H. Rubahn | Syddansk Universitet (original) (raw)

Papers by H. Rubahn

The fundamentals of optical second harmonic generation (SHG) is briefly described in this paper. ... more The fundamentals of optical second harmonic generation (SHG) is briefly described in this paper. By measuring the reflected SHG intensities following surface near-resonant excitation, the 3m symmetry of a clean Si(111)7×7 surface is modified.The experimental results are obtained with a light beam at normal incidence and at an 45° angle of incidence geometry. Explanation and discussion of the experimental results are also given in the paper.

Faraday Discussions, 1991

Journal of Quantitative Spectroscopy and Radiative Transfer, 1991

Applied Surface Science, 2004

ABSTRACT The dynamics of short pulsed laser-induced desorption of alkali atoms from poly-dimethyl... more ABSTRACT The dynamics of short pulsed laser-induced desorption of alkali atoms from poly-dimethylsiloxane (PDMS) coated glass prism surfaces is investigated. Kinetic energies of desorbing sodium atoms of several hundred meV are found, increasing with increasing laser fluence. Evanescent wave measurements suggest that the desorbed atoms stem from a subsurface layer of the PDMS film, resulting in diffusional characteristics for the total yield of the desorption process.

Applied Surface Science, 2001

ABSTRACT

Applied Physics A, 2004

Excimer laser (193 nm and 157 nm) induced ablation and structure formation in poly-dimethylsiloxa... more Excimer laser (193 nm and 157 nm) induced ablation and structure formation in poly-dimethylsiloxane (PDMS) thin films is demonstrated. Ellipsometric measurements provide values of the optical constants of the films as well as their thicknesses, which are below 1 μm. At fluences above 160 mJ/cm 2 two pulses of UV light induce gratings with at minimum 1-μm periods and crossed gratings with 4-μm periods. The structure heights are between 10 nm and 20 nm with ridge widths of several hundred nanometres. The ablation occurs after a single incubation pulse with a threshold that increases logarithmically with the ablation wavelength increasing from 157 nm to 1064 nm. At 193 nm the ablation rate for 2 J/cm 2 is 127 nm/pulse.

Solid State Communications, 2001

We demonstrate via measurement of normal-incidence, near-resonant, angular dependent second harmo... more We demonstrate via measurement of normal-incidence, near-resonant, angular dependent second harmonic generation a strong change of Si(111)7 £ 7 surface symmetry as a consequence of submonolayer adsorption of sodium atoms. The results are interpreted by assuming that a new surface structure with lower symmetry coexists on the Na/Si(111)7 £ 7 interface with the 3m-symmetry of the clean Si(111) surface. This new surface structure could be due to an Na-induced surface reconstruction.

Thin Solid Films, 2000

Linear and non-linear optical spectroscopies are applied to monitor the formation of large, sever... more Linear and non-linear optical spectroscopies are applied to monitor the formation of large, several 10-nm-sized alkali clusters near epitaxially grown Au films on mica substrates. The Au films are covered by ultrathin, self-assembled alkane thiol films. The growth and morphology of these clusters are initially monitored in ultrahigh vacuum by comparison of calculated with measured Ž. polarization-dependent extinction spectra. We find that at low surface temperatures 150 K the cluster growth is very similar to growth directly on insulating substrates. With increasing surface temperature the size distribution of the clusters changes. A Ž. quantitative evaluation of ambient-air measurements with scanning force microscopy SFM supports the conclusions from Ž. optical spectroscopy. Field-enhancement effects at the surface of the clusters facilitate the observation of second harmonic SH light. From angular-and polarization-dependent SH-measurements we deduce ratios of Ž2.-tensor components for the cluster-films.

Optics Letters, 1997

Two-photon laser-induced f luorescence measurements of transition frequencies and lifetimes of la... more Two-photon laser-induced f luorescence measurements of transition frequencies and lifetimes of laser-excited Na atoms are presented as a function of distance (2-3 nm) to metal surfaces of different roughness. We characterized the surface roughness by fitting a Gaussian height-height correlation function to force microscopic data. An unusually small red shift of the Na transition frequency of a few hundred megahertz was observed. In the framework of a quantum-mechanical linear optical response formalism we fitted the lifetime and the frequency shift data simultaneously to obtain effective parameters of a selvedge dielectric function. In that way we are able to take into account both surface roughness and nonlocality, which in turn result in the observed small frequency shifts.

Nano Letters, 2006

Nanoscaled, needle-shaped frequency doublers have been generated via self-assembled surface growt... more Nanoscaled, needle-shaped frequency doublers have been generated via self-assembled surface growth from functionalized quaterphenylene molecules with a designed large hyperpolarizability. The nanofiber frequency doublers exhibit very weak fluorescence centered around 430 nm but emit a strong, resonance-enhanced second-harmonic signal when excited with infrared 80 fs laser pulses. The frequency doublers are employed to correlate second-harmonic response and morphology via two-dimensional true second-harmonic images of individual nanoaggregates obtained with the help of a femtosecond laser scanning microscope.

Materials Letters, 2009

1,4‴-Dimethoxy-4,1′:4′,1″:4″,1‴-quaterphenylene crystallizes in space group Pbca with a=7.4989(5)... more 1,4‴-Dimethoxy-4,1′:4′,1″:4″,1‴-quaterphenylene crystallizes in space group Pbca with a=7.4989(5) Å, b=6.1544(3) Å, c=40.359(2) Å, V=1862.6(2) Å3, and Z=4. In the crystal structure the molecules are arranged in layers, which are stacked in the direction of the crystallographic c-axis. The torsion between the molecules in a layer amounts to 69.06° resulting in a herringbone structure. The phenyl rings of the molecule are approximately planar with a torsion angle of 4.64° between the outer and the center rings. UV/Vis transmission measurements of the colorless crystal reveal a band gap of ~3.2eV.

The Journal of Chemical Physics, 1990

ABSTRACT

The Journal of Chemical Physics, 1988

A beam of highly vibrationally excited (vless than or equal to20) Liâ molecules has been generate... more A beam of highly vibrationally excited (vless than or equal to20) Liâ molecules has been generated via Franck--Condon pumping using a cw ring dye laser. The linearly polarized light from a second cw dye laser is used to determine the polarization of the remaining unscattered molecules of the beam after it has passed through a scattering cell filled with krypton gas. The dependence of the state selective total integral cross section on orientation is obtained from the anisotropy of the angular momentum distribution of the beam molecules described by 1+aâPâ(cos theta), where theta is the angle between the angular momentum vector and the velocity vector. The measured polarization increases from aâ = -0.019 +- 0.007 for Liâ(v = 0)--Kr collisions to aâ = -0.074 +- 0.016 for Liâ(v = 20)--Kr collisions. The measurements are interpreted in a vibrationally adiabatic approximation in terms of the potential anisotropy parameter qâ for an assumed simple anisotropic potential model V = +- (C/sub s/R/sup s/) (1+qâxPâ(cos ..gamma..)), where ..gamma.. is the angle between bond distance r and distance between center of mass R. For the v = 20 vibrationally excited molecules we obtain qâ = 1.38 +- 0.31 compared to qâ = 0.47 +- 0.15 for the v = 0 molecules. This large increase in anisotropy with vibrational excitation is compared with that obtained from a previously proposed model for Liâ(v)--He (J. Chem. Phys. 85, 7062 (1986)).

The Journal of Chemical Physics, 1986

ABSTRACT An HF nozzle beam and a cw HF laser beam in a coaxial and antiparallel arrangement facil... more ABSTRACT An HF nozzle beam and a cw HF laser beam in a coaxial and antiparallel arrangement facilitated exciting HF molecules vibrationally in v=1,J up to the saturation limit. A beam consisting exclusively of monomeric HF was generated by heating the beam source to 600 K and was confirmed using a mass spectrometer. Energy‐flux measurements made using a bolometer indicate that the vibrational energy distribution initiated by the laser is conserved, confirming that V–T energy transfer to monomers is slow. The maximum measured flux of HF(v=1,J) molecules was 1.5×1017 molecules s−1 sr−1 at 670 mbar stagnation pressure. Beams consisting of a mixture of monomeric and polymeric HF were generated using appropriate combinations of source temperature and pressure. In the presence of dimers, the excited monomers relax rapidly by a fast V–V transfer and subsequent predissociation of the metastable dimers. The loss of beam energy due to energy transport by the departing dimer fragments was detected bolometrically. The laser‐induced depletion of dimers was determined over a wide source pressure range at T0=300 K using a mass spectrometer. From a kinetic model of these dimer‐depletion data, the product of cross section and transfer probability for V–V energy transfer from HF (v=1) monomers to dimers was deduced to be 14 Å2. The terminal‐dimer‐concentration equation for HF nozzle beams was refined. An analysis of the energy fluxes measured by the bolometer with no laser excitation of the beam revealed that, for an HF nozzle beam, the flow from the source to the detector is, in general, nonisenthalpic due to the onset of dimerization. Mach‐number focusing enriches the dimer mole fractions detected by the bolometer. An analysis of the excess enthalpy due to the presence of polymers in the beam results in a new method for the experimental determination of the dissociation energy of HF dimers, determined to be 0.36 eV.

The Journal of Chemical Physics, 1992

Rate constants for inelastic processes in Na2 (v,j)–Ne, Ar, Na collisions are determined at a mea... more Rate constants for inelastic processes in Na2 (v,j)–Ne, Ar, Na collisions are determined at a mean collision energy of the order of 1 meV in a supersonic seeded beam arrangement. The initial vibrational state v and rotational state j is varied in the range 4≤v≤35 and 5≤j≤25. The Na2 molecules are excited by Franck–Condon pumping near the transition region to free molecular flow and the flux of molecules which survive in the level (v,j) is monitored far downstream behind a small aperture. It is shown that elastic processes do not contribute to the observed signal. The magnitude of the cross section for the sum of all inelastic processes is as large as several hundred A˚2. It is found that the vibrational motion of the molecule, which carries an energy of up to 500 meV, strongly influences the collision dynamics. The rate constant increases with v by about 35% within the range of vibrational levels studied here. Some state-to-state rate constants are also reported. The experimental results are in good agreement with data from quasiclassical trajectory calculations on a semiempirical bond-distance-dependent potential surface. The observations can be rationalized by realizing that the long range attraction increases with vibrational excitation since the polarizability of the molecule increases. Furthermore, the vibrational excitation in combination with the relatively long interaction time due to the low collision energy enhances the probability for a change of the initial quantum state of the molecule.

Journal of Applied Physics, 1999

The influence of excimer laser irradiation (193 nm, 20 ns pulses and 248 nm, 20 ns and 500 fs pul... more The influence of excimer laser irradiation (193 nm, 20 ns pulses and 248 nm, 20 ns and 500 fs pulses) on the structure and morphology of plain and gold-film coated mica surfaces has been investigated for fluences between 8 mJ/c2 and 19 J/cm2. Surfaces treated with laser fluences below the ablation threshold (lambda=248 nm) are well suited for controlled growth

Zeitschrift f�r Physik D Atoms, Molecules and Clusters, 1993

Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. K... more Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energyEkin

Chemical Physics, 1988

ABSTRACT

Molecular Physics, 1993

The effect of high initial vibrational (v = 0-25) and rotational (j = 0-100) excitation of the re... more The effect of high initial vibrational (v = 0-25) and rotational (j = 0-100) excitation of the reactant molecule on the cross-section sigmaR for the reaction Li2 + Na --> LiNa + Li at a relative translational energy (Etrans) of 0·5 eV has been investigated using the three-dimensional quasiclassical trajectory method on the recently proposed Varandas-Morais-Pais (VMP) potential. sigmaR increases

The fundamentals of optical second harmonic generation (SHG) is briefly described in this paper. ... more The fundamentals of optical second harmonic generation (SHG) is briefly described in this paper. By measuring the reflected SHG intensities following surface near-resonant excitation, the 3m symmetry of a clean Si(111)7×7 surface is modified.The experimental results are obtained with a light beam at normal incidence and at an 45° angle of incidence geometry. Explanation and discussion of the experimental results are also given in the paper.

Faraday Discussions, 1991

Journal of Quantitative Spectroscopy and Radiative Transfer, 1991

Applied Surface Science, 2004

ABSTRACT The dynamics of short pulsed laser-induced desorption of alkali atoms from poly-dimethyl... more ABSTRACT The dynamics of short pulsed laser-induced desorption of alkali atoms from poly-dimethylsiloxane (PDMS) coated glass prism surfaces is investigated. Kinetic energies of desorbing sodium atoms of several hundred meV are found, increasing with increasing laser fluence. Evanescent wave measurements suggest that the desorbed atoms stem from a subsurface layer of the PDMS film, resulting in diffusional characteristics for the total yield of the desorption process.

Applied Surface Science, 2001

ABSTRACT

Applied Physics A, 2004

Excimer laser (193 nm and 157 nm) induced ablation and structure formation in poly-dimethylsiloxa... more Excimer laser (193 nm and 157 nm) induced ablation and structure formation in poly-dimethylsiloxane (PDMS) thin films is demonstrated. Ellipsometric measurements provide values of the optical constants of the films as well as their thicknesses, which are below 1 μm. At fluences above 160 mJ/cm 2 two pulses of UV light induce gratings with at minimum 1-μm periods and crossed gratings with 4-μm periods. The structure heights are between 10 nm and 20 nm with ridge widths of several hundred nanometres. The ablation occurs after a single incubation pulse with a threshold that increases logarithmically with the ablation wavelength increasing from 157 nm to 1064 nm. At 193 nm the ablation rate for 2 J/cm 2 is 127 nm/pulse.

Solid State Communications, 2001

We demonstrate via measurement of normal-incidence, near-resonant, angular dependent second harmo... more We demonstrate via measurement of normal-incidence, near-resonant, angular dependent second harmonic generation a strong change of Si(111)7 £ 7 surface symmetry as a consequence of submonolayer adsorption of sodium atoms. The results are interpreted by assuming that a new surface structure with lower symmetry coexists on the Na/Si(111)7 £ 7 interface with the 3m-symmetry of the clean Si(111) surface. This new surface structure could be due to an Na-induced surface reconstruction.

Thin Solid Films, 2000

Linear and non-linear optical spectroscopies are applied to monitor the formation of large, sever... more Linear and non-linear optical spectroscopies are applied to monitor the formation of large, several 10-nm-sized alkali clusters near epitaxially grown Au films on mica substrates. The Au films are covered by ultrathin, self-assembled alkane thiol films. The growth and morphology of these clusters are initially monitored in ultrahigh vacuum by comparison of calculated with measured Ž. polarization-dependent extinction spectra. We find that at low surface temperatures 150 K the cluster growth is very similar to growth directly on insulating substrates. With increasing surface temperature the size distribution of the clusters changes. A Ž. quantitative evaluation of ambient-air measurements with scanning force microscopy SFM supports the conclusions from Ž. optical spectroscopy. Field-enhancement effects at the surface of the clusters facilitate the observation of second harmonic SH light. From angular-and polarization-dependent SH-measurements we deduce ratios of Ž2.-tensor components for the cluster-films.

Optics Letters, 1997

Two-photon laser-induced f luorescence measurements of transition frequencies and lifetimes of la... more Two-photon laser-induced f luorescence measurements of transition frequencies and lifetimes of laser-excited Na atoms are presented as a function of distance (2-3 nm) to metal surfaces of different roughness. We characterized the surface roughness by fitting a Gaussian height-height correlation function to force microscopic data. An unusually small red shift of the Na transition frequency of a few hundred megahertz was observed. In the framework of a quantum-mechanical linear optical response formalism we fitted the lifetime and the frequency shift data simultaneously to obtain effective parameters of a selvedge dielectric function. In that way we are able to take into account both surface roughness and nonlocality, which in turn result in the observed small frequency shifts.

Nano Letters, 2006

Nanoscaled, needle-shaped frequency doublers have been generated via self-assembled surface growt... more Nanoscaled, needle-shaped frequency doublers have been generated via self-assembled surface growth from functionalized quaterphenylene molecules with a designed large hyperpolarizability. The nanofiber frequency doublers exhibit very weak fluorescence centered around 430 nm but emit a strong, resonance-enhanced second-harmonic signal when excited with infrared 80 fs laser pulses. The frequency doublers are employed to correlate second-harmonic response and morphology via two-dimensional true second-harmonic images of individual nanoaggregates obtained with the help of a femtosecond laser scanning microscope.

Materials Letters, 2009

1,4‴-Dimethoxy-4,1′:4′,1″:4″,1‴-quaterphenylene crystallizes in space group Pbca with a=7.4989(5)... more 1,4‴-Dimethoxy-4,1′:4′,1″:4″,1‴-quaterphenylene crystallizes in space group Pbca with a=7.4989(5) Å, b=6.1544(3) Å, c=40.359(2) Å, V=1862.6(2) Å3, and Z=4. In the crystal structure the molecules are arranged in layers, which are stacked in the direction of the crystallographic c-axis. The torsion between the molecules in a layer amounts to 69.06° resulting in a herringbone structure. The phenyl rings of the molecule are approximately planar with a torsion angle of 4.64° between the outer and the center rings. UV/Vis transmission measurements of the colorless crystal reveal a band gap of ~3.2eV.

The Journal of Chemical Physics, 1990

ABSTRACT

The Journal of Chemical Physics, 1988

A beam of highly vibrationally excited (vless than or equal to20) Liâ molecules has been generate... more A beam of highly vibrationally excited (vless than or equal to20) Liâ molecules has been generated via Franck--Condon pumping using a cw ring dye laser. The linearly polarized light from a second cw dye laser is used to determine the polarization of the remaining unscattered molecules of the beam after it has passed through a scattering cell filled with krypton gas. The dependence of the state selective total integral cross section on orientation is obtained from the anisotropy of the angular momentum distribution of the beam molecules described by 1+aâPâ(cos theta), where theta is the angle between the angular momentum vector and the velocity vector. The measured polarization increases from aâ = -0.019 +- 0.007 for Liâ(v = 0)--Kr collisions to aâ = -0.074 +- 0.016 for Liâ(v = 20)--Kr collisions. The measurements are interpreted in a vibrationally adiabatic approximation in terms of the potential anisotropy parameter qâ for an assumed simple anisotropic potential model V = +- (C/sub s/R/sup s/) (1+qâxPâ(cos ..gamma..)), where ..gamma.. is the angle between bond distance r and distance between center of mass R. For the v = 20 vibrationally excited molecules we obtain qâ = 1.38 +- 0.31 compared to qâ = 0.47 +- 0.15 for the v = 0 molecules. This large increase in anisotropy with vibrational excitation is compared with that obtained from a previously proposed model for Liâ(v)--He (J. Chem. Phys. 85, 7062 (1986)).

The Journal of Chemical Physics, 1986

ABSTRACT An HF nozzle beam and a cw HF laser beam in a coaxial and antiparallel arrangement facil... more ABSTRACT An HF nozzle beam and a cw HF laser beam in a coaxial and antiparallel arrangement facilitated exciting HF molecules vibrationally in v=1,J up to the saturation limit. A beam consisting exclusively of monomeric HF was generated by heating the beam source to 600 K and was confirmed using a mass spectrometer. Energy‐flux measurements made using a bolometer indicate that the vibrational energy distribution initiated by the laser is conserved, confirming that V–T energy transfer to monomers is slow. The maximum measured flux of HF(v=1,J) molecules was 1.5×1017 molecules s−1 sr−1 at 670 mbar stagnation pressure. Beams consisting of a mixture of monomeric and polymeric HF were generated using appropriate combinations of source temperature and pressure. In the presence of dimers, the excited monomers relax rapidly by a fast V–V transfer and subsequent predissociation of the metastable dimers. The loss of beam energy due to energy transport by the departing dimer fragments was detected bolometrically. The laser‐induced depletion of dimers was determined over a wide source pressure range at T0=300 K using a mass spectrometer. From a kinetic model of these dimer‐depletion data, the product of cross section and transfer probability for V–V energy transfer from HF (v=1) monomers to dimers was deduced to be 14 Å2. The terminal‐dimer‐concentration equation for HF nozzle beams was refined. An analysis of the energy fluxes measured by the bolometer with no laser excitation of the beam revealed that, for an HF nozzle beam, the flow from the source to the detector is, in general, nonisenthalpic due to the onset of dimerization. Mach‐number focusing enriches the dimer mole fractions detected by the bolometer. An analysis of the excess enthalpy due to the presence of polymers in the beam results in a new method for the experimental determination of the dissociation energy of HF dimers, determined to be 0.36 eV.

The Journal of Chemical Physics, 1992

Rate constants for inelastic processes in Na2 (v,j)–Ne, Ar, Na collisions are determined at a mea... more Rate constants for inelastic processes in Na2 (v,j)–Ne, Ar, Na collisions are determined at a mean collision energy of the order of 1 meV in a supersonic seeded beam arrangement. The initial vibrational state v and rotational state j is varied in the range 4≤v≤35 and 5≤j≤25. The Na2 molecules are excited by Franck–Condon pumping near the transition region to free molecular flow and the flux of molecules which survive in the level (v,j) is monitored far downstream behind a small aperture. It is shown that elastic processes do not contribute to the observed signal. The magnitude of the cross section for the sum of all inelastic processes is as large as several hundred A˚2. It is found that the vibrational motion of the molecule, which carries an energy of up to 500 meV, strongly influences the collision dynamics. The rate constant increases with v by about 35% within the range of vibrational levels studied here. Some state-to-state rate constants are also reported. The experimental results are in good agreement with data from quasiclassical trajectory calculations on a semiempirical bond-distance-dependent potential surface. The observations can be rationalized by realizing that the long range attraction increases with vibrational excitation since the polarizability of the molecule increases. Furthermore, the vibrational excitation in combination with the relatively long interaction time due to the low collision energy enhances the probability for a change of the initial quantum state of the molecule.

Journal of Applied Physics, 1999

The influence of excimer laser irradiation (193 nm, 20 ns pulses and 248 nm, 20 ns and 500 fs pul... more The influence of excimer laser irradiation (193 nm, 20 ns pulses and 248 nm, 20 ns and 500 fs pulses) on the structure and morphology of plain and gold-film coated mica surfaces has been investigated for fluences between 8 mJ/c2 and 19 J/cm2. Surfaces treated with laser fluences below the ablation threshold (lambda=248 nm) are well suited for controlled growth

Zeitschrift f�r Physik D Atoms, Molecules and Clusters, 1993

Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. K... more Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energyEkin

Chemical Physics, 1988

ABSTRACT

Molecular Physics, 1993

The effect of high initial vibrational (v = 0-25) and rotational (j = 0-100) excitation of the re... more The effect of high initial vibrational (v = 0-25) and rotational (j = 0-100) excitation of the reactant molecule on the cross-section sigmaR for the reaction Li2 + Na --> LiNa + Li at a relative translational energy (Etrans) of 0·5 eV has been investigated using the three-dimensional quasiclassical trajectory method on the recently proposed Varandas-Morais-Pais (VMP) potential. sigmaR increases