion functions was derived from general invariance properties. From variational calculations on these diabatic potential surfaces we obtained the quasibound vibration-rotation-spin levels of the CO–He complex in its lowest triplet state. Only the lower spin–orbit levels of this complex with approximate quantum number Ω=0 of the CO(a 3Π) monomer were found to be stable with respect to dissociation into He and triplet CO. The potential and the bound van der Waals levels of the ground state CO(X 1Σ+)–He complex were recalculated and used in combination with the triplet excited state wave functions to compute the line strengths and the bound–bound part of the singlet–triplet excitation spectrum of the CO–He complex. The spin-forbidden singlet–triplet transitions access mainly the higher spin–orbit levels with |Ω|=1, but these were found to undergo rapid predissociation. The companion Paper II explicitly studies this process, predicts the excited state lifetimes, and generates the bound-continuum part of the CO–He singlet–triplet spectrum.

Production and application of translationally cold molecules

Inspired by the spectacular successes in the field of cold atoms, there is currently great interest in cold molecules. Cooling molecules is useful for various fundamental physics studies and gives access to an exotic regime in chemistry where the wave property of the molecules becomes important. Although cooling molecules has turned out to be considerably more difficult than cooling atoms, a number of methods to produce samples of cold molecules have been demonstrated over the last few years. This paper aims to review the application of cold molecules and the methods to produce them. Emphasis is put on the deceleration of polar molecules using time-varying electric fields. The operation principle of the array of electrodes that is used to decelerate polar molecules is described in analogy with, and using terminology from, charged-particle accelerators. It is shown that, by applying an appropriately timed high voltage burst, molecules can be decelerated while the phase-space density, i.e. the number of molecules per position-velocity interval, remains constant. In this way the high density and low temperature in the moving frame of a pulsed molecular beam can be transferred to the laboratory frame. Experiments on metastable CO in states that are either repelled by or attracted to high electric fields are presented. Loading of slow molecules into traps and storage rings is discussed.

Identification of Defect Sites on MgO(100) Surfaces

Methodologies are reported that allow the identification of extended defects on MgO(100) surfaces. The techniques used include low-energy electron diffraction (LEED), metastable impact electron spectroscopy (MIES), D2O and CO temperature-programmed desorption (TPD), and MIES of adsorbed Xe. LEED and MIES data for MgO(100), TPD spectra of D2O and CO, and MIES spectra for adsorbed Xe are significantly altered as a function of the density of the extended defects. NO is shown to be an ineffective probe molecule for titration of defect sites on MgO(100).

Alternate gradient focusing and deceleration of a molecular beam.

Neutral dipolar molecules can be decelerated and trapped using time-varying inhomogeneous electric fields. This has been demonstrated only for molecules in low-field seeking states, but can, in principle, be performed on molecules in high-field seeking states as well. Transverse stability is then much more difficult to obtain, however, since molecules in high-field seeking states always experience a force towards the electrodes. Here we demonstrate that an array of dipole lenses in alternate gradient configuration can be used to maintain transverse stability. A pulsed beam of metastable CO in high-field seeking states is accelerated from 275 to 289 m/s as well as decelerated from 275 to 260 m/s.

Kinetic energy distributions for O − and metastable CO * produced by electron stimulated desorption from condensed CO 2

We report kinetic energy (KE) distributions of O − anions and metastable CO * produced by 10–30 eV electron stimulated desorption from multilayer CO 2 films condensed on polycrystalline platinum at 20 K. The O − KE distributions are obtained from energy analysis with a hemispherical electrostatic deflector and the CO * KEs from time-of-flight measurements. Both anion and metastable energy distributions are broadened and the energy shifted is compared to those of the gas phase. The narrow (∼160 meV) and low energy (∼60 meV) peaked CO * KE distributions reveal that post-dissociation interactions play a minor role in the broadening, whereas the wide (>1 eV) KE distributions of O − result mainly from multiphonon excitation of the substrate by the departing stable fragment anion.

Infrared Spectra of Matrix-Isolated [OC···X2] and [CO···X2] Complexes (X = Cl, Br) and ab Initio Calculations

Co-deposition of argon or nitrogen dilute samples of natural and isotopic carbon monoxide and halogen molecules (Cl2, Br2, BrCl) produced new narrow absorptions lines assigned to X2···CO complexes. With BrCl, in an argon matrix two 1:1 complexes were observed, one stronger than the Br2···CO complex, the other one weaker than the Cl2···CO complex. In the nitrogen matrix, one complex was only observed, namely, the ClBr···CO complex. For all complexes, observed shifts were larger in nitrogen than in argon matrices and similar to available gas-phase results for Cl2···CO and Br2···CO complexes. Irradiation of the samples with a filtered xenon lamp (λ > 360 nm) induced in an argon matrix (not in a nitrogen matrix) the isomerization of the complexes into a metastable form CO···X2 identified for the first time. Back isomerization was observed by weak temperature increase. The structure and energetics of the four complexes formed between CO and BrCl were investigated by DFT ab initio calculations and compared to c...

Singlet and triplet states of the CO 3 and CO 4 molecules

The C 2v isomers of CO 2 and CO 3 and the metastable D 2d isomer of CO 4 are shown to have a similar electronic nature in both their ground singlet and excited triplet states. A simple MO model explaining the electronic structure of the species containing the cyclic CO 2 fragment was suggested. On the basis of qualitative analysis and the results of MP2/6-31G* calculations, it is shown that the radiationless singlet–triplet decay of the lowest vibrational levels of the metastable CO 4(1 1A 1, D 2d) state is hardly probable, but that such a decay seems to be possible in the case of higher-lying vibrational levels of this state.

Electron-impact desorption of metastable particles from CO films

The impact of monochromatic low-energy electrons (4–52 eV) is found to induce desorption of metastable particles (MP) from multilayer films formed by condensing CO gas on a Pt(111) substrate held at 20 K. The results are compared with metastable production from gaseous CO. Metastable CO (CO*) is identified as the major desorbed species, its desorption is discussed in terms of intramolecular to molecule-surface vibrational energy transfer. Primary electronic excitation proceeds via the states, a′3Σ+, d3Δ, e3Σ−, I1Σ−, and D1Δ which contribute to MP desorption directly or by first decaying in high vibrational levels of the a3Π configuration. The energy threshold of 8.0 eV for CO* desorption indicates that CO in low vibrational levels of the a3Π state does not desorb. Electronic excitation occurs either near the surface of the CO film or in the bulk. In the latter case, excitonic motion to the surface can be followed by CO* desorption.

Nanoscale Island Formation during Oxidation of Carbon Monoxide Adlayers at Ordered Electrochemical Interfaces: A Dipole-Coupling Analysis of Coverage-Dependent Infrared Spectra

Model dipole−dipole coupling calculations of infrared spectra for various ordered CO packing arrangements on Pt(111) have been undertaken as a function of the spatial dimensions of isolated nanoscale adlayer domains. Comparisons of some of these predictions with coverage (θCO)-dependent infrared spectra obtained upon partial adlayer oxidation on low-index Pt-group surfaces in aqueous electrochemical environments enable approximate estimates of densely packed CO island sizes to be obtained. The evidence for CO island formation, described in several earlier reports, is based on the observation of C−O stretching bands, νCO, whose frequency, band shape, and relative intensity exhibit markedly smaller alterations upon progressive electrooxidative removal in comparison with corresponding νCO frequency-coverage data obtained for increasing θCO by means of dosing with dilute CO solutions. The dipole-coupling calculations indicate a significant sensitivity of the νCO frequency and related parameters to island size even for domains containing sizable numbers (100−200) of CO molecules, suggesting the value of the procedure for providing at least rough estimates of nanoscale island dimensions. While the domain size-dependent νCO spectra should be dependent inevitably upon the microscopic CO packing geometry, the (relative) changes in the νCO frequency for atop (i.e. terminal) adsorbate are predicted to be dependent on the island dimensions, yet relatively insensitive to the adlayer structure, suggesting the practical utility of this parameter for island size diagnosis. The resulting estimates of average island dimensions, along with their stability as gleaned from experimental time-dependent spectra, are seen to vary widely on different low-index Pt-group electrodes. Interestingly, the metastable CO domains formed on Pt(100), estimated to contain 20−50 atop CO's, have dimensions roughly compatible with the restructured substrate “mesas” seen to form by scanning tunneling microscopy.

HX elimination from Ir(H)2X(PBu2tPh)2 promoted by CO coordination: assessment of X ligand influence

The coordinatively unsaturated complexes Ir(H)2X(PBu2tPh)2 [X=Cl, Br, I, N3, NCNSiMe3, NHC(O)CH3, OC(O)CF3, OSO2CF3, OC(O)CH3, SPh, OPh, F] all react within the mixing time in arene solvents to bind carbon monoxide. Subsequent reactivity of these CO adducts reductively to eliminate HX is dependent on the magnitude of the inherent destabilization caused by filled-filled repulsions between the ligand pπ orbitals and metal dπ orbitals. This destabilization is not sufficient to promote the loss of HX when X=Cl, Br, I, N3, NCNSiMe3, NHC(O)CH3 or OC(O)CF3. When X=OC(O)CH3 or SPh, metastable CO adducts are formed that ultimately lose HX. The complexes containing OPh or F quickly lose HX upon reaction with CO. The unusual iridium(I) complexes IrH(CO)2(PBu2tPh) and IrH(CO)(PBu2tPh)2 have been characterized by multinuclear NMR and IR spectroscopy. The reaction of Ir(H)2(F)(PBu2tPh)2 with CO in a glass vessel yields crystalline [Ir(H)2(CO)2(PBu2tPh)2][SiF5] and [Ir(CO)2(PBu2tPh)2][SiF5]·C6D6, both characterized by X-ray diffraction. The latter, although approximately square planar, has a C–Ir–C angle of only 162.7°. Crystallographic data (Pc at -165 °C) for [Ir(H)2(CO)2(PBu2tPh)2][SiF5], a=8.293(2), b=12.462(5), c=16.333(7) A, β=98.21(2)° with Z=2. Crystallographic data (P21/n at -172 °C) for [Ir(CO)2(PBu2tPh)2][SiF5]·C6D6, a=13.041(7), b=12.998(5), c=22.553(13) A, β=97.50(2)° with Z=4.

Observation of the electronic energy transfer reaction [formula omitted] in molecular beams

Metastable CO(a 3Π) molecules in a molecular beam were allowed to interact with ground-state NO molecules under single collision conditions. Optical emission from the collision region was spectrally analyzed at a resolution of 1.5 Å FWHM. In the wavelength range 290–400 nm, NO β bands having an anomalous contour were observed. Superimposed on a broad intensity distribution were pairs of narrow “spikes”. The broadband emission is due to NO(B) formed by energy transfer from CO(a) to NO(X). The line emission is evidence of another reaction branch, giving NO(a) products. Only those NO(a) molecules can emit which originate in specific “gateway” levels, exhibiting some NO(B) character as a result of S/O perturbation. The gateway emission was filtered out of the broadband background utilizing magnetic field quenching.

State-to-state scattering of metastable CO molecules from a LiF (100) surface

Scattering of electronically excited, state-selected metastable CO(aΠ3) molecules from a cleaved LiF(100) surface is studied experimentally. Internal state distributions, fluorescence profiles, time-of-flight (TOF) profiles and angular distributions of the surviving metastable CO molecules are measured. Relative and absolute survival probabilities are determined for various impact velocities. The dependence of translation and rotational temperature on the velocity of the incoming beam unambiguously indicates a direct inelastic scattering process, even though the angular distributions are broad, both in plane and out of plane. The internal state distribution after scattering shows an overpopulation of the initially prepared Ω=1-component relative to the other spin components.

Confining CO molecules in stable orbits

A scheme is presented for the confinement of neutral molecules in stable orbits on the basis of the linear Stark effect in a properly designed electrostatic trap. Based on Stark shift measurements in electric fields up to 140 kV/cm, an electrode configuration which is, under realistic experimental conditions, capable of confining metastable CO(a 3Π) molecules with velocities up to 22 m/s is described. Metastable CO molecules can be laser-prepared inside the trap and their trajectories can be directly visualized via the spatially resolved detection of their fluorescence.

State-to-state Scattering of Metastable CO Molecules from a LiF(100) Surface

Survival of electronically excited, state-selected $\mathrm{CO}({a}^{3}\ensuremath{\Pi})$ molecules scattering from a cleaved LiF(100) surface is studied. Absolute survival probabilities are determined for various impact energies. Time-of-flight profiles, angular distributions, and internal state distributions of the surviving metastable CO molecules are measured. The dependence of their translational and rotational temperatures on the incoming velocity unambiguously indicates a direct inelastic scattering process even though the angular distributions are broad, both in plane and out of plane.

Radiative lifetimes, vibrational band strengths, relaxation mechanisms, and electronic transfer rate constants for the perturbed (CO(a′ 3Σ +, υ′ = 31 and 35) levels

One-photon, laser-induced fluorescence has been used to study the CO(a′ 3Σ +, υ′ = 31 and 35), states which are strongly perturbed over a range of low rotational levels by the CO( b 3 Σ +, υ′ = 0 and 1) states, respectively. The effects of these perturbations on the radiative lifetimes, the radiative branching to the CO( a 3 Π, υ″) levels, and the collisional relaxation of these CO(a′ and b) states in He are discussed. The CO( b 3 Σ +, ν′ = 0, 1) and CO( a′ 3 Σ +, υ′ = 31, 35) molecules were generated by pulsedlaser excitation from metastable CO( a 3 Π) molecules that were prepared by a dc discharge through a flow of CO 2 in He.

Resonance-enhanced multiphoton ionization spectroscopy of X 1Σ+ and a 3Π carbon monoxide using electron stimulated desorption as a source for rovibronically excited species

Electron stimulated desorption from CO monolayers chemisorbed on transition metal surfaces and from condensed CO layers were utilized as new sources for the production of nonthermal rovibrational distributions of CO probed by resonance-enhanced multiphoton ionization spectroscopy (REMPI). Highly vibrationally excited ground state CO molecules were studied by (1+1)-REMPI via the A 1Π and D′ 1Σ+ intermediate states, and metastable CO via the b 3Σ+ and c 3Π intermediate states. Rotationally resolved (1+1) spectroscopy via A 1Π (v′≥20) allowed the verification of the intermediate term energies, including the existence of a potential barrier for the A 1Π state. (1+1) spectroscopy via D′ 1Σ+ is shown to provide the possibility to directly probe the D′ 1Σ+ potential curve. Additional spectral features accompanying the b 3Σ+←a 3Π (1+1) spectra which cannot be explained by known perturbations were found. In gas phase (2+1) spectroscopy via the B 1Σ+ and C 1Σ+ intermediate states, O- and S-branches could be detected even with linearly polarized light at very low CO densities (some 10−6 Pa).

Electron stimulated desorption of O− and metastable CO* from physisorbed CO2

We report electron stimulated desorption (ESD) measurements of O− yields produced by dissociative electron attachment (DEA) to physisorbed CO2. The molecules are condensed at about 17–20 K on polycrystalline Pt, either as pure multilayer films, or in submonolayer (ML) quantities onto thick rare gas substrates. For the pure disordered multilayer solids, we observe four peaks in the O− yield function at incident electron energies, E(e), of about 4.1, 8.5, 11.2, and 15 eV. The lowest two are assigned, respectively, to the 2Πu and 2Πg resonance states of CO−2, which dissociate into O−(2P)+CO(X 1Σ+), and are known to dominate the gas phase DEA O− production cross section for E(e)≤20 eV. Measurements of ESD CO* metastable yields from similar CO2 multilayer solids on Pt(111), also presented here, suggest that the 11.2 and 15 eV O− peaks are associated with the manifold of close-lying CO2*− states which dissociate into O−(2P)+CO* (a 3Π, a′ 3Σ+, or d 3Δ). For 0.15 ML of CO2 physisorbed on 20 ML thick rare gas substrate films significant sharp enhancements (fwhm ≤0.5 eV) are observed in the ESD O− yields at about 0.3–0.4 eV below the lowest substrate exciton energy. These enhancements are attributed to a coupling of the (electron plus exciton) core-excited anion resonances of the rare gas atoms to the dissociative Rydberg anion states of the coadsorbed CO2 at the solid’s surface. This is followed by a transfer of the charge and excitation energy to the coadsorbate.

Two-dimensional imaging of metastable CO molecules

Direct time and spatially resolved detection of metastable CO molecules, prepared in selected quantum states via pulsed laser excitation, is experimentally demonstrated in a molecular beam machine. Characterization of the molecular beam in terms of parallel and perpendicular velocity distributions and rotational temperatures is performed. A direct two-dimensional (2D) demonstration of the mass-focusing effect in binary gas mixtures is given. Two-dimensional imaging of the spatial distribution of the metastable a 3Π CO molecules in the beam after passage through a hexapole field is used to study hexapole focusing performance. Structured 2D images demonstrate the dependence of the focusing characteristics on the magnitude of the Λ-doubling and on the angular dependence of the focusing force in a hexapole consisting of cylindrical rods.