Manipulating the torsion of molecules by strong laser pulses

Transcription

1 Manipulating the torsion of molecules by strong laser pulses Christian Bruun Madsen AMO physics seminar,

2 1. Introduction Laser induced alignment Outline 2. Scheme to manipulate the torsion DFDBrBPh as a test molecule Two pulse laser scheme for manipulation of torsion Semi-classical model 3. Results (proof-of-principle) Testing theory vs. experiments Proof of torsional manipulation 4. Perspectives Application to molecular junctions Time-resolved study of de-racemization 5. Summary and outlook 1. Laser induced alignment 2. Strategy to manipulate torsion 3. Results (proof-of-principle) 4. Perspectives 5. Summary and outlook

3 Laser induced alignment A strong non-resonant laser pulse can align a gas of molecules. Unaligned Aligned Oriented Laser pol. axis

4 Laser induced alignment The non-resonant laser creates a broad superposition of (rotational) states in E through Non-essential states (N) all the states in N: Ψ(t) = E c E (t)ψ E, H 0 ψ E = ɛ E ψ E ω ω Essential states (E) The coupling of states in E is well-described by an effective Hamiltonian: i Ψ(t) = H eff Ψ(t) t H eff = H 0 1 α ij F j (t)f i (t) 4 ij Intensity profile Dynamic polarizability

5 Laser induced alignment Revivals Short pulse (impulsive) Increasing pulse duration Degree of alignment Intensity profile Long pulse (adiabatic) Torres, de Nalda and Marangos PRA 72, (2005)

6 Scheme to manipulate the torsion

7 DFDBrBPh as a test molecule Markers of ring (2) F Br Markers of ring (1) (2) (1) F Br Sa Mirror Ra Torsion

8 Torsional potential (mev) Sa φ d Ra 0!90! ! (degrees) d

9 Two pulse laser scheme for manipulation of torsion

10 Two pulse laser scheme for manipulation of torsion 1. Long ns pulse fixes the C-C axis. Long pulse pol. axis

11 Two pulse laser scheme for manipulation of torsion 1. Long ns pulse fixes the C-C axis. Long pulse pol. axis

12 Two pulse laser scheme for manipulation of torsion 1. Long ns pulse fixes the C-C axis. Long pulse pol. axis

13 Two pulse laser scheme for manipulation of torsion 1. Long ns pulse fixes the C-C axis. Kick pulse pol. axis Long pulse pol. axis

14 Two pulse laser scheme for manipulation of torsion 1. Long ns pulse fixes the C-C axis. 2. Short (fs) kick pulse induces change of φ Br and φ F. φ F φ Br Kick pulse pol. axis Long pulse pol. axis Kick pulse pol. axis

15 Semi-classical model H = 1 2I 2 Φ 2 1 2I rel 2 φ 2 d + V tor (φ d ) φ d = φ Br φ F, Φ = (1 η)φ Br + ηφ F φ d φ F φ Br Overall rotation on a ns time scale. Torsion on a ps time scale. Torsional potential (mev) Φ 0!90! ! d (degrees) Treat slow dynamics clasically and fast dynamics quantum mechanically.

16 Semi-classical model H = 1 2I 2 Φ 2 1 2I rel 2 φ 2 d + V tor (φ d ) φ d = φ Br φ F, Φ = (1 η)φ Br + ηφ F φ d φ F φ Br Overall rotation on a ns time scale. Torsion on a ps time scale. Torsional potential (mev) Φ 0!90! ! d (degrees) Treat slow dynamics clasically and fast dynamics quantum mechanically.

17 Semi-classical model H = 1 2I 2 Φ 2 1 2I rel 2 φ 2 d + V tor (φ d ) +V kick (Φ, φ d,t) Non-resonant kick laser couples states of nuclear motion φ d φ F φ Br φ d = φ Br φ F, Φ = (1 η)φ Br + ηφ F kick pulse pol. Φ

18 Semi-classical model H = 1 2I 2 Φ 2 1 2I rel 2 φ 2 d + V tor (φ d ) +V kick (Φ, φ d,t) Non-resonant kick laser couples states of nuclear motion φ d φ F φ Br φ d = φ Br φ F, Φ = (1 η)φ Br + ηφ F Quantum treatment of torsion: χ ν χ Φ ν (t) = ν c Φ ν (t)e ie ν (t t 0 ) χ ν kick pulse pol. ċ Φ ν = i ν c Φ ν (t)e i(e ν E ν )(t t 0 ) χ ν V kick (Φ,t) χ ν Φ

19 Semi-classical model H = 1 2I 2 Φ 2 1 2I rel 2 φ 2 d + V tor (φ d ) +V kick (Φ, φ d,t) Non-resonant kick laser couples states of nuclear motion φ d φ F φ Br φ d = φ Br φ F, Φ = (1 η)φ Br + ηφ F Quantum treatment of torsion: χ ν χ Φ ν (t) = ν c Φ ν (t)e ie ν (t t 0 ) χ ν kick pulse pol. ċ Φ ν = i ν c Φ ν (t)e i(e ν E ν )(t t 0 ) χ ν V kick (Φ,t) χ ν Φ Classical treatment of overall rotation: Φ(t) =Φ 0 t 1 I ( Φ dt V kick (Φ,t ) ) Φ=Φ 0

20 Results (proof-of-principle)

21 AMO physics seminar, Do experiment and theory agree? ps Fixed C-C axis and uniform distr. of rings prior to kick. F+ Exp.: Br+ F Long pulse pol. axis Theory: Br 1.47 ps 2.47 ps

22 AMO physics seminar, Do experiment and theory agree? ps Fixed C-C axis and uniform distr. of rings prior to kick. F+ Exp.: kick pulse pol., 5x1012 W/cm2, FWHM=700 fs Br+ F Long pulse pol. axis Theory: Br 1.47 ps 2.47 ps

23 AMO physics seminar, Do experiment and theory agree? ps Fixed C-C axis and uniform distr. of rings prior to kick. F+ Exp.: Br+ F Theory: Br 1.47 ps 2.47 ps

24 AMO physics seminar, Do experiment and theory agree? ps Fixed C-C axis and uniform distr. of rings prior to kick. F+ Exp.: Br+ F Theory: Br 1.47 ps 2.47 ps

25 AMO physics seminar, Do experiment and theory agree? ps Fixed C-C axis and uniform distr. of rings prior to kick. F+ Exp.: Br+ F Theory: Br 1.47 ps 2.47 ps

26 AMO physics seminar, Do experiment and theory agree? ps Fixed C-C axis and uniform distr. of rings prior to kick. F+ Exp.: Br+ F Theory: Br 1.47 ps 2.47 ps

27 AMO physics seminar, Do experiment and theory agree? ps Fixed C-C axis and uniform distr. of rings prior to kick. Maximal confinement of F-ring around 1.47 ps. F+ Exp.: Br+ F Theory: Br 1.47 ps 2.47 ps

28 AMO physics seminar, Do experiment and theory agree? ps Fixed C-C axis and uniform distr. of rings prior to kick. Maximal confinement of F-ring around 1.47 ps. F+ Exp.: Br+ F Theory: Conclusion: Experiment and theory agree! Br 1.47 ps 2.47 ps

29 AMO physics seminar, Do we induce torsion? ps The Br-rings and F-rings do not align maximally at the same time ps F+ Exp.: Br+ The dihedral angle is not fixed at ±39. Kick pulse pol ps

30 AMO physics seminar, Do we induce torsion? ps The Br-rings and F-rings do not align maximally at the same time ps F+ Exp.: Br+ The dihedral angle is not fixed at ±39. So there must be torsion... Kick pulse pol ps

31 AMO physics seminar, Do we induce torsion? 1.47 ps 2.47 ps F+ D rif to ff -r in g Drift of F+ motion due to rotation, but recurrent dips every ~ 1 ps. Kick pulse pol.

32 AMO physics seminar, Do we induce torsion? 1.47 ps 2.47 ps to ff -r in g Drift of F+ motion due to rotation, but recurrent dips every ~ 1 ps rif 1.2 ps Kick pulse pol. 20 0!90 D Predicted torsional motion for -geometry!φd " (degrees) Torsional potential (mev) Explained by theory: Periodic torsional motion.! !d (degrees) 90

33 AMO physics seminar, Do we induce torsion? 1.47 ps 2.47 ps to ff -r in g Drift of F+ motion due to rotation, but recurrent dips every ~ 1 ps. D 1.2 ps!φd " (degrees) -geometry can be realized using an elliptically pol. ns pulse. rif Explained by theory: Periodic torsional motion.

34 Perspectives

35 Use in molecular junctions Axially chiral molecules can act as a switch in a molecular junction: Flow of charge Rings: Charge passes. Rings: Charge cannot pass. Left lead Right lead Torsional control Ultrafast charge modulations.

36 Time-resolved studies of processes involving torsion Sa Ra Torsion by Time-resolved study of the conversion between enantiomers. De-racemization: Selective conversion from a racemate (1:1 mixture of Sa and Ra) to a pure compound (e.g., Sa s only).

37 Prerequisites for de-racemization 1. Overcome the torsional barrier Pre-alignment of rings and higher kick laser intensity. ns pulse pol. side view 2. Selectivity x Alignment of C-C axis insufficient. end view Kick pulse pol. R a S a molecule R a S a

38 Prerequisites for de-racemization 1. Overcome the torsional barrier Pre-alignment of rings and higher fs laser intensity. ns pulse pol. side view 2. Selectivity Ensured by orientation. end view Kick pulse pol. R a S a molecule R a S a

39 Example of de-racemization with initial racemate of DFDBrBPh exposed to an elliptically polarized ns pulse and a kick pulse of intensity 1.2x10 13 W/cm 2 and FWHM=1 ps: (a) P rob. dens. 6 99% of Ra is converted into S a R a 5!d (degrees) (b) Ra S a % of S a is converted into R a Sa tp (ps) 0

40 Summary We have extended the use of non-resonant strong laser fields to the manipulation of the torsion of molecules. Experimental and theoretical proof-of-principle results. Perspectives include molecular switches and time-resolved study of de-racemization. Outlook More and better experiments (pre-alignment of rings with elliptically pol. ns pulse, increased kick strength, orientation). Use a pure quantum model and do quantum optimal control theory. The use of axially chiral molecules as molecular switches.

41 Acknowledgements physics (LTC) Lars Bojer Madsen Exp. strong-field physics (Femtolab) Henrik Stapelfeldt Simon Stenfeldt Viftrup Lotte Holmegaard Vinod Kumarappan Quantum chemistry Mikael Peter Johansson Synthetic chemistry Thomas Bjørnskov Poulsen Karl Anker Jørgensen