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Characterization of modification of organic layers on the surface by means of

Imaging Ellipsometry

The quality mapping modifications of the optical properties of organic layer by means of Imaging Ellipsometry is  illustrated here by  presenting studies of a model polymeric layer (polybutadiene, PBD). It is clearly marked by a mesh of connected regions showing optical parameters being deviated from the base level. (see left panel image below)
An image of the modulation of the refractive index spanning an imaged field sized about
200 µm x 300 µm was obtained by fitting the ellipsometric parameters.
They were acquired performing frame wise focus re-adjustment data acquisition based on nulling ellipsometry. The measurments were performed using a EP3 nanofilm ellipsometer.

The modification of the surface layer (polybutadine, PBD) was impressed by a plasma treatment. The connected regions which were found to show no activation thereby were achieved covering the layer by a micro-mesh within the plasma processing.

The derivatized polymeric layer was provided by Bernd Zimmermann and Thorsten Mayer in cooperation with the Photoelectron Spectroscopy Group (AG Prof. Dr. M. Neumann) at the University of  Osnabrück. Thorsten Meyer has been supported within the scope of this academic work by (WEROS Technologie GmbH, Germany)

The results were presented in a direct comparison with Atomic Force Microscopy phase contrast scans and depth resolving Angle Resolved x-ray photo electron spectroscopic (AR-XPS) studies at the Spring meeting of the German Society of Physics (DPG) in 2006.

Map of the index of refraction (left side panel) of a polybutadiene (PBD) layer adsorbed on a silicon surface. The image was retrieved  by fitting the ellipsometric parameters which were obtained by frame wise focus adjusted data acquisition based on the nulling ellipsometry technique.The quality of the modulation is documented by a histogram analysis of this  map (right side panel).
The PBD layer was adsorbed by using spin coating technique, modified by an Argon plasma processing and investigated by means of AFM and  AR-XPS studies in the course of a Diploma thesis of Bernd Zimmermann and a PhD Thesis of Thorsten Meyer (University Osnabrück).


Investigating the homogeneity of ultra-thin organic monolayers

by means of imaging ellipsometry based techniques

The homogeneity of ultra-thin organic layers was studied on the micrometer scaled domain based on ellipsometric imaging measurements. Whereat the raw DELTA parameter values being acquired  (spanning about 200 µm x  300 µm) were analysed referring to the particle defect density. An example showing the threshold settings applied therefore is given in the figure below.

Raw ellipsometric DELTA-parameter image (left side panel) of a self assembled monolayer of iodo-phenylphenol prepared on etched silicon by oxidative attachment. A binary image  demonstrating the threshold settings to illustrated the density of local defects is shown in the right side panel of this figure. The binary maps were compared to the ones obtained from measurements of reference layers formed by bromo-undecene (see data). These layers were immobilized to the etched silicon surface by a hydosilylation.


Functional ultra-thin organic layers on the surface

Resorcin[4]arene based calyx skeleton compounds exhibiting photo reactive anthracene functionalities were immobilized on the etched silicon surface. The organic monolayers were studied by means of x-ray photo electron spectroscopy (XPS).
Modifications of the C2s valence band XPS (VB-XPS) signature of an organic monolayer formed by anthracene functionalities which were caused by a UV irradiation were found to correspond to the ones observed by studying a polymer (polybutadine, PBD) backboned antharcene containing sample. (see the figure below for spectra before and after irradiation)

This graphical abstract is linked to the publication M Michelswirth, M Räkers, Ch. Schaefer et al. (2010) Photocycloaddition of Anthracene-Functionalized Monolayers on Silicon (100) Surface, in J. Phys. Chem. B 114, 3482 (2010).

The photocycloaddition reaction on the surface was driven by an irradiation without the presence of ambient air. The samples were irradiated by a Nitrogen plasma emission. The layers were protected by a thin AF35 glass sheet (pgo, Präzisionsglas & Optik GmbH, Germany) within this process.
The polymeric sample was adsorbed on BaF2 FT-IR window material substrates. The anthracene functionalized organic monolayers were linked to the etched silicon surface substrates by a wet chemical preparation based on a hydrosylilation reaction of  unsaturated carbon chains.
The observed appearance of an additional area amount modifying the  (C-C) typical 'poly ethylene'-like C2s VB-XPS double feature signature  by a filling  in between until the occurrence of an additional third feature at 14.5 eV, like it was observed in the case of the monolayer measurements, turned out to match SVWN/6-31G(dp) DFT modelled spectra.
These simulated spectra were retrieved from the single molecule modellings of the functionality and from the ones retrieved of oligomer structures showing N=8 anthracene functionalities. The SVWN DFT calculations in both cases  were performed based on B3LYP DFT optimized dimer and non-dimer structures.
Modelled modifications of the vibrational signature, like they were associated to the single molecule structure conformers were found to correspond to Fourier-Transform Infra Red (FTIR) measurements of the polymer linked functional layers before and after the ultraviolet irradiation. Anthracene specific modes including linking sites of dimer formation localized at 1,366 cm-1 and 1,643 cm-1 were observed to be suppressed in the irradiated situation. This was seen to imply a reinforcement affecting to this linking sites due to the dimerization of the anthracene units.
The XPS measurements were performed by Michael Raekers in the course of his PhD thesis at the Photoelectron Spectroscopy Group (AG Prof. Dr. M. Neumann, Osnabrück University). The anthracene functionalized resorcin[4]arene compound and the model anthracene functionalities to be immobilized on the surface were synthesized in the course of the PhD thesis of Christian Schäfer at the Department of Chemistry, (Bielefeld University Organic Chemistry (OCI)) in assistance by Prof. Dr. J. Mattay.[Schäf2004] [Schäf2008] [Schäf2008]
The FTIR measurements were performed by Burkhard Matschke in association with Prof. Dr. Ulrich Giese at  the  Deutsches Institut für Kautschuk e.V.TechnologieInstitut (DIK).
The molecular modelings were performed using GAUSSIAN03 quantum chemical software.
We are indebted to R. Brodbeck and U. Manthe (Department of Theoretical Chemistry, Bielefeld) of for guidance on molecular modeling questions and access to computation hardware.


Formation of heterodimeric H-bond capsules on the surface

Tetra-pyridyl resorcin[4]arenes were achieved to get assembled to counterpart carboxyl functionalized Resorcinarenes forming H-bond capsules. The nature of coupling being investigated by means of x-ray spectroscopic Nitrogen N1s studies referenced to signatures obtained form benzoic acid labelled  branched polyethyleneimine (PEIb) and  polyethyleneimine-silane layers. Whilst the PEIb provides primary, secondary and tertiary amines which were effectively weighted (25:50:50) and the PEIs provides just only secondary and tertiary ones to get labelled by a carboxyl functionality.
An analysis by fit of both layer types N1s spectra revealed an appropriate  model  to investigate the organic monolayers formed from the  tetra-pyridyl resorcinarene compounds. Whereat the character of a labelling by the simple benzoic acids could be discriminated from the situation of capsule formation.
The PEIs/benzoic acid assemblies were found to get formed by almost equally weighted (57:43) hydrogen bond and a nitrogen protonating interaction character.  The monolayers of pyridyl(propyl)triethoxisilanes shows a N....O(C=O) coupling character being mixed by ratio of about (61:39). Whereat the tetra-pyridyl resorcinarenes turned out to get labelled by a clearly hydrogen bond dominated interaction of a (78:22) weighting using these simple individual carboxyl labellings units. Thus the more complex template was observed to get coupled essentially by a modification of the nature of the assembling interaction in support of the hydrogen bond.
The N....O(C=O) coupling efficiency in contrast to this observations gets mainly affected not till then the labelling counterpart's (carboxyl units) availability is restricted  structurally by the resorcinarene calyx frame like this is the case forming the cavitands capsules.
The efficiency of labelling per pyridyl functionality of the tetra-pyridyl resorcinarenes was found to get reduced from 0.43 (pyridyl silane) to 0.26 thereby.

This graphical abstract is linked to the publication Martin Michelswirth, Michael Räkers, Björn Schnatwinkel et al. (2011) Formation of Heterodimeric Resorcin[4]arene Capsules on Surfaces: An X-Ray Photoelectron Spectroscopy Investigation, in ChemPhysChem 12, 785 (2011).

The stable conformer of the hydrogen bond formed assembly of the complementary functionalized  resorcin[4]arene cavitands shown here was retrieved from PBE/6-31(dp) DFT dimer optimizations. Whilst a C2v symmetric structure was imposed therein. The counterpoise corrected energy of the optimized dimer was found to be E=-5721.80 Hartree. The hydrogen bond distance obtained in the symmetrysized case was calculated to be about 0.168 nm in comparison to H-bond distances varying between 0.184-0.185 nm like they were obtained performing non-symmetrysized calculations (E=5721.76 Hartree).
The DFT molecular modellings were performed using GAUSSIAN03 quantum chemical software. Pre-optimizations were obtained from atomic charge calculations based on the electronegativity equalization method (EEM) [Bul2002] using the Ambfor module implemented in the MOLDEN package. The structure was visualized using MOLDEN (vers. 4.8).[Schaf2000]
To illustrate the 3D geometry of the  dimer assembly a  *.vmrl  visualization file is available  (here: capsule_non_symm, capsule_2cv )
The XPS measurements were performed by Michael Raekers in the course of his PhD thesis at the Photoelectron Spectroscopy Group (AG Prof. Dr. M. Neumann, Osnabrück University). The resorcin[4]arene compounds were synthesized in the course of the PhD thesis of Björn Schnatwinkel at the Department of Chemistry, (Bielefeld University Organic Chemistry (OCI)) in assistance by Prof. Dr. J. Mattay.[Schna2008][Schrö2010]
We are indebted to R. Brodbeck and U. Manthe (Department of Theoretical Chemistry, Bielefeld) of for guidance on molecular modeling questions and access to computation hardware.


S0/S1 activation of phenyl systems on the  surface

The S1 deactivation of spectroscopic iodine marked bi-phenyls adsorbed on the etched silicon surface was studied by mean of time resolved core-level photoelectron spectroscopy.  High Harmonic Generation (HHG) [Li1989] [LHu1993] [Wah1993] based pulsed EUV radiation (95.1 eV) was used for probing. Thus the observed modifications within the core level signature of the iodine (I4d) were ensured to be assigned to the direct environmental phenyl unit due to the nature of the core-level photo ionization.[Sif2001] [Dre2002] [Dre2004] [Sif2004]
The excitation was driven by pulsed 266 nm radiation obtained from the fundamental NIR laser system one (800 nm, 50 fs) by a  third harmonic generation (THG) process. In contrast to a balanced singlet (pipi*)/triplet n-sigma* excitation, like it is seen to be consistent to Configuration Interaction semi-empiric modellings being performed, the excitation of the bi-phenylic and of even more extended polycyclic hydrocarbons (PAH)  were found to be  dominated  by the  S0->S1(pipi*) UV band  excitation. The characters of the activations of this little series of prototype-like marked compounds build of conjugated units were retrieved from SVWN/MIDI TD-DFT calculations. They were assigned to the corresponding excitation wavelengths and oscillation strengths, which were obtained at semi-empiric (MNDO) level. The orbital representation of the first singlet excitation of a polyphenyl chain shown below was visualized based on TD-DFT calculations performed for a iodine marked oligomer formed of N=8 conjugated units. The excitation energies associated to the S0/S1 activations turned out to match the Rustagi–Ducuing relation [Her1974] [vFa2004] corresponding to a conjugation limit being localized at 314.6 nm.


Visualization of the TD-DFT modeled HOMO (left side panel)  and LUMO (right side panel) orbitals of the linear conjugated oligo-phenyl chain (N=8). 

This graphic and excitation analysis listed as follows (see both tables) are associated to the publication Martin Michelswirth, Hatem Dachraoui, Jochen Mattay et al. (2012) Radiationless S 1 → S 0 phenyl deactivation pathway: an investigation of iodine-marked bi-phenyl on a silicon surface by means of time resolved core-level photoelectron spectroscop, in Molecular Physics 110 (4), 207 (2012)


Table: Singlet-singlet excitations of iodine terminated benzene,
bi-phenyl and a linear oligophenyl chain (N=8).

iodine

exc. orb.

wavel.

osc.

excitation

marked

LUMO+

[nm]

strength

character

benzene






0(B2)

251.8

0.8439

pipi*


1(A2)

226.3

0.2112

pipi*


2(A1)

256.3

0.8535

nsigma*

bi-phenyl






0(B)

279.7

1.452

pipi*


1-2(A)

272.4

0.0005

pipi*


3(A)

260.7

0.1219

nsigma*

oligophenyl






0-3(A)

305.1

2.489

pipi*


4(A)

282.8

0.0486

 pipi*

The S0->S1 activation near the UV-band edge of the bi-phenyl system, like it is adsorbed on the silicon surface could be verified to show a comparable signature of excitations. A fragmentary layer approximation was used optimizing the adsorbed structure considering periodic bounding conditions (PBC) therefore. The result of a single point Configuration Interaction calculation (CI-MNDO) based on the PM6 [Ste2009] optimized adsorbates of a marked iodo-benzene and -biphenyl listed below. To illustrate the 3D geometry of the  immobilized iodo-phenylphenol a  *.vmrl  visualization file is available  (here: Silicon_substrate, adsorbed_iodophenol, adsorbed_iodophenylphenol).


Visualization of the PM6 semi-empiric level optimized iodo-phenylphenol structure adsorbed on a fragmentary layer approximated silicon substrate. Periodic bounding condition (PBC) were considered. The calculations were performed by use of MOPAC (vers. 2009)
Table: Activation of iodine terminated benzene and bi-phenyl:
A comparative list of singlet-singlet excitations retrieved from
CI-MNDO modellings of the adsorbed and the non-adsorbed
structures.


adsorbed

linear chain

excitation


wavelength

wavelength

character


[nm]

[nm]


benzene





266.3

251.8

pipi*


268.6

256.3

nsigma*

bi-phenyl





276.5

279.7

pipi*

conj. limit





287.5

314.6

pipi*

The excitation characters listed here were assigned based on the correspondence to the first singlet-singlet excitations of the non-adsorbed structures. In contrast to the single molecule chains calculations the UV band edge excisions in the adsorbed situation revealed no adequate fit to the Rustagi–Ducuing relation [Her1974] [vFa2004], which describes a simple excitation approach just assuming conjugated building units. A disruption of the phenyl-conjugation character due to the oxidative immobilization is seen to affect the localization of the UV band edge S0->S1 excitation signature.
The excitation signature of the substrate was found to provide a spectroscopic gap centered at 276 nm and spanning 25-32 nm, like it is obtained from the CI-MNDO simulation.
Thus a significant spectroscopic interference to the S0->S1 activating excitations of the biphenyl adsorbate is proposed to be excluded thereby.
The TD-DFT calculations were were performed using GAMESS quantum chemistry software. The structure optimizations on semi empiric level were performed using MOPAC2009.


S1/S0 deactivation reaction pathways

Deactivation dynamics showing an ultrafast proceeding deactivations were observed by means of the UV pump/EUV photo electron probe experimental studies. The associated transient modifications of the core-level spectroscopic I4d signature (BE: 47.3 eV, 45.6 eV) were found to be combined of a decrease of the relative 5/2:3/2 compounds’ ratio within this signature and a decrease of the cross-section detecting this signature. Both sub-processes were observed to vanish on a time-scale of 0.25–0.4ps. This is about more than 4–8 times of the duration of the activating UV radiation pulses.
Thus both modifications observed in corresponding occurrence were assigned to reflect the environmental molecular response to the driven activation. (see [Mic2012] for further details)

Apart from this 'fast' proceeding response which was assigned to a deactivation reaction pathway along an internal conversion a manifold of transient modifications of the marker signature were observed which span time constants between 4 ps and 148 ps.[Dac2011]
Deactivation evolving on such 'slow' time scales are seen to give advice to a presence of further reaction pathways avoiding the direct prefuvene  mode reaction coordinated one funneling the respective conical intersection.

In the case of prototype benzene units a similar manifold of transient decays was observed experimentally by M. Clara et al. performing gas phase time resolved UV photoionization spectroscopic  measurements. [Cla2000] Different ps time constants of decay (20-100 ps) were revealed to show dependence on the provided excess energy of the activating pulsed UV excitation by the authors. The obtained dynamics  were assigned  to the combined mode 611n and  the 711n progression driving the excitation of benzene. A minimum barrier (< 4,000 cm-1) pathway showing a reaction sole coordinate apart from the direct one was predicted based on dynamic calculations taking 6 active modes of the benzene into account.[Pen2009]


Inspire to gain a deeper understanding of the

Controlling of the sole-coordinate of phenyl S1/S0 deactivation reaction pathways

Referring to the idea of controlling the reaction sole coordinate with dependence on the provided excess energy the excess energy spectrum of benzene was analysed revealing the structure and cross sections of the 611n and  the 711n signatures. A significance of these  progressions activating the most simple building unit of conjugated poly aromatic hydrogen carbons (PAH) is seen to be given taking the respective bathochromatic shift and the bandwidth of the pulsed laser laser based radiation into account. The a pulsed uv (266 nm, 50 fs) radiation shows a (FWHM) bandwidth of about 3,560 cm-1(~24 nm). Thus the laser based activation centered about 2,120 cm-1 beyond the band edge of the S0(Bpi)->S1(Bpi*) excitations (see CI/MNDO calculations presented above) ensures excess energies tightly reaching the 711n progression (excess energy ~4,000 cm-1)activation.
Resuming the dependences observed in the gas phase measurements of benzene [Cla2000] the ultrafast proceeding S1(pi*)->S0(pi) phenyl deactivation (<300 fs) is assigned to this band. Whilst the 6113 (excess energy ~3,290 cm-1) and 6114 (excess energy ~4,210 cm-1)  ones were found to be associated to picoseconds dynamics. The 711n progression implies a (v7+v2) combined mode where the  v2 fundamental represents the anti-symmetric counterpart to the v6 one, which was thought about to initiate the dynamics calculations [Pen2009] revealing damping constants corresponding to a time scale of ~150 fs.
Whereat the 611n bands seen to initiate the 'slow' proceeding reaction pathways implies a  v6+v19 combined mode. Where the v19 fundamental is associated to an anti-symmetric breathing vibration preparing a in-plane deformation of the C-Ring skeleton in addition to the pure v6 one.
To assess the relevance and the cross-sections of the respective 611n and 711n bands the excess energy spectrum of the benzene was analyzed in detail. A Fourier Transform UV acquired spectrum of benzene [Fal2009] provided by the authors was processed therefore. Line affectings were erased by an exponential weighting apodization of reconstructed interferogram corresponding to a decay of about 4.6 waves.

Processing of FT-UV spectrum The reconstructed signature in the interferogram domain was apodized erasing 'over-shooting' effects until showing a phase decay which corresponds to 4.61 waves (red curve: left side panel), while the reconstruction (blue curve) was found to show an decay of  approximately 5.73 waves. The obtained FT-UV spectrum (red curve, right side panel) was aligned to an axis of excess energy with respect to the 6112 bands maximum (2367 cm-1) [Cla2000]

The resolution of the processed FT-UV spectrum retrieved based on a sinc profile  analysis was found to be about 0.089 lpcm (line pairs per inverse cm). This corresponds to a resolveables feature separation of ~11.2 cm-1. The 611n bands and the 6114, 7111combined band were analysed by fit separately. A curve assembled from the performed band fits was auto-leveled using a  GIFTS algorithm. (see green curve in the right panel above)
The absorption cross sections obtained by fitting were normalized to the 6112 bands maximum. Thus a value of about 0.32  was found to get matched to the 6114 maximum. It turned out to show a higher significance than 7111 showing a cross section of about 0.09.  Further features of  711n progression turned out  to be not observable.
Table: S0(pi)->S1(pi*) excitation bands of benzene. 
band excess energy (cm-1)
relative
cross section
(a. u.)		 excess energy (cm-1)
[Cla2000]		 decay time
[Cla2000]
6111 1445+/-1.5 0.810+/-0.080 1444 >500 ps

1358+/-0.8 0.144+/-0.005


1284+/-0.4 0.284+/-0.019


1249+/-2.4
 0.136+/-0.007

6112 2367* 1 -- >500 ps

2276+/-1.0 0.177+/-0.006


2206+/-0.4
 0.337+/-0.038


2172+/-2.8
 0.152+/-0.014


2103+/-2.0
 0.105+/-0.006


2044+/-1.2
 0.103+/-0.004

6113 3289+/-0.6 0.670+/-0.056 3290     20 ps,




>500 ps

3200+/-1.0
 0.113+/-0.005


3128+/-0.7
 0.232+/-0.005

6114 4209+/-0.9 0.318+/-0.026 4213   550 fs,




   30 ps

4109+/-1.2
 0.059+/-0.003


4048+/-0.6
 0.096+/-0.002

7111
 4002+/-0.5
 0.090+/-0.003
 4000
 <300 fs,




   20 ps

*The axis of excess energy was aligned with respect to this
feature.

 

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Associated publications

[Sif2004] P. Siffalovic, M. Michelswirth, P. Bartz, B. Decker, C. Agena, C. Schäfer, S. Molter, R. Ros, M. Bach, M. Neumann, D. Anselmetti, J. Mattay, M. Drescher, U. Heinzmann, Journal of Biotechnology 2004, 112, 139.

[Dac2011] H. Dachraoui, M. Michelswirth, P. Siffalovic, P. Bartz, C. Schäfer, B. Schnatwinkel, J. Mattay, W. Pfeiffer, M. Drescher, U. Heinzmann, Physical Review Letters 2011, 106, 107401.


Own publications

[Mic2010] M. Michelswirth, M. Räkers, C. Schaefer, J. Mattay, M. Neumann, U. Heinzmann, J. Phys. Chem. B 2010, 114, 3482.

[Mic2011] M. Michelswirth, M. Räkers, B. Schnatwinkel, R. Brodbeck, J. Mattay, M. Neumann, U. Heinzmann, ChemPhysChem 2011, 12, 785 .

[Mic2012] M. Michelswirth, H. Dachraoui, J. Mattay, U. Heinzmann, Molecular Physics 2012, 110, 207.