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Over the course of this thesis, a variety of lasers, wavelengths and pulse durations were employed. A schematic diagram of our laser table, detailing the systems used and how the laser beam was directed into the interaction region of the anion beam machine, is presented in Figure 2.3. In order to investigate ultrafast dynamics on the femtosecond scale by PE spectroscopy, it is necessary to employ a laser system capable of operating on these timescales. For this purpose, we utilise a femtosecond Titanium:Sapphire oscillator coupled to a regenerative chirped pulse amplifier to generate pulses of 800 nm with a pulse width of ~35 fs duration.
The initial pulse is generated by a mode-locked Spectra Physics “Tsunami” Ti:Sapphire oscillator, which is pumped by the second harmonic (532 nm) of a 5 W Spectra Physics “Millennia” continuous wave Nd:YAG laser. This produces ~35 fs pulses of the Ti:Sapphire fundamental (800 nm) at a repetition rate of ~76 MHz with a power of 0.5 W. This output is then used to seed a Spectra Physics “Spitfire Pro XP” regenerative chirped pulse amplifier; a Ti:Sapphire crystal in the amplifier is pumped by the second harmonic (527 nm) of a 30 W Spectra Physics “Empower” pulsed Nd:YLF laser. A pulse from the seed laser is selected every millisecond by a series of Pockels’ cells to stimulate emission from the crystal, enhancing the output power to 3W.
The output beam of the Spitfire XP Pro is initially split by a 60:40 beam splitter. 40% of the beam is used for one of two purposes: It may be used to pump a Light Conversion TOPAS-C optical parametric amplifier, which produces laser light of wavelengths between 1140 nm - 2600 nm. This may be achieved through non-linear splitting of a small percentage of the pump beam into two separate wavelengths. This can subsequently be used for sum frequency generation with the pump beam through use of a β-barium borate (BBO) crystal. Alternately, the 800 nm beam may be directed into the machine as a secondary harmonic generation line. The remaining 60% of the
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output of the amplifier is passed through a second 50:50 beam splitter, where half of the output beam is removed for use in another experiment. This line is passed through a retroreflector mounted upon a Physik Instrumente delay stage, which is computer controlled and used to generate the various delays used in time-resolved experiments, and into a home-built harmonic generation stage. This stage can be used to produce wavelengths of 400 nm, 266 nm and 200 nm through harmonic generations in BBO crystals. Both lines converge on a raised stage where the beams are recombined through the use of dichroic mirrors. The combined pulse train is subsequently directed into region 6 of the anion beam machine through a CaF2 window in order to interact with the ion packets and liberate PEs.
Additionally, we also employ a nanosecond system which may produce a wide range of wavelengths (190 nm - 2700 nm). In this, a “Continuum Surelite II” Q- switched Nd:YAG laser is coupled to a “Continuum Horizon” OPO. The OPO works on the same principle as TOPAS-C, in that part of the pump beam undergoes non-linear splitting to produce both a ‘signal’ and ‘idler’ beam (the sum of the two wavelengths will recover the wavelength of the pump beam), which can then be combined with the pump beam through sum-frequency generation. Through this, the system is capable of producing pulses of tunable wavelength of up to 50 mJ for UV wavelengths (190 nm – 400 nm) and 130 mJ for the signal/idler wavelengths (400 nm – 2700 nm) at peak efficiency. The pulses produced are ~ 6 ns in length, so cannot be usefully combined or used with the output of the femtosecond laser. The nanosecond laser is therefore primarily employed for single photon experiments.
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Figure 2.4 Typical performance of the Horizon OPO using a P800 pump laser. Reproduced from Continuumlasers.com.18
The OPO has two ports through which the laser pulse is emitted, one for the signal/idler wavelengths and another for the UV wavelengths. A prism is fixed in front of each port to direct the beam into a periscope and subsequently into the interaction
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region in region 6. In order to maintain a single beam path into the interaction region, the prism in front of the UV port (the closest to the periscope) is mounted on a removable magnetic stage, so as not to block the beam path from the signal/idler port.
2.6 References
(1) Roberts, G. M. Development and Construction of an new photoelectron
Imaging Spectrometer for Studying the Spectroscopy and Ultrafast Dynamics of Molecular Anions, PhD Thesis, Durham University, 2010.
(2) Lecointre, J.; Roberts, G. M.; Horke, D. A.; Verlet, J. R. R. Journal of
Physical Chemistry A 2010, 114, 11216.
(3) Horke, D. A.; Verlet, J. R. R. Physical Chemistry Chemical Physics
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(6) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M.
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(7) Wiley, W. C.; McLaren, I. H. Review of Scientific Instruments 1955, 26,
1150.
(8) Paul, W.; Steinwedel, H. Zeitschrift Fur Naturforschung Section a-a
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(9) Horke, D. A.; Roberts, G. M.; Lecointre, J.; Verlet, J. R. R. Review of
Scientific Instruments 2012, 83, 063101.
(10) Eppink, A.; Parker, D. H. Review of Scientific Instruments 1997, 68,
3477.
(11) Piani, G.; Becucci, M.; Bowen, M. S.; Oakman, J.; Hu, Q.; Continetti, R.
E. Physica Scripta 2008, 78, 058110.
(12) Peláez, R. J.; Blondel, C.; Delsart, C.; Drag, D. Journal of Physics B:
Atomic, Molecular and Optical Physics 2009, 42, 125001.
(13) Hanstorp, D.; Gustafsson, M. Journal of Physics B: Atomic, Molecular
and Optical Physics 1992, 25, 1773.
(14) Berry, R. S.; Reimann, C. W. Journal of Chemical Physics 1963, 38,
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(15) Roberts, G. M.; Nixon, J. L.; Lecointre, J.; Wrede, E.; Verlet, J. R. R.
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(16) Zhao, K.; Colvin, T.; Hill, W. T.; Zhang, G. Review of Scientific
Instruments 2002, 75, 3044.
(17) Garcia, G. A.; Nahon, L.; Powis, I. Review of Scientific Instruments
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(18) In Horizon OPO
Datasheet,http://www.continuumlasers.com/images/stories/products/specifications/Tun
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