© Imperial College LondonPage 1 Probing nuclear dynamics in molecules on an attosecond timescale 7 th December 2005 J. Robinson, S. Gundry, C. A. Haworth, L. Chipperfield, H. Teng, J. Lazarus, M. Hohenberger, R. A. Smith, M. Lein, C. Ciprila, R. Torres, J.W.G. Tisch, and J.P. Marangos A powerful new technique exploiting the temporal chirp of a harmonic spectrum to allow molecular dynamics to be monitored on a single shot basis.

© Imperial College LondonPage 2 Outline of talk Introduction 1. Using HHG to probe ultrafast nuclear motion 2. Exploiting the temporal chirp of a harmonic spectrum 3. A new technique for probing ultrafast nuclear motion The experiment 3. Probing nuclear motion: experimental details 4. Results for D 2 /H 2 5. Analysis of results: comparison to theory 6. Results for CD 4 /CH 4 Conclusions and outlook for this technique

© Imperial College LondonPage 3 High harmonic generation involves the collision of an electron wavepacket with the ion from which it was born - Corkum’s semiclassical model [PRL, 71, 1994 (1993)] Using HHG to probe ultrafast nuclear dynamics

© Imperial College LondonPage 4 Electron transit time depends on the point in the field at which ionisation occurs. Electron trajectories split in to two groups, ‘long’ and ‘short’ trajectories. Photon energy = Electron energy + Ip < 3.17 Up + Ip Exploiting the temporal chirp of an harmonic spectrum

© Imperial College LondonPage 5 Exploiting the temporal chirp of an harmonic spectrum Lein’s proposal [Phys. Rev. Lett., 94, (2005).]: harmonic signal includes contribution from nuclear part of wavefunction: where Wavefunction describing nuclear wavepacket

© Imperial College LondonPage 6 The time the electron wavepacket spends in the continuum can be used as an pump-probe time delay on the (sub)-femtosecond time scale. Exploiting the temporal chirp of an harmonic spectrum

© Imperial College LondonPage 7 Exploiting the temporal chirp of an harmonic spectrum 2 predictions: 1.Stronger harmonic signal in a heavy isotope 2.Ratio between signals D 2 :H 2 increases with harmonic order Phys. Rev. Lett., 94, (2005) Strong order-to-order variation due to interference between short and long trajectories.

© Imperial College LondonPage 8 A new technique for probing ultrafast nuclear motion Phys. Rev. Lett., 94, (2005): Fig. 3 Red: time evolution as reconstructed from harmonic spectra Dotted: time evolution in exact potential The increasing ratio can be used to gain information about the nuclear motion. In theory, this represents a feasible new method of probing nuclear motion at the time of wavepacket birth, with sub-femtosecond time resolution, and gaining information on the shape of relevant potential surfaces.

© Imperial College LondonPage 9 pulsed valve Off-Axis Parabolic Mirror Soft X-ray “Flat-Field” Grating Microchannel Plate Imaging Detector Experimental Setup for HHG 

© Imperial College LondonPage 10 Experimental approach Requires short (<~15fs) pulses to avoid “disturbance” of molecule prior to ionisation. Otherwise, experimentally relatively simple: 1 kHz, 7-8 fs, 0.25 mJ.

© Imperial College LondonPage 11 Results 1 – Stronger harmonic signal obtained in D 2 as compared to H 2. We have verified Lein’s first prediction (stronger harmonic signal in isotope with slowest nuclear motion)– and shown that with modest averaging this is a measurable effect. D2D2 H2H2  

© Imperial College LondonPage 12 Results 2 – Ratio of harmonic signals increases with electron return time Ratio D 2 /H 2 Ratio H 2 /H 2 We have verified Lein’s second prediction (Ratio between signals D2:H2 increases with harmonic order), and made a measurement of nuclear motion with 100 as time resolution? Effect robust to variations in gas jet backing pressure. Re-absorption cannot account for increase observed

© Imperial College LondonPage 13 Results 3 – Investigations in CH 4 /CD 4 Technique is not limited to simple diatomic molecules… however retrieval of nuclear motion more complicated. Measurement consistent with previous results: 1.Signal stronger in heavier isotope. 2.Ratio increases with harmonic order. Experimental set-up as for H2/D2, but with different backing pressure correction ratio.

© Imperial College LondonPage 14 Investigations in CH 4 /CD 4 Possible we are probing the re-arrangement of the hydrogen nucleii as they move between configurations? CH 4 + does not have the tetrahedral structure of CH 4 : CH 4 + CH 4

© Imperial College LondonPage 15 Conclusions We have demonstrated a new technique for probing fast nuclear motion, verifying that the method proposed by Lein is experimentally feasible. This method operates on a (in principle) single shot basis. We have achieved a time resolution of ~100 as, using 8 fs pulses. We have demonstrated that this technique may be applicable to more complex molecules. Outlook Retrieval of evolution of internuclear separation from experimental data. Extension to longer time delays: requires laser with longer wavelength. Application to other molecules / atoms