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Alignment, orientation and conformational control: Applications in ultrafast imaging. Henrik Stapelfeldt. Department of Chemistry University of Aarhus Denmark. Ultra-fast Dynamic Imaging of Matter II April 30 – May 3, 2009. Purpose of this talk.
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Alignment, orientation and conformational control: Applications in ultrafast imaging Henrik Stapelfeldt Department of Chemistry University of Aarhus Denmark Ultra-fast Dynamic Imaging of Matter II April 30 – May 3, 2009
Purpose of this talk Recent progress in laser based alignment, orientation and conformer selection methods List potential examples of ultrafast dynamic imaging
1-D Alignment Order of the molecular geometry with respect to a space fixed axis
1-D Alignment Order of the molecular geometry with respect to a space fixed axis
3-D Alignment 3-dimensional order of the molecular geometry Z Y X
3-D Orientation 1-D Orientation Breaking the head for tail symmetry
How to align molecules • Use an intense (but not too intense) nonresonant pulse • and rotationally cold molecules • Long pulse Adiabatic alignment • Short pulse Nonadiabatic alignment • (transient / impulsive)
Classical picture of alignment Potential energy - Linear molecule - Strong, linearly polarized laser field, High rotational energy Low rotational energy
Quantum Mechanical picture of alignment Solve the rotational Schrödinger equation Zon (1976), Friedrich + Herschbach (1995), Seideman (1995) Pendular states : linear combination of field free rotational states For a linear molecule :
Adiabatic alignment slow turn-on of the alignment field Alignment pulse = nanosecond pulse Pendular states Field-free states J = 2 J = 1 32 30 J = 0 22 20 21 11 10 00
light F+ I+ F+ C6H3n+ Measurement of the spatial orientation of the molecules Coulomb explosion : light n+ m+
Experimental Setup I+ I+ Supersonic expansion Alignment pulse YAG : 9 ns 1064 nm Molecular beam 25 fs ionization pulse 2-D ion detector CCD camera
1D Alignment of iodobenzene (C6H5I) I+ images
1D Alignment of Iodobenzene (C6H5I) intensity and temperature dependence
3D alignment Elliptically polarized long pulse Larsen et al. PRL 2000 Tanji et al. PRA 2005 Perpendicularly-polarized pulse pair Lee et al. PRL 2006 Viftrup et al. PRL 2007 Short elliptically polarized long pulse Rouzée et al. PRA 2008
3D alignment - Elliptically polarized long pulse 2,6 dFIB End-view I+ F+
Rotational state selection of polar molecules by electrostatic deflection Strongly improved laser induced orientation and alignment
EYAG Es Setup and idea
Deflection of iodobenzene 10 kV 5 kV 0 kV
Alignment and orientation of iodobenzene = 90 100 110 120 135 150 EYAG I+ - C6H52+ I+ - C6H5+ Estatic = 90 80 70 60 45 30 EYAG Estatic EYAG Undeflected molecules = 90 100 110 120 135 150 Estatic EYAG = 90 80 70 60 45 30 Estatic
Alignment and orientation of iodobenzene = 90 100 110 120 135 150 EYAG I+ - C6H52+ I+ - C6H5+ Estatic = 90 80 70 60 45 30 EYAG Estatic EYAG = 90 100 110 120 135 150 Estatic EYAG = 90 80 70 60 45 30 Estatic
Alignment and orientation of iodobenzene = 90 100 110 120 135 150 EYAG I+ - C6H52+ I+ - C6H5+ Estatic = 90 80 70 60 45 30 EYAG Estatic EYAG Undeflected molecules = 90 100 110 120 135 150 Estatic EYAG = 90 80 70 60 45 30 Estatic
Orientation by mixed fields Combine static electric field and laser field 1999: Friedrich + Herschbach 2001: Buck 2003: Sakai 2> Laser induced potential 1> Static electric field mixes the pendular states: ”+” combination: 2> + 1> localization at = 0o “-” combination: 2> - 1> localization at = 180o
BUT ! Different initial states orient in opposite directions Averaging over the Boltzman distribution strongly diminishes the overall degree of orientation Ideal target: All the molecules initially populated in the rotational ground state [or in the same rotational state (Marc Vrakking, Nat. Phys. 2009)]
Up-down asymmetry Phys. Rev. Lett. 102, 023001 (2009)
Deflection of iodobenzene seeded in He or in Ne F. Filsinger et al., arXiv:0903.5413v1 (2009)
Up-down asymmetry F. Filsinger et al., arXiv:0903.5413v1 (2009)
Latest improvements capacitor plates
3D alignment - Elliptically polarized long pulse 1:2 1:4 Linear 2,6 dFIB
3D alignment - Elliptically polarized long pulse 1:2 1:4 Linear Undeflected 2,6 dFIB Deflected
3D orientation See also: Sakai, PRA (2005) Undeflected Deflected
Cis and trans conformers of 3-aminophenol trans-3AP cis-3AP p = 2.3 D p = 0.7 D
Cis / transconfomer selection Cis fraction
Anti and gauche conformers of 1,2-diiodoethane (C2H4I2) anti gauche Side-view End-view p ~ 2 D p = 0 D
11.0mm 10.1mm 9.7mm YAG Cou 008 007 009 003 006 004+005 011 012 013 Coulomb explosion of 1,2-diiodoethane I+ images gauche anti Parallel fields Perpendicular fields
CONCLUSIONS 1D and 3D aligned or oriented molecules are available for experiments Adiabatic alignment provides strongest alignment and orientation BUT it is not field-free conditions rapid truncation of alignment field [Stolow PRL (2003), Sakai PRL (2008)] Quantum state selection can strongly enhance the degree of (adiabatic) alignment and orientation and alignment / orientation can be induced at lower fields! Electrostatic beam deflection control of stereo isomers (conformers)
OUTLOOK Strong laser field phenomena - High harmonic generation - Electron diffraction Selection of a single rotational quantum state (Marc Vrakking: NO and hexapole, Nat. Phys. March 2009) Time resolved studies of chirality [PRL 102,/ 073007 (2009) ] Steric effects in reactive scattering (SN2: Trippel and Wester) Photoelectron angular distribution from fixed-in-space molecules [PRL 100, 093006 (2008) , Science 320, 1478 (2008) , Science 323, 1464 (2009)] Aligned molecules as targets for free electron lasers - FLASH: Photoelectron spectroscopy (angular distributions) - LCLS: x-ray diffraction
OUTLOOK • x-ray diffraction with free-electron laser sources Calculations by Henry Chapman
X-ray diffraction from aligned molecules Calculations by Henry Chapman Planned target molecule
Deflection project Fritz Haber Institute, Berlin Lotte Holmegaard Jens H. Nielsen Iftach Nevo Jonas L. Hansen Frank Filsinger Jochen Küpper Gerard Meijer
Alignment beam Beam overlap at LCLS ! Molecular beam X-ray beam