Femtosecond Laser Spectroscopy of C 60 Nieuwegein, The Netherlands August 21, 2001 Eleanor Campbell, Göteborg University & Chalmers, Sweden R.D. Levine, Fritz Haber Center, Hebrew University M. Boyle, K. Hoffmann, R. Stoian, C.P.Schulz & I.V. Hertel

Max-Born-Institut, Mark Boyle Outline 1.) Observed Rydberg Structure in photo-electron spectra What do we learn about the Rydberg states from photoelectron spectroscopy? Excitation and ionization process 2.) Femtosecond Pulse Shaping on C 60 interested in impulsively exciting vibrational modes

Max-Born-Institut, Mark Boyle Experimental TOF Apparatus Electron TOF e-e- Ion + C 60 -Oven Double µ-Metal Shielding Wiley-McLaren Reflectron TOF x y z

Max-Born-Institut, Mark Boyle Experimental Variable Parameters Intensity W/cm 2 Wavelength- 800nm, 400nm, 660nm Bandwidth Limited Pulse Duration Chirp Polarization

Max-Born-Institut, Mark Boyle Intensity Dependence of Photoelectron Spectra  = 800 nm,  = 1.5 ps electron yield / log units 1.5x10 12 W/cm 2 e - -energy / eV ,51,5 1 electron yield *10 4 / arb. units 1.1x10 12 W/cm 2 2.2x10 12 W/cm 2 3.6x10 12 W/cm

Max-Born-Institut, Mark Boyle Wavelength Dependence of Binding Energy ***assuming 1-photon ionization IP

Max-Born-Institut, Mark Boyle Bandwidth Limited Pulse Duration Electron Spectra Comparison  E*  =0.441 Rydberg spectra seen for pulse durations as short as 25 fs Indicates very fast population process  meas =  laser   decay   laser  radiative decay <<  laser Lifetimes of Rydberg states are 400 fs or longer

Max-Born-Institut, Mark Boyle Photoelectron Spectra for two chirps Shift=energy bandwidth of laser IP

Max-Born-Institut, Mark Boyle Effect of different polarization e - TOF Electrons emitted directly within pulse duration

Max-Born-Institut, Mark Boyle Experimental Summary Intensity - R.S. emerges from background in low intensity pulses Wavelength - indicate a non-resonant excitation process Pulse duration - indicates a very fast process Chirp - indicates electron emitted within one pulse duration Polarization - indicates the electrons are emitted directly

Max-Born-Institut, Mark Boyle Modeling of Rydberg Series in C r [a.u.] Solved the Schrödinger Equation for the bound energies of C 60 assuming a simple two particle system the potential as shown below Resultant BE values were in agreement with literature values of Puska and Nieminen, Phys. Rev. A 47, 1181 (1993)

Max-Born-Institut, Mark Boyle Calculated Rydberg Series

Max-Born-Institut, Mark Boyle Solid points from Calculation Open points from fitting of exp. Results from Calculation and fitting l=5 l=7 l=9l=3 l=1

Max-Born-Institut, Mark Boyle Excitation of Rydberg Series in C 60 Resolved Rydberg Structure has been observed with Laser Photoelectron Spectroscopy of C 60. Electrons populate the Rydberg states with a four photon process, and are then single photon emitted within the same pulse. Results of calculations using a simple two particle model show excellent agreement to experimental spectra.

Max-Born-Institut, Mark Boyle Next Experimental Steps 1.) Two color pump probe 4.) C 70, NC 59,... 2.) angular distribution of Rydberg electrons 3.) cold C 60 source

Max-Born-Institut, Mark Boyle Comparison to C 70

Max-Born-Institut, Mark Boyle Femtosecond Pulse shaping and C 60 Idea: To excite C 60 using trains of pulses with frequency equal to the vibrational frequencies. (Impulsive Excitation) The two frequencies of highest interest are the two energetically lowest Raman active frequencies. A g (1) with energy 496 cm -1 and oscillation period 67 fs H g (1) with energy 271 cm -1 and oscillation period 123 fs

Max-Born-Institut, Mark Boyle Motivation for Excitation Diameter of C 60 versus time following a 12 fs laser pulse with a fluence of 0,44 J/cm 2 at T=300K B. Torralva and Roland E. Allen Proceeding of 24th International Conference on the Physics of Semiconductors Diameter (Å) For impulsive excitation to occur, Periods of oscillation: 67fs, 123 fs Our pulse duration: 30fs

Max-Born-Institut, Mark Boyle Schematic of Pulse Shaping Apparatus Input waveform Output waveform Fourier synthesis spectral modulation Diverse Applications: optical fiber communications, coherent quantum control micro machining Usable with pulse durations from picoseconds to below 10 fs

Max-Born-Institut, Mark Boyle How the crystals change the pulse shape Frequency Response Time Domain: e out (t)=  dt´h(t-t ´)e in (t ´) Frequency Domain: E out (w)=H(w)E in (w) h(t) Impulse Response e in (t) e out (t) H(  ) E in (  ) E out (w)

Max-Born-Institut, Mark Boyle Examples of Shaped Pulses  ~ 1000 fs  ~ 500 fs  ~ 250 fs Double Triple Pump-probe measurements simplified

Max-Born-Institut, Mark Boyle Ion Results from shaped pulses  Memory of Signal

Max-Born-Institut, Mark Boyle Schematic of Optimization Experiment

Max-Born-Institut, Mark Boyle Optimization with C 60 Optimize certain fragmentation or ionization patterns or discriminate between competing paths. Regions of interest low energy- primarily ionization, C 2 loss mid-energy-fragmentation and ionization high energy-bimodal fragmentation pattern Pulse shape gives additional information

Max-Born-Institut, Mark Boyle Conclusions Introduction of Pulse Shaping and use on C 60 1.) Impulsive Excitation-creating pulse trains of varying separation to excite C 60 2.) Using optimization feedback control to learn information about C 60