Metastable CO Research Articles

Surface triggered stabilization of metastable charge-ordered phase in SrTiO3

Charge ordering (CO), characterized by a periodic modulation of electron density and lattice distortion, has been a fundamental topic in condensed matter physics, serving as a potential platform for inducing novel functional properties. The charge-ordered phase is known to occur in a doped system with high d-electron occupancy, rather than low occupancy. Here, we report the realization of the charge-ordered phase in electron-doped (100) SrTiO3 epitaxial thin films that have the lowest d-electron occupancy i.e., d1-d0. Theoretical calculation predicts the presence of a metastable CO state in the bulk state of electron-doped SrTiO3. Atomic scale analysis reveals that (100) surface distortion favors electron-lattice coupling for the charge-ordered state, and triggering the stabilization of the CO phase from a correlated metal state. This stabilization extends up to six unit cells from the top surface to the interior. Our approach offers an insight into the means of stabilizing a new phase of matter, extending CO phase to the lowest electron occupancy and encompassing a wide range of 3d transition metal oxides.

High-fidelity Reaction Kinetic Modeling of Hot-Jupiter Atmospheres Incorporating Thermal and UV Photochemistry Enhanced by Metastable CO(a3Π)

A detailed modeling of simultaneous UV-photochemical and thermochemical processes in exoplanet atmosphere-like conditions is essential for the analysis and interpretation of a vast amount of current and future spectral data from exoplanets. However, a detailed reaction kinetic model that incorporates both UV photochemistry and thermal chemistry is challenging due to the massive size of the chemical system as well as the lack of understanding of photochemistry compared to thermal-only chemistry. Here, we utilize an automatic chemical reaction mechanism generator to build a high-fidelity thermochemical reaction kinetic model later then incorporated with UV photochemistry enhanced by metastable triplet-state carbon monoxide (a3Π). Our model results show that two different photochemical reactions driven by Lyα photons (i.e., H2 + CO(a3Π) → H + HCO and CO(X1Σ+) + CO(a3Π) → C(3P) + CO2) can enhance thermal chemistry resulting in significant increases in the formation of CH4, H2O, and CO2 in H2-dominated systems with trace amounts of CO, which qualitatively matches the observations from previous experimental studies. Our model also suggests that at temperatures above 2000 K, thermal chemistry becomes the dominant process. Finally, the chemistry simulated up to 2500 K does not produce any larger species such as C3 species, benzene, or larger (i.e., PAHs). This might indicate that the photochemistry of C2 species such as C2H2 might play a key role in the formation of organic aerosols observed in a previous experimental study.

Production of Metastable CO3+ through the Strong-Field Ionization and Coulomb Explosion of Formic Acid Dimer

Femtosecond laser pulses are utilized to drive multiple ionization of formic acid dimers and the resulting ions are studied using time-of-flight mass spectrometry. The interaction of formic acid dimer with 200 fs linearly polarized laser pulses of 400 nm with intensities of up to 3.7 × 1015 W/cm2 produces a metastable carbon monoxide trication. Experimental kinetic energy release (KER) measurements of the ions are consistent with molecular dynamics simulations of the Coulomb explosion of a formic acid dimer and suggest that no significant movement occurs during ionization. KER values were recorded as high as 44 eV for CO3+, in agreement with results from a classical Molecular Dynamics simulation of fully ionized formic acid dimers. Potential energy curves for CO3+ are calculated using the multireference configuration interaction (MRCI+Q) method to confirm the existence of an excited metastable 2Σ state with a significant potential barrier with respect to dissociation. This combined experimental and theoretical effort reveals the existence of metastable CO3+ through direct observation for the first time.

Approaching the transit time limit for high-precision spectroscopy on metastable CO around 6 μm.

As molecular spectroscopy makes its comeback to the limelight of fundamental sciences, scientists need ever better coherent light sources and diagnostic methods. Of particular importance for molecular spectroscopy is the mid infrared spectral region, where strong and narrow ro-vibrational excitations have their fundamental transition frequencies. Unfortunately, much technology in some portions of this spectral region is still rather pioneering. Here we present a high-resolution spectroscopy experiment, based on a molecular beam setup, which pushes the measured linewidth close to the transit time limit, on the order of 100 kHz. Moreover, we discuss the issue of frequency-noise characterization and the linewidth measurement of the ultrastable infrared laser used in the experiment.

Single-Molecule Dynamics in the Presence of Strong Intermolecular Interactions.

In contrast to conventional spectroscopic studies of adsorbates at high coverage that provide only spatially averaged information, we have characterized the laterally confined shuttling dynamics of a single molecule under the influence of intermolecular interactions by vibrational spectroscopy using a scanning tunneling microscope. The bridge sites on Pt(111) are only occupied by a CO molecule that is surrounded by four other CO molecules at on-top sites. The bridge-site CO undergoes laterally confined shuttling toward an adjacent on-top site to transiently occupy a metastable site, which is slightly displaced from the center of an on-top site through repulsive interaction with adjacent on-top CO molecules. Analysis of action spectra for the shuttling events reveals the C-O stretch frequency of the metastable CO. We also constructed a modified potential energy surface incorporating the intermolecular interaction, which reveals the underlying mechanism and provides a new way to experimentally determine detailed information on the energetics of the metastable state.

Adsorbate enhancement of electron emission during the quenching of metastable CO at metal surfaces.

When electronically excited CO(a(3)Π) collides with a Au(111) surface, electron emission can be observed with a quantum efficiency of 0.13. We have studied the influence of Ar, Kr and Xe adsorption on the electron emission efficiency resulting from CO(a(3)Π) quenching. Surprisingly, a single monolayer (ML) of rare gas dramatically enhances electron emission. For Ar and Kr bilayers, emission efficiency is further enhanced and approaches unity. The quenching mechanism involves electron transfer from the metal to the CO(a(3)Π) molecule followed by electron emission from the molecule. The enhanced emission efficiency is due to the long range nature of the initial electron transfer process and the rare gas adlayer's ability to reflect the electron emitted by the transient CO anion. This work shows that CO(a(3)Π) quenching is a useful model system for investigating the fate of electronically excited molecules at surfaces.

Activation of nitrogen Brønsted acids: synthesis and reactivity of a new class of nitrogen acid complexes.

The nitrogen acids RC(O)NHNO2, N-nitroamide, R = CH3 (1), C2H5 (2) and N-nitrocarbamate, R'OC(O)NHNO2, R' = CH3 (3), C2H5 (4) are a class of primary N-nitrocarboxamide compounds that oxidatively add to trans-Ir((I))(Cl)(N2)(PPh3)2 to give six-coordinate Ir((III))(η(2)-(NO2)-nitrogen acid)(H)(Cl)(PPh3)2 complexes 5-8. Unexpected fluxional behavior of the complexes in solution is observed by (1)H NMR spectroscopy. Reaction intermediates of the oxidative addition reactions were also observed and monitored using (31)P and (1)H NMR and solution IR spectroscopies. Complex 5 reacts with methyl triflate in CH3CN to generate bis(acetonitrile) complex (9) from a net loss of the nitrogen acid anion. P(CH3)2Ph reacts with 5 to give phosphine-substituted and P(CH3)2Ph addition isomers (10). Reactivity studies of 5 with CO gave metastable CO adduct isomer 11, which loses CO on prolonged standing in solution.

CO (a3Π) quenching at a metal surface: Evidence of an electron transfer mediated mechanism.

We observe a strong influence of molecular vibration and surface temperature on electron emission promoted by the de-excitation of metastable CO(a(3)Π) on a clean Au(111) surface using a molecular beam surface scattering apparatus. The de-excitation is independent of incidence translational energy. These observations appear incompatible with existing theories of metastable particle de-excitation on metal surfaces, which are based on the Auger effect. Instead, they strongly suggest a mechanism involving formation of a transient anion whose lifetime is similar to the vibrational period of the CO molecule.

Ramsey-type microwave spectroscopy on CO ([formula omitted

Using a Ramsey-type setup, the lambda-doublet transition in the J=1,Ω=1 level of the a3Πstate of CO was measured to be 394064870(10)Hz. In a molecular beam apparatus, a beam of metastable CO is prepared in a single quantum level by expanding CO into vacuum and exciting the molecules using a narrow-band UV laser system. After passing two microwave zones that are separated by 50cm, the molecules are state-selectively deflected and detected 1 meter downstream on a position sensitive detector. In order to keep the molecules in a single mJB level, a magnetic bias field is applied. We find the field-free transition frequency by taking the average of the mJB=+1→mJB=+1 and mJB=-1→mJB=-1 transitions, which have an almost equal but opposite Zeeman shift. The accuracy of this proof-of-principle experiment is a factor of 100 more accurate than the previous best value obtained for this transition.

Alignment Selection of the Metastable CO(a 3Π1) Molecule and the Steric Effect in the Aligned CO(a 3Π1) + NO Collision

The aligned metastable CO(a (3)Π1) molecular beam was generated by an electronic excitation through the Cameron band (CO a (3)Π1 ← X (1)Σ(+)) transition. Beam characterization of the aligned molecular beam of CO(a (3)Π1) was carried out by (1 + 1) REMPI detection via the b (3)Σ(+) state. The REMPI signals showed the clear dependence on the polarization of the pump laser, and the experimental result was well reproduced by the theoretical simulation. This agreement confirms that aligned metastable CO(a (3)Π1) can be generated and controlled by rotating polarization of the pump laser. By using this technique, a single quantum state of CO(a (3)Π1) can be selected as a metastable molecular beam. The steric effect in the energy-transfer collision of CO(a (3)Π1) with NO forming the excited NO was carried out with this aligned CO(a (3)Π1) molecular beam. We find that the sideways orientation of CO(a (3)Π1) is more favorable in the formation of the excited NO(A (2)Σ(+), B (2)Π) than that for the axial collisions. The obtained steric effect was discussed with the aid of the spatial distribution of CO(a (3)Π1) molecular orbitals, and we find that specific rotational motion of CO(a (3)Π1) in each state may not be a dominant factor in this energy-transfer collision.

Optical Emission from Microwave Activated C/H/O Gas Mixtures for Diamond Chemical Vapor Deposition

The spatial distributions and relative abundances of electronically excited H atoms, OH, CH, C(2) and C(3) radicals, and CO molecules in microwave (MW) activated CH(4)/CO(2)/H(2) and CO/H(2) gas mixtures operating under conditions appropriate for diamond growth by MW plasma enhanced chemical vapor deposition (CVD) have been investigated by optical emission spectroscopy (OES) as a function of process conditions (gas mixing ratio, incident MW power, and pressure) and rationalized by reference to extensive 2-dimensional plasma modeling. The OES measurements clearly reveal the switch in plasma chemistry and composition that occurs upon changing from oxygen-rich to carbon-rich source gas mixtures, complementing spatially resolved absorption measurements under identical plasma conditions (Kelly et al., companion article). Interpretation of OES data typically assumes that electron impact excitation (EIE) is the dominant route to forming the emitting species of interest. The present study identifies a number of factors that complicate the use of OES for monitoring C/H/O plasmas. The OH* emission from EIE of ground state OH(X) radicals can be enhanced by excitation energy transfer from metastable CO(a(3)Π) molecules. The CH* and C(2)* emissions can be boosted by chemiluminescent reactions between, for example, C(2)H radicals and O atoms, or C atoms and CH radicals. Additionally, the EIE efficiency of each of these radical species is sensitively dependent on any spatial mismatch between the regions of maximal radical and electron density, which itself is a sensitive function of elemental C/O ratio in the process gas mixture (particularly when close to 1:1, as required for diamond growth) and the H(2) mole fraction.

Producing translationally cold, ground-state CO molecules

Carbon monoxide molecules in their electronic, vibrational, and rotational ground state are highly attractive for trapping experiments. The optical or ac electric traps that can be envisioned for these molecules will be very shallow, however, with depths in the sub-milliKelvin range. Here, we outline that the required samples of translationally cold CO (X(1)Σ(+), v'' = 0, N'' = 0) molecules can be produced after Stark deceleration of a beam of laser-prepared metastable CO (a(3)Π(1)) molecules followed by optical transfer of the metastable species to the ground state via perturbed levels in the A(1)Π state. The optical transfer scheme is experimentally demonstrated and the radiative lifetimes and the electric dipole moments of the intermediate levels are determined.

An electrostatic elliptical mirror for neutral polar molecules

Focusing optics for neutral molecules finds application in shaping and steering molecular beams. Here we present an electrostatic elliptical mirror for polar molecules consisting of an array of microstructured gold electrodes deposited on a glass substrate. Alternating positive and negative voltages applied to the electrodes create a repulsive potential for molecules in low-field-seeking states. The equipotential lines are parallel to the substrate surface, which is bent in an elliptical shape. The mirror is characterized by focusing a beam of metastable CO molecules and the results are compared to the outcome of trajectory simulations.

Metastable states during dissociation of carbon dioxide hydrates below 273 K

In this study, the dissociation of isolated carbon dioxide hydrate particles of sizes in the range 0.25–2.5 mm was investigated. It was found that below the ice melting point, the hydrates dissociated into supercooled water (metastable liquid) and gas. The formation of the liquid phase during CO 2 hydrate dissociation was visually observed, and the pressures of the hydrate dissociation into supercooled water and gas were measured in the temperature range 249–273 K. These pressures agreed well with the calculated data for the supercooled water–hydrate–gas metastable equilibrium ( Istomin et al., 2006). In the P– T area on the phase diagram bounded by the ice–hydrate–gas equilibrium curve and the supercooled water–hydrate–gas metastable equilibrium curve, hydrates could exist for a long time because the metastable phase and their stability are not connected to the self-preservation effect. The growth of the metastable CO 2 hydrate film on the surface of supercooled water droplets formed during the hydrate dissociation was observed at pressure above the three-phase supercooled water–hydrate–gas metastable equilibrium pressure but still below the three-phase ice–hydrate–gas equilibrium pressure. It was found that the growth rate of the metastable CO 2 hydrate film was higher by a factor of 25 and 50 than that for methane hydrate and propane hydrate, respectively.

Slowing polar molecules using a wire Stark decelerator

We have designed and implemented a new Stark decelerator based on wire electrodes, which is suitable for ultrahigh vacuum applications. The 100 deceleration stages are fashioned out of 0.6 mm diameter tantalum and the array's total length is 110 mm, approximately 10 times smaller than a conventional Stark decelerator with the same number of electrode pairs. Using the wire decelerator, we have removed more than 90% of the kinetic energy from metastable CO molecules in a beam.

Ferrihydrite reactivity toward carbon dioxide

The reaction of ferrihydrite with gaseous CO 2 was investigated with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and density functional theory (DFT) calculations. ATR-FTIR results show that CO 2 reacts with ferrihydrite resulting in surface adsorbed carbonate species. The carbonate species experimentally observed in view of theoretical calculations are shown to be in large part monodentate binuclear complexes. These carbonate complexes exist as both inner-sphere and outer-sphere hydrogen-bonded complexes. Under “dry” conditions CO 2 reacts with free OH sites on the ferrihydrite surface resulting in a metastable bent CO 2 (bicarbonate-like) complex. Removal of the gaseous reactant leads to the loss of this metastable surface complex. The reaction of CO 2 with hydrated ferrihydrite results in only carbonate formation (no bicarbonate). In this circumstance, experiments and theoretical calculations suggest that hydrogen bound water on surface OH sites prevents the formation of the metastable bicarbonate species. Ferrihydrite that was allowed to react with atmospheric levels of CO 2 and water vapor resulted in the formation of surface carbonate coordinated as both inner and outer-sphere complexes.

A Stark decelerator on a chip

A microstructured array of 1254 electrodes on a substrate has been configured to generate an array of local minima of electric field strength with a periodicity of 120 μm about 25 μm above the substrate. By applying sinusoidally varying potentials to the electrodes, these minima can be made to move smoothly along the array. Polar molecules in low-field seeking quantum states can be trapped in these traveling potential wells. Recently, we experimentally demonstrated this by transporting metastable CO molecules at constant velocities above the substrate (Meek et al 2008 Phys. Rev. Lett. 100 153003). Here, we outline and experimentally demonstrate how this microstructured array can be used to decelerate polar molecules directly from a molecular beam. For this, the sinusoidally varying potentials need to be switched on when the molecules arrive above the chip, their frequency needs to be chirped down in time and they need to be switched off before the molecules leave the chip again. Deceleration of metastable CO molecules from an initial velocity of 360 m s-1 to a final velocity as low as 240 m s-1 is demonstrated in the 15–35 mK deep potential wells above the 5 cm long array of electrodes. This corresponds to a deceleration of almost 105 g, and about 85 cm-1 of kinetic energy is removed from the metastable CO molecules in this process.

Testing the time-invariance of fundamental constants using microwave spectroscopy on cold diatomic radicals

The recently demonstrated methods to cool and manipulate neutral molecules offer new possibilities for precision tests of fundamental physics theories. We here discuss the possibility of testing the time-invariance of fundamental constants using near degeneracies between rotational levels in the fine structure ladders of molecular radicals. We show that such a degeneracy occurs between the J = 6, Q2 1 and the J = 8, Q = 0 levels of the various natural isotopomers of carbon monoxide in its a 3pi state. As a result, the 2-photon transition that connects these states is 300 times more sensitive to a variation of m(p)/m(e) than a pure rotational transition. We present a molecular beam apparatus that might be used to measure these transitions with a fractional accuracy of 10(-12). Ultimately, the precision of an experiment on metastable CO will be limited by the lifetime of the a 3pi state. We will discuss other possible molecules that have a suitable level structure and can be cooled using one of the existing methods.

Trapping Molecules on a Chip in Traveling Potential Wells

A microstructured array of over 1200 electrodes on a substrate has been configured to generate an array of local minima of electric field strength with a periodicity of 120 microm about 25 microm above the substrate. By applying sinusoidally varying potentials to the electrodes, these minima can be made to move smoothly along the array. Polar molecules in low field seeking quantum states can be trapped in these traveling potential wells. This is experimentally demonstrated by transporting metastable CO molecules in 30 mK deep wells that move at constant velocities above the substrate.

The radiative lifetime of metastable CO (aΠ3, v=)

We present a combined experimental and theoretical study on the radiative lifetime of CO in the aΠ1,23, v=0 state. CO molecules in a beam are prepared in selected rotational levels of this metastable state, Stark-decelerated, and electrostatically trapped. From the phosphorescence decay in the trap, the radiative lifetime is measured to be 2.63±0.03ms for the aΠ13, v=0, J=1 level. From the spin-orbit coupling between the aΠ3 and the AΠ1 states a 20% longer radiative lifetime of 3.16ms is calculated for this level. It is concluded that coupling to other Π1 states contributes to the observed phosphorescence rate of metastable CO.