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Revolution in VUV and X-ray physics
Free electron lasers (FEL)
FLASH ­ Free Electron Laser in Hamburg (2005) : flux 1013 photons/pulse Intensity 1013 W/cm2 Photon energy 13 eV ­ 100 eV Pulse duration 10-50 fs LCLS - Linac Coherent Light Source (2009): flux 1012 photons/pulse Photon energy 800 eV ­ 8000 eV Pulse duration 100-200 fs XFEL at DESY (2013) Intensity 1016 W/cm2 Photon energy 200 eV - 10 keV Pulse duration 100 fs


Free electron laser


Density modulation of the electron beam (simulation) along the undulator:

At the entrance

In the middle

At the exit

FLASH ­ Free Electron Laser in Hamburg

Peak power 1-10 GW, pulse duration 10-50 fs


Why it is so exciting? Perspectives
EUV lithography (nano!) Bio ­ imaging (bio!) Study of deluted samples or/and processes with low cross sections Pump-probe experiments on femtosecond or even attosecond scale, including two colour experiments Non-linear processes in VUV and X-ray region


Problems:
a. Spectrum of the radiation

b. Time-structure of pulses. Jitter

Solutions:
...
Seeded FEL


Non-linear processes in atoms
Multiphoton ionization (MPI)
A
+

Prediction: M. Goeppert-Mayer, Ann. d.Phys. 9, 273, 1931

!!
A

First observation: G.S. Voronov and N.B. Delone, JETP Letters 1, 66, 1965

Simple arguments: Let suppose that photons are absorbed sequentially,
independently with equal probability then

Peculiarities of VUV and X-ray range:
1. Dominance of transitions in continuum 2. Big role of inner-shell electrons


Multiple ionization of atoms
A.A. Sorokin et al. PRL 99, 213002, 2007 FLASH: I=10
16

W/cm2, 13.3 nm (93 eV)

Markis et al. PRL 102, 033002, 2009

To produce Xe21+ more than 5 keV or 57 photons of 93 eV are necessary Sequential ionization ­ peeling of outermost electrons
Space averaging over FEL intensity profile!


Two-photon double ionization of atoms
Example: He
Energy conservation: Sequential ionization

Direct (non-sequential) ionization

In direct ionization electron energies are continuously distributed so that
38.5 eV < Eph < 54.4 eV (D)

In sequential ionization electron energies are fixed:

Electron-electron correlations are not necessary for sequential double ionization


Direct two photon double ionization of He
Total angle-integrated cross section
L. Nikolopoulos and P. Lambropoulos 2007 J. Feist et al. PRA 77, 043420, 2008

HHG

FLASH

Experiment : HHG ( Hasegawa et al. 2005) FLASH ( A. Sorokin et al. 2007 )


Angular distributions of photoelectrons

TDCS at Eph = 42 eV, E1 = E2 = 2.5 eV Blue: Feist et al. PRA 77, 043420, 2008 Red: Hu et al. J. Phys. B 38, L35, 2005 Green: Ivanov et al. PRA 75, 033411, 2007 Strong angular correlations between electrons Strong model dependence

Up to now no experimental data !!


Sequential two-photon double ionization
5a. Angular distributions of photoelectrons
Example: Ne
1st step
62.6 eV

Two-step approach :

2

nd

step

Ne

++

Theory: S. Fritzsche, A. Grum-Grzhimailo, E. Gryzlova and N.M.K. J.Phys. B 41, 165601, 2008
21.6 eV

For completely uncorrelated ionizations: But the intermediate state is aligned (!) along the linear polarization direction, then

Ne Ne

+

for the 2

nd

electron

Our analysis has shown that for the 1st electron angular distribution has the same form !!


Angular distributions of photoelectrons ( FLASH experiments )
M. Braune, U. Becker et al (not published yet)


Angular distributions of photoelectrons: Theory versus experiment
Experiment: M. Braune et al. (not published) Ar (38 eV), Ne (48 eV), Kr (48 eV) M. Kurka, A. Rudenko et al. J. Phys. B, 42, 141002, 2009 Ne (44 eV) Theory: S.Fritzsche et al. J. Phys. B, 41, 165601, 2008 A. Kheifetz J. Phys. B, 40, F313, 2007 Example: Ar 2nd step in Ar

2nd step in Ne


5b. Angular correlations between photoelectrons
Calculations by S. Fritzsche et al. J. Phys. B, 41, 165601, 2008 Coincidence ! LS (a) 3P0 (b) 3P1 (c) 3P2 (d) 1D
2

(e) 1S0


Calculations by E. Gryzlova et al. Uzhgorod University Scientific Herald, 24, 73, 2009.


Angular correlations: Experiment versus theory
Experiment: M. Kurka, A. Rudenko et al. J. Phys. B 42, 141002, 2009 Ne, Eph = 44 eV

Theory: S. Fritzsche et al.



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