Polar molecules in femto- and attosecond pulses. Lars Bojer Madsen 2010

Transkript

1 Polar molecules in femto- and attosecond pulses Lars Bojer Madsen 2010

2 Thanks Christian Bruun Madsen Christian P.J. Martiny Henriette A. Leth Jan C. Baggesen Adam Etches Henrik Stapelfeldt Christer Bisgaard Lotte Holmegaard Jonas Hansen Line Kalhøj Sofie Kragh Mahmoud Abu-samha Darko Dimitrovski Yong-Chang Han Jochen Küpper Frank Filsinger Gerard Meijer DDK: Lundbeck, FNU Klaus Mølmer

3 Outline Characteristics of polar systems Photoelectron angular distributions High-order harmonic generation Nuclear motion Summary and outlook

4 Characteristics Asymmetric charge distribution. Permanent dipole moment.

5 Characteristics Unique targets: Can be oriented and fixed in space due to dipole and polarizabilities a S E probe e O y x E stat z Detector

6 Characteristics Energy shifts (Stark shifts)

7 Characteristics Dipole of the molecular potential μ p Role of the dipole of the unrelaxed cation V (r) r = 1/r + μ p ˆr/r 2 +

8 Characteristics Dipole moment of molecule Control of external orientation Dipole moment of orbital (HOMO) Ionization step. Ionization potential Dipole of unrelaxed cation (multipole) Ionization step. Propagation step V (r) r = 1/r + μ p ˆr/r 2 +

9 Key question How do these different dipoles affect the dynamics? Can their effects be separated out?

10 Outline Characteristics of polar systems Photoelectron angular distributions High-order harmonic generation Nuclear motion Summary and outlook

11 Holmegaard et al., Nat. Phys. 6, 428 (2010).

12 1. Can we understand the up-down asymmetry? 2. What does it tell us about the molecule? 3. Experiment is in the tunneling regime 4. The electron adiabatically adjust to the instantaneous magnitude and direction of the field

13 I p eff (θ) = I p0 +(μ OCS+ μ OCS )E probe cosθ E probe 2 [{(α OCS α OCS + ) (α OCS α OCS + )}cos 2 θ + (α OCS α OCS + )] (1) OCS+ I eff ( θ = 0) > I p0 p I p0 I p eff ( θ = π) < I p0 OCS S O Less likely More likely

14 Less likely More likely

15

16

17 Imprints of angular nodes on 2D momentum distributions Martiny et al, PRA 2010

18 Which dipole was at play? Dipole of the HOMO for OCS in circularly polarized fields Experiments explained without taking the dipole of the molecular potential into account

19 MFPADs with the TDSE Linearly polarized field

20 MFPADs with the TDSE No asymmetry in the case of one-photon ionization in the high-energy limit M fi ( k)= d re i k r ɛ rφ 0 [ ] M fi ( k)= d re i k r ɛ rφ 0 = M fi ( k)

21 MFPADs with the TDSE Asymmetries are due to the difference between a plane wave and an exact scattering state, i.e., phase shifts induced by the asymmetric molecular potential Such potential effects may be enhanced in a linearly polarized field due to multiple rescatterings off the core Effects beyond first-born-like approaches (Strong-field-approx.)

22 Asymmetry due to orbital dipole 0.65 (a) HF 0.6 (b) LiF IP H (E) [a.u.] 0.6 IP H (E) [a.u.] D 1.9D E (a.u.) E (a.u.)

23 Asymmetry due to dipole from the potential 0 0 (a) HF (b) LiF D D V(z M ) (a.u.) V(z M ) (a. u.) V(z L ) + z L E (a.u.) z M (a.u.) (a) HF 0.5 V(z L ) z L E (a.u.) z M (a.u.) (b) HF z L (a.u.) z L (a.u.) V(z L ) + z L E (a.u.) (c) LiF V(z L ) z L E (a.u.) (d) LiF z L (a.u.) z L (a.u.)

24 HF LiF

25 HF LiF

26 Conclusion on MFPADs MFPADs can be obtained for a broad range of species Time-dependent phenomena. Sensitive to valence electrons. Charge migration Strong-field ionization by circularly polarized fields minimizes rescattering and reflect the orbital Strong asymmetries in the photoelectron angular distributions with linearly polarized fields highlighting the role of the dipole term of the molecular potential, and the dipole of the orbital.

27 Outline Characteristics of polar systems Photoelectron angular distributions High-order harmonic generation Nuclear motion Summary and outlook

28 High-order harmonic generation Atomic Antenna. M.Yu.Kuchiev. JETP Lett. 45(7), (1987)

29 S n (ω) = n e iωt d dt v dip(t) dt 2 v λ (R,t) = i t 0 dτ dkv rec,λ (R, k,t)e is λ(k,t,t τ) F(t τ) d ion,λ (R, k,t τ) +c.c., t ( ) 1 S λ (k,t,t τ) = 2 [k + A(t )] 2 E λ dt t τ

30 S n (ω) = n e iωt d dt v dip(t) dt 2 v λ (R,t) = i t 0 dτ dkv rec,λ (R, k,t)e is λ(k,t,t τ) F(t τ) d ion,λ (R, k,t τ) +c.c., t ( ) 1 S λ (k,t,t τ) = 2 [k + A(t )] 2 E λ dt t τ

31 S n (ω) = n e iωt d dt v dip(t) dt 2 v λ (R,t) = i t 0 S λ (k,t,t τ) = dτ dkv rec,λ (R, k,t)e is λ(k,t,t τ) F(t τ) d ion,λ (R, k,t τ) +c.c., t t τ ( ) 1 2 [k + A(t )] 2 E λ dt For a polar molecule E λ = E (0) λ μi λ F i 1 2 αij λ F if j

32 (a) Paral l el geomet ry F P1 x E (0) i F i 1 2 ij F i F j F P2 x (b)antiparallel geometry F A1 x E (0) i F i 1 2 ij F i F j F A2 x

33 System-induced gating Without Stark shift With Stark shift

34 Harmonic intensity (arb. units) Harmonic order Etches and LBM, JPB (2010)

35 Outline Characteristics of polar systems Photoelectron angular distributions High-order harmonic generation Nuclear motion Summary and outlook

36 Nuclear motion: Dissociative ionization Theoretical frame-work that treats both electrons and nuclei when the pulse lengths are such that the nuclei move during the pulse (Example FLASH 10 fs - 50 fs)

37 Dissociative double ionization Full problem not understood Nuclei: Exact on electronic bound state curves Electrons: Exact for frozen nuclei Born-Oppenheimer and existing electronic rates in a single theoretical framework.

38 Dissociative double ionization

39 Monte Carlo Wave Packet Describes open quantum systems Non-hermitian evolution of wave functions Equivalent to the master equation Stochastic jumps between states (Hilbert spaces)

40 Dissociative double ionization Theory Wave function in a basis of electronic states Rates are R-dependent Input Electronic BO curves Ionization rates as a function of R and intensity Dipole moment functions within a given system (charge state) g u

41 Results Final result Averaged over many runs and focalvolume Includes many different instants of ionization Experiment Staudte et al., Phys. Rev. Lett 98, (2007)

42 AU Oxygen in FEL LTC Ionization of Oxygen Born Oppenheimer No cross sections cross sections

43 Conclusion: Dissociative ionization Methodology seems promising Room for improvements: Include more electronic state, improved rates, Other systems than H 2 Heteronuclear systems Leth, LBM, Mølmer, PRL (2009), PRAs (2010).

44 Summary and outlook Polar molecules in femto- and attosecond pulses Dipole of the HOMO (MFPADs and HHG) Dipole of the molecular potential (TDSE: ionization and HHG). SFA? Dipole of the active orbital in"attosecond streaking photoelectron spectroscopy", Baggesen, LBM, PRL (2010) Nuclear motion