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Femtosecond infrared strong field ionisation of metal clusters produced by kHz laser ablation
Femtochemistry and Femtobiology M.M. Martin andJ.T. Hynes (editors) © 2004 Elsevier B.V. All rights reserved.
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Femtosecond infrared strong field ionisation of metal clusters produced by kHz laser ablation Marc Sniits% C.A. de Lange% Adrian Pegoraro, D. M. Rayner and Albert Stolow National Research Council, 100 Sussex Drive, Ottawa, Ontario, Canada, Kl A 0R6 a) Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht 127129, 1018 WS Amsterdam, The Netherlands 1. INTRODUCTION Modem chirped pulse amplification (CPA) femtosecond laser systems permit the study of high order non-linear optical phenomena such as ultra-high harmonic generation. Coulomb explosion, and strong field control of chemical processes. In the quasi-static non-resonant regime, strong field ionisation occurs when the electric part of the laser field suppresses the core potential barrier such that the electron can tunnel out or, at even higher intensities, can escape over the barrier. Tunnelling can only take place when the Keldysh parameter y is less than unity, i.e. the tunnelling time is less than the laser period [1]. Most characteristics of non-linear ionisation of rare gas atoms are well understood and are successfully described by ADK theory [2]. This theory is based on the single active electron approach and on the adiabatic picture of electron dynamics (the electronic motion is faster than the timescale of the laser field oscillation). Irrespective of the value of the Keldysh parameter, these assumptions are not necessarily valid for molecules or clusters, where many doubly excited states might contribute to the polarisation of the molecule, and where the spatial extent of the molecular orbital might increase the timescale of electronic motion. Although recent studies have proposed a modification of the original Keldysh parameter [3], these studies still neglect the electron dynamics within the potential. In previous publications [4] we show that in the long wavelength limit, ionisation of all studied molecules is harder than expected from any of these theories. We speculated that this was due to a d3mamic screening effect due to the polarisation of all bound electron in the laser field, shielding the 'active' electron and thus suppressing ionisation. In this study we demonstrate the existence of these multielectron dynamics by studying non resonant strong field ionisation of small Nickel clusters. Nickel is a typical transition metal with electronic configuration [Ar] 3d^ 4s\ Nickel clusters as a model system resemble more closely a zero range potential, due to the compact geometry. Furthermore it provides us with a variable parameter: the number of atoms per cluster.
62 2. EXPERIMENTAL SETUP For this experiment, a stable kHz rate molecular beam ablation source was developed, matching the kHz repetition rate of most amplified femtosecond lasers [5]. Our vacuum apparatus is presented in Fig 1 and shows the three differentially pumped regions and a Campargue type molecular beam source. The regions are pumped by respectively a Roots Blower (P<10"^ Torr), a diffusion pump (P<10^ Torr), and a turbo molecular pump (P<10'^ Torr). The first region contains a modified Smalley type cluster source [6] that is employed with a glass growth channel and nozzle. The source is based upon a strong non-resonant interaction of a dithering laser focus with a rotating and translating solid rod, hydrodynamic transport of the ablated material in helium and subsequent supersonic expansion. In the second region residual ions after recondensation of the plasma are removed from the molecular beam by two deflection plates. The final region contains a standard two region linear time of flight Wiley-McLaren mass spectrometer, where the repeller and attracter plates are kept at negative voltage and the final plate and flight tube are kept at ground. Compensation for the potentially high lab-frame molecular beam velocity is provided by two deflection plates, mounted perpendicular to the molecular beam and typically operate at a potential difference of up to 100 V. A narrow sHt (<500 |^m), kept at ground, is mounted perpendicular to laser propagation, on top of these deflection plates to collect ions only from within a part of the focus where the axial intensity is constant. The ions are detected using microchannel plates, fed directly into a pulse amplifier and discriminator. Mass spectra are individually recorded for each laser shot using a PCI- based multichannel scaler (FastComtec).
M-^
kHz Ablation Source
r
AVJ
i
-~-- iicz% tsma
yr,
y
10-6 to
lo-^c 20001/s
Fig. 1. kHz molecular beam ablation setup with a linear time of flight mass spectrometer.
63 The energy of the laser pulse is measured using an integrating sphere and a (GeGaAs) photodiode, integrated by a boxcar. To ensure that mass spectra and laser energies are recorded in pairs, a home built amplitude to time converter is used to write the energy as a time delay at the end of each mass spectrum. Our laser system includes a femtosecond Ti:Sa oscillator (Spectra Physics), and both a regenerative amplifier and a two pass linear amplifier (Positive Light). Part of the amplified femtosecond laser pulses are used to pump an optical parametric amplifier (TOPAS, Quantronix/LightConversion). Either the output of the amplified Ti:Sa (800 nm, 2.5 mJ, 80 fs) or the optical parametric amplifier (L5 jjm, 150 fiJ, 90 fs) are used as the source for strong field ionisation. The beam diameter is approximately 1 cm and the light is focused into the mass spectrometer using a 150 mm anti-reflection coated lens (i.e. f#=15). Intensities of over 10^^ W/cm^ can be reached for either wavelengths. Absolute laser intensities (± 30%) are determined by our calibration method: the so-called saturation intensity of a reference standard (xenon) is measured using an intensity scan and is calibrated to the ADK-value as discussed in detail elsewhere [7]. The light of a Nd:YLF (Positive Light Merlin) ablation laser, synchronised with the femtosecond laser setup (Stanford Research Delay Generator), is focused onto the target rod using a 300 mm lens. The laser energies vary from 5 to 15 mJ per pulse, depending on the ablation characteristics of the metal. Before the first use, the metal rod is quickly sanded and exposure to oxygen is minimised. 3 RESULTS In Fig. 2 we present one of our preliminary intensity scans of Nis. The saturation intensity is found at the intersection of the extrapolated linear part of the ion signal and the intensity axis, and is an independent measure for the ease of ionisation. Using the measured ionisation potential of Ni clusters, [8] and assuming a Gaussian beam profile (t=90 fs, 1=1500 nm), we calculate the saturation intensity with the ADK theory. So far, most measured saturation intensities for Ni clusters are a factor of approximately 3 times higher than the values predicted by ADK theory. This suppression of ionization clearly shows the anticipated multielectron shielding effect in metal clusters, due to a polarization of the cluster by the remaining electrons. We are currently working on models to include multielectron polarisability into our calculations by describing the clusters as a macroscopic conducting sphere.
64
Intensity (W/cm^) Fig 2. Saturation scan of Nig and calibration scan with Xe. REFERENCES [1] L.V. Keldysh, Sov. Phys. JETP 20, 1307 (1965). [2] M.V. Ammosov, N.B. Delone, and V.P. Krainov, Sov. Phys. JETP 64, 1191 (1986). [3] M.J. DeWitt, and R.J. Levis, Phys. Rev. Lett. 81, 5101 (1998) [4] M. Lezius, V. Blanchet, D.M. Rayner, D.M. Villeneuve, A. Stolow, and M.Y. Ivanov, Phys. Rev. Lett 86, 51 (2001) [5] M. Smits, S. Ulrich, T. Schulz, M. Schmitt, J.G. Underwood, J.P. Shaffer, C.A. de Lange, A.J. Alcock, D.M. Rayner, and A. Stolow, Rev. Sci. Instrum (in press, nov. 2003) [6] M. E. Geusic, M. D. Morse, S. C. O'Brien, and R. E. Smalley, Rev. Sci. Instrum. 56, 2123 (1985); M. D. Morse, M .E. Geusic, J. R. Heath, and R.E. Smalley, J. Chem. Phys. 83, 2293 (1985) [7] S. Hankin, D. Villeneuve, P. Corkum, and D. Rayner, Phys. Rev. A 64 (2001) [8] M.B. Knickelbein, S. Yang, and S.J. Riley, J. Chem. Phys 93, 94 (1990)
61
Femtosecond infrared strong field ionisation of metal clusters produced by kHz laser ablation Marc Sniits% C.A. de Lange% Adrian Pegoraro, D. M. Rayner and Albert Stolow National Research Council, 100 Sussex Drive, Ottawa, Ontario, Canada, Kl A 0R6 a) Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht 127129, 1018 WS Amsterdam, The Netherlands 1. INTRODUCTION Modem chirped pulse amplification (CPA) femtosecond laser systems permit the study of high order non-linear optical phenomena such as ultra-high harmonic generation. Coulomb explosion, and strong field control of chemical processes. In the quasi-static non-resonant regime, strong field ionisation occurs when the electric part of the laser field suppresses the core potential barrier such that the electron can tunnel out or, at even higher intensities, can escape over the barrier. Tunnelling can only take place when the Keldysh parameter y is less than unity, i.e. the tunnelling time is less than the laser period [1]. Most characteristics of non-linear ionisation of rare gas atoms are well understood and are successfully described by ADK theory [2]. This theory is based on the single active electron approach and on the adiabatic picture of electron dynamics (the electronic motion is faster than the timescale of the laser field oscillation). Irrespective of the value of the Keldysh parameter, these assumptions are not necessarily valid for molecules or clusters, where many doubly excited states might contribute to the polarisation of the molecule, and where the spatial extent of the molecular orbital might increase the timescale of electronic motion. Although recent studies have proposed a modification of the original Keldysh parameter [3], these studies still neglect the electron dynamics within the potential. In previous publications [4] we show that in the long wavelength limit, ionisation of all studied molecules is harder than expected from any of these theories. We speculated that this was due to a d3mamic screening effect due to the polarisation of all bound electron in the laser field, shielding the 'active' electron and thus suppressing ionisation. In this study we demonstrate the existence of these multielectron dynamics by studying non resonant strong field ionisation of small Nickel clusters. Nickel is a typical transition metal with electronic configuration [Ar] 3d^ 4s\ Nickel clusters as a model system resemble more closely a zero range potential, due to the compact geometry. Furthermore it provides us with a variable parameter: the number of atoms per cluster.
62 2. EXPERIMENTAL SETUP For this experiment, a stable kHz rate molecular beam ablation source was developed, matching the kHz repetition rate of most amplified femtosecond lasers [5]. Our vacuum apparatus is presented in Fig 1 and shows the three differentially pumped regions and a Campargue type molecular beam source. The regions are pumped by respectively a Roots Blower (P<10"^ Torr), a diffusion pump (P<10^ Torr), and a turbo molecular pump (P<10'^ Torr). The first region contains a modified Smalley type cluster source [6] that is employed with a glass growth channel and nozzle. The source is based upon a strong non-resonant interaction of a dithering laser focus with a rotating and translating solid rod, hydrodynamic transport of the ablated material in helium and subsequent supersonic expansion. In the second region residual ions after recondensation of the plasma are removed from the molecular beam by two deflection plates. The final region contains a standard two region linear time of flight Wiley-McLaren mass spectrometer, where the repeller and attracter plates are kept at negative voltage and the final plate and flight tube are kept at ground. Compensation for the potentially high lab-frame molecular beam velocity is provided by two deflection plates, mounted perpendicular to the molecular beam and typically operate at a potential difference of up to 100 V. A narrow sHt (<500 |^m), kept at ground, is mounted perpendicular to laser propagation, on top of these deflection plates to collect ions only from within a part of the focus where the axial intensity is constant. The ions are detected using microchannel plates, fed directly into a pulse amplifier and discriminator. Mass spectra are individually recorded for each laser shot using a PCI- based multichannel scaler (FastComtec).
M-^
kHz Ablation Source
r
AVJ
i
-~-- iicz% tsma
yr,
y
10-6 to
lo-^c 20001/s
Fig. 1. kHz molecular beam ablation setup with a linear time of flight mass spectrometer.
63 The energy of the laser pulse is measured using an integrating sphere and a (GeGaAs) photodiode, integrated by a boxcar. To ensure that mass spectra and laser energies are recorded in pairs, a home built amplitude to time converter is used to write the energy as a time delay at the end of each mass spectrum. Our laser system includes a femtosecond Ti:Sa oscillator (Spectra Physics), and both a regenerative amplifier and a two pass linear amplifier (Positive Light). Part of the amplified femtosecond laser pulses are used to pump an optical parametric amplifier (TOPAS, Quantronix/LightConversion). Either the output of the amplified Ti:Sa (800 nm, 2.5 mJ, 80 fs) or the optical parametric amplifier (L5 jjm, 150 fiJ, 90 fs) are used as the source for strong field ionisation. The beam diameter is approximately 1 cm and the light is focused into the mass spectrometer using a 150 mm anti-reflection coated lens (i.e. f#=15). Intensities of over 10^^ W/cm^ can be reached for either wavelengths. Absolute laser intensities (± 30%) are determined by our calibration method: the so-called saturation intensity of a reference standard (xenon) is measured using an intensity scan and is calibrated to the ADK-value as discussed in detail elsewhere [7]. The light of a Nd:YLF (Positive Light Merlin) ablation laser, synchronised with the femtosecond laser setup (Stanford Research Delay Generator), is focused onto the target rod using a 300 mm lens. The laser energies vary from 5 to 15 mJ per pulse, depending on the ablation characteristics of the metal. Before the first use, the metal rod is quickly sanded and exposure to oxygen is minimised. 3 RESULTS In Fig. 2 we present one of our preliminary intensity scans of Nis. The saturation intensity is found at the intersection of the extrapolated linear part of the ion signal and the intensity axis, and is an independent measure for the ease of ionisation. Using the measured ionisation potential of Ni clusters, [8] and assuming a Gaussian beam profile (t=90 fs, 1=1500 nm), we calculate the saturation intensity with the ADK theory. So far, most measured saturation intensities for Ni clusters are a factor of approximately 3 times higher than the values predicted by ADK theory. This suppression of ionization clearly shows the anticipated multielectron shielding effect in metal clusters, due to a polarization of the cluster by the remaining electrons. We are currently working on models to include multielectron polarisability into our calculations by describing the clusters as a macroscopic conducting sphere.
64
Intensity (W/cm^) Fig 2. Saturation scan of Nig and calibration scan with Xe. REFERENCES [1] L.V. Keldysh, Sov. Phys. JETP 20, 1307 (1965). [2] M.V. Ammosov, N.B. Delone, and V.P. Krainov, Sov. Phys. JETP 64, 1191 (1986). [3] M.J. DeWitt, and R.J. Levis, Phys. Rev. Lett. 81, 5101 (1998) [4] M. Lezius, V. Blanchet, D.M. Rayner, D.M. Villeneuve, A. Stolow, and M.Y. Ivanov, Phys. Rev. Lett 86, 51 (2001) [5] M. Smits, S. Ulrich, T. Schulz, M. Schmitt, J.G. Underwood, J.P. Shaffer, C.A. de Lange, A.J. Alcock, D.M. Rayner, and A. Stolow, Rev. Sci. Instrum (in press, nov. 2003) [6] M. E. Geusic, M. D. Morse, S. C. O'Brien, and R. E. Smalley, Rev. Sci. Instrum. 56, 2123 (1985); M. D. Morse, M .E. Geusic, J. R. Heath, and R.E. Smalley, J. Chem. Phys. 83, 2293 (1985) [7] S. Hankin, D. Villeneuve, P. Corkum, and D. Rayner, Phys. Rev. A 64 (2001) [8] M.B. Knickelbein, S. Yang, and S.J. Riley, J. Chem. Phys 93, 94 (1990)