Orientation Of CO Molecules Research Articles

Comparative studies of the degrees of orientation of CO molecules pumped by intense femtosecond two-color pulses based on high-order harmonic generation and Coulomb explosion imaging

With an intense femtosecond two-color pulse optimized for the generation of even-order harmonics from CO molecules, we directly measure the actual degrees of orientation by utilizing the Coulomb explosion imaging technique with appropriate probe polarization. We find that the macroscopic orientation of CO molecules is negligible even when significant even-order harmonics are observed. This finding shows that the generation of even-order harmonics cannot be ascribed to the macroscopic orientation of CO molecules. The rotational wave packet of CO molecules created with an intense femtosecond two-color pulse is thought to be in an uninvestigated quantum state, which cannot be explained by the theoretical model based on the Born–Oppenheimer approximation, without inversion symmetry at any of the three steps of high-order harmonic generation, leading to the generation of even-order harmonics.

Field-free orientation of CO molecule by combining two-color femtosecond laser pulse with time-delayed positively chirped laser pulse

The orientation of CO molecule controlled by two-color femtosecond laser pulse and positively chirped laser pulse is investigated theoretically. Compared with the single two-color or positively chirped laser pulse, the combination of two laser pulses with an optimized delay time can greatly improve the degrees of the positive and negative molecular orientation, respectively. The effects of intensity of two-color femtosecond laser pulse on molecular orientation are discussed, and the molecular orientation can be achieved by a calculated intensity. Additionally, the delay time between two-color femtosecond laser pulse and positively chirped laser pulse can significantly enhance the molecular orientation. Furthermore, by varying the carrier frequency and the chirp value of the positively chirped laser pulse, it is shown that the molecular orientation can be changed to some degree. The positive and negative molecular orientation can also be manipulated by adjusting the relative amplitude of the second harmonic field with respect to the fundamental.

Field-Free Molecular Orientation Induced by a Single-Cycle THz Laser Pulse Train

AbstractWe investigate theoretically the field-free orientation of CO molecules induced by a single-cycle THz laser pulse train. It is shown that the molecular orientation can be obviously enhanced by applying the pulse train. The laser intensity and pulse number have some effects on the molecular orientation. The high degree of field-free molecular orientation |⟨cosθ⟩|max=0.9246 is obtained at temperature T=0 K. The variations of the maximum orientation degree with the experimentally available pulse number and peak intensity are given. Temperature T has a considerable influence on the field-free molecular orientation. The maximal field-free molecular orientation at T=0, 10, 20 and 30 K for N=14 and E0=1.8 MV/cm are |⟨cosθ⟩|max=0.9188, 0.7338, 0.6055 and 0.5154 in order, and the corresponding effective duration times of molecular orientation are Δt=0.759, 0.432, 0.297 and 0.117 ps.

CO Molecules on a NaCl(100) Surface: Structures, Energetics, and Vibrational Davydov Splittings at Various Coverages

In this work, we study the adsorption of CO from low to high coverage at a defect-free NaCl(100) surface by means of cluster and periodic models, using highly accurate wave function-based QM:QM embedding as well as density functional theory. At low coverages, the most accurate methods predict a zero-point-corrected adsorption energy of around 13 kJ/mol, and the CO molecules are found to be oriented perpendicular to the surface. At higher coverages, lower-energy phases with nonparallel/upright, tilted orientations emerge. Besides the well-known p(2 × 1)/antiparallel phase (T/A), we find other tilted phases (tilted/irregular, T/I; tilted/spiral, T/S) as local minima. Vibrational frequencies for CO adsorbed on NaCl(100) and Davydov splittings of the C–O stretch vibration are also determined. The IR spectra are characteristic fingerprints for the relative orientation of CO molecules and may therefore be used as sensitive probes to distinguish parallel/upright from various tilted adsorption phases.

Field-free orientation dynamics of CO molecule by combining two-color shaped laser pulse with THz laser pulse train

Field-free orientation of CO molecule is studied theoretically by combining a nonresonant two-color shaped laser pulse with a time-delayed THz laser pulse train. The molecular positive and negative orientation degrees can be significantly enhanced by the combination of two-color shaped laser pulse and THz laser pulse train compared with the single THz laser pulse train. The effects of the pulse number of THz laser pulse train and the delay time between two-color shaped laser pulse and THz laser pulse train on molecular orientation are discussed. Further, the influence of the initial rotational temperature on molecular orientation is investigated. The maximal positive and negative orientation degrees obtained are 〈cos θ〉pmax=0.91 and 〈cos θ〉nmax=0.89, respectively.

Field-free orientation of CO molecules induced by a chirped pulse

We theoretically show that the field-free molecular orientation induced by a single-color or dual-color pulse can be manipulated by changing the pulse linear chirp. It is found that the maximum degrees of molecular orientation created by a single-color chirped field exhibit periodic changes with respect to the increasing of the chirp value. It is also shown that the enhancement or suppression of the molecular orientation excited by a dual-color chirped pulse can be achieved by varying the chirp values of the fundamental field and/or the second harmonic field.

Erratum: Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields [Phys. Rev. Lett. 103, 153002 (2009)

Erratum: Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields [Phys. Rev. Lett. 103, 153002 (2009)

Transition between Mechanisms of Laser-Induced Field-Free Molecular Orientation

The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 × 10(13) W/cm(2), the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrating absolute rates of nonperturbative strong-field molecular ionization.

Efficiently field-free orientation of CO molecules by a femtosecond pulse and a THz pulse train

We theoretically study the field-free orientation dynamics of CO molecules in three cases, with the orientation steered (I) by a single-cycle THz pulse, (II) by a single-cycle THz pulse train L1, and (III) by the combination of an off-resonant femtosecond pulse L0 with L1. In case (I), the strong intensity of the single-cycle THz pulse is required to realize efficient molecular orientation. In case (II), by using a pulse train L1 with experimentally accessible intensity 1 MV cm−1, the maximum orientation degree |〈cosθ〉|max = 0.906 is achieved when pulse number N = 25. In case (III), by introducing the Raman excitations of L0, the pulse train L1 with only N = 3 can realize a high degree of orientation, |〈cosθ〉|max = 0.896. Moreover, we investigate the dependence of the thermally averaged orientation on the initial rotational temperature in cases (II) and (III). The maximum orientation degree |〈〈cosθ〉〉|max decreases with the increase of rotational temperature in the two cases. However, case (II) is more robust against the rotational temperature: |〈〈cosθ〉〉|max > 0.5 can still be achieved when the rotational temperature is as high as T = 30 K.

Field-free molecular orientation induced by combined femtosecond single- and dual-color laser pulses: The role of delay time and quantum interference

The coherent control of field-free molecular orientation of CO with combined femtosecond single- and dual-color laser pulses has been theoretically studied. The effect of the delay time between the femtosecond single- and dual-color laser pulses is discussed, and the physical mechanism of the enhancement of molecular orientation with pre-alignment of the molecule is investigated. It is found that the basic mechanism is based on the creation of a rotational wave packet by the femtosecond single-color laser pulse. Furthermore, we investigate the interference between multiple rotational excitation pathways following pre-alignment with femtosecond single-color laser pulse. It is shown that such interference can lead to an enhancement of the orientation of CO molecule by a factor of 1.6.

Efficient field-free orientation of CO molecules using a three-step excitation scheme

An efficient three-step excitation scheme is proposed to achieve the strong molecular orientation. By taking the CO molecule for example, via the Raman excitation by the femtosecond pulse L 1 and subsequently resonant excitations by a single-cycle THz pulse L 2, the high degree of the field-free orientation of |⟨ cosθ ⟩ |max = 0.8921 can be obtained at the rotational temperature T = 0 K. An additional single-cycle THz pulse L 3 is introduced to further regulate the population distribution on rotational states and decrease the reduction of the orientation originating from the offset phase. With the increase of the central frequency ω 3 of L 3, the optimal |⟨ cosθ ⟩ |max increases monotonically. When ω 3 = 1.0 THz, the degree of the field-free orientation reaches |⟨ cosθ ⟩ |max = 0.9401. Moreover, our scheme is robust against the rotational temperature. Although the molecular orientation weakens with the rise of rotational temperature, |⟨ ⟨ cosθ ⟩ ⟩ | > 0.5 is still obtained until T = 22.8 K.

Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields

We report the first experimental observation of nonadiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction, and molecular frame photoelectron emission.

Photoemission Study of CO2adsorbed on K/Cu(110). Analysis of Adsorbate Induced Structures

Improved adsorption characteristics of a K predosed Cu(110) surface with a coverage corresponding to a point before the work function minimum have been confirmed. Analysis of the CO-induced orbitals in the photoelectron spectra of the CO/K/Cu(110) interface for low coverages of carbon monoxide adsorbed at 118 K has been done. Noticeable changes of the parameters of the orbitals with increasing CO coverage have been registered. Elongation of the C-O bond without dissociation of the molecule has been deduced from the energetic separation of 3.4 eV between the 4σ and the 1π orbital. A weakening of the CO-interface bond with coverage has been found on the base of decreasing 5σ-1π separation with increasing 4σ/5σ intensity ratio. Perpendicular orientation of CO molecules to the plane of the substrate surface has been concluded from analysis of the constant initial state spectra of the 4σ and 5σ/1π orbitals. PACS numbers: 79.60.Jν, 33.60.—q, 82.80.Pv

Thermal and Structural Properties of N2 or CO on Ag(111)

Low-energy electron diffraction (LEED) measurements were made for N2 and CO adsorption on Ag(111) at temperatures (T) >25 K. The results indicate that at coverages up to a saturated monolayer, N2 and CO each forms a rotated incommensurate hexagonal structure. The first layer of CO is somewhat more strongly bound to the surface than that of N2, and has a monolayer lattice spacing that is ∼5% smaller. This result is interpreted as being due to a more perpendicular orientation of the CO molecules. The isobaric expansion coefficient for the CO monolayer is considerably smaller than that of N2, which may also be related to a more perpendicular orientation of CO molecules. The epitaxial rotation angle measured for N2 is close to the Novaco-McTague prediction for the lowest temperatures measured; the angle for CO, on the other hand, is very different from the predictions. No additional order was noted upon adsorption of the N2 second layer. Adsorption of a CO second layer, however, produces an oblique and mutual...

Photoemission Study of Co Adsorbed on K/Cu(110). Analysis of Adsorbate Induced Structures

Improved adsorption characteristics of a K predosed Cu(110) surface with a coverage corresponding to a point before the work function minimum have been confirmed. Analysis of the CO-induced orbitals in the photoelectron spectra of the CO/K/Cu(110) interface for low coverages of carbon monoxide adsorbed at 118 K has been done. Noticeable changes of the parameters of the orbitals with increasing CO coverage have been registered. Elongation of the C-O bond without dissociation of the molecule has been deduced from the energetic separation of 3.4 eV between the 4σ and the 1π orbital. A weakening of the CO-interface bond with coverage has been found on the base of decreasing 5σ-1π separation with increasing 4σ/5σ intensity ratio. Perpendicular orientation of CO molecules to the plane of the substrate surface has been concluded from analysis of the constant initial state spectra of the 4σ and 5σ/1π orbitals. PACS numbers: 79.60.Jν, 33.60.—q, 82.80.Pv

Coupled harmonic oscillator models of carbon monoxide adsorbed on stepped, platinum surfaces

Harmonic oscillator models are used to explain recent experimental data on infrared absorption by CO molecules adsorbed on two stepped platinum surfaces. These data reveal only a lower frequency band at low coverage and only a higher frequency band at high coverage. Both bands exist over a range of intermediate coverages. The data are explained by a coupled-dipole model which includes the effects of electronic polarizability, the tilted orientation of CO molecules at step sites, and the electric field enhancement at step sites. The lower-frequency band is associated with CO molecules adsorbed on step sites and the higher-frequency band is associated with two-dimensional islands consisting of both step and terrace CO. The model explains the observed variation of frequency and intensity with coverage for CO adsorption on Pt(533) and Pt(432) surfaces. The model calculations indicate that the wavenumber for a single, linearly bonded CO molecule is about 9 cm −1 higher on a terrace site than on a step site.

Interaction of CO molecules adsorbed on stepped platinum surfaces.

Recent reflection-absorption infrared data of CO adsorbed on Pt(533) and Pt(432) surfaces are explained by a calculation of normal modes based on a dipole coupling model. The model includes electronic polarizability and field-enhancement effects, and allows for the tilted orientation of CO molecules on step sites. The low-frequency band arises from one-dimensional arrays of CO molecules on step sites; the onset of the higher-frequency band signals a transition from one-dimensional to two-dimensional arrays of CO molecules.

Characterisation of a Low Temperature Surface Phase of CO on Pt{110}

Adsorption of CO on a Pt{110} (1 × 2) surface precooled to 160 K is found to proceed without loss of the (1 × 2) reconstruction, in contrast to adsorption at higher temperatures. Using high resolution electron energy loss spectroscopy, we find that adsorption occurs initially into a linear species, with the additional occupation of bridged sites at fractional coverages above 0.2. At saturation, the ratio of bridged to linear species is 1:3. The absorption intensity from the linear species is shown to be 2.5 times that of the bridged species. Heating the saturated surface to temperatures between 280 and 340 K results in an irreversible transformation of the bridged species to linear, and a lifting of the (1 × 2) reconstruction to (1 × 1). The orientation of CO molecules in the low temperature metastable phase on the (1 × 2) surface was determined using angle-resolved photoemission, with electron collection in the plane orthogonal to the incidence plane. The CO molecules are found to be tilted away from the macroscopic surface normal, along the [100] azimuth, by 15 ± 5°. This contrasts with our previous findings for the stable phase of CO on the annealed (1 × 1) surface, where the tilt angle was found to be 26 ± 2° along the [211] azimuth.

ANGLE-RESOLVED PHOTOEMISSION FROM SURFACES AND ADSORBATES

In the application of angle-resolved photoemission spectroscopy to the study of surface systems, two basic kinds of information are sought : (1) the positions of atoms in the surface and in adsorbed species ; (2) the nature of the electronic states characteristic of free surfaces and those involved in the chemical binding of adsorbates. This paper surveys recent progress in both of these atomic and electronic aspects of the field. An example of an atomic-structure investigation using angle-resolved photoemission is the determination of the orientation of CO molecules adsorbed on Ni. In many studies in the valence band region, the atomic and electronic information is not readily separable, since a complete description of the electronic structure (and therefore the valence band spectra) requires a knowledge of the atomic positions. The role here of semiempirical calculations of two-dimensional band structures (E(k||) dispersion relations) using the Linear-Combination of Atomic Orbitals (LCAO) or pseudopotential methods is emphasized. As examples, angle-resolved photoemission measurements of the E(k||) relations for surface states on GaAs(110) and Cl-induced states on Si(111) are compared with model calculations. Parity considerations and also anisotropies in core level photoemission from two-dimensional systems are discussed.