5 Time-Resolved Photoelectron Spectroscopy for Excited State Dynamics
105
few hundreds of picoseconds to avoid non-linear processes and damage in the gain
medium. Single pass amplifiers can be used to increase the pulse intensity further.
After amplification, the amplified pulses are compressed back to durations of a few
tens of femtoseconds. The amplifier is usually pumped by a nanosecond laser operating around 525 nm.
Harmonics of the fundamental output of the Ti:S amplifier can be obtained by
frequency-doubling and sum-frequency generation processes in β-barium borate
(BBO) crystals generating pulses around 400 nm, 267 nm and 200 nm. Optical parametric amplifiers (OPAs) [53, 54] are employed to generate femtosecond pulses that
are tuneable throughout the UV to around 235 nm. In an OPA, the fundamental output of the Ti:S amplifier is focussed onto a Sapphire crystal to generate a white light
continuum, which is then mixed with the fundamental in a BBO crystal, providing
tuneable light in the range 1150 nm to 2630 nm; shorter wavelengths are generated
by subsequent sum-frequency mixing and higher order harmonic generation.
The duration of a laser pulse is limited to the cycle of the laser field, which
for 800 nm is a few femtoseconds but can be less than a hundred attoseconds in
the XUV [55]. Attosecond XUV pulses are produced by high harmonic generation
(HHG) in nonlinear gases. In principle, it is possible to use any gas, however the
ionization potential of the atom or molecule has to be high enough for the HHG
to compete with multiphoton ionization. The energy range and intensity profile of
the XUV spectrum depends on the gas; in Ar, the spectrum ranges from 10 eV
(125 nm) to around 100 eV (12.5 nm). An XUV monochromator or filters can be
used to select the photon energy range for the experiment. The efficiency of HHG
is usually relatively low (∼ 10 −6 ) yielding photon fluxes of around 10 8 photons per
pulse. XUV laser sources have sufficient photon energy to ionize the ground-state
of any molecule, but they also have sufficient photon energy to ionise inner valence
electrons, which promises to yield complementary information.
Free electron lasers (FELs) can produce very short laser pulses with high photon energy and high brilliance [56]. FELs work by generating high quality electron
pulses which are accelerated to relativistic energies and compressed to femtosecond
duration before being fed into an undulator. The periodic trajectory of the electrons
in the magnetic field of the undulator results in the emission of synchrotron radiation. Due to the interaction between the emitted radiation and the electrons the radiation is amplified coherently, thus, producing coherent electromagnetic radiation
with a mean photon energy that is controlled by the electron energy and the undulator parameters. There are several international FEL facilities in the world, which
include the European soft x-ray FEL Free-Electron Laser in Hamburg (FLASH) [57]
and the USA x-ray FEL (XFEL) the Linac Coherent Light Source (LCLS) [58]. The
FLASH facility generates laser pulses in the wavelength range 6.8 nm to 47 nm and
pulse durations from 10 fs to 70 fs. The LCLS facility operates in the wavelength
range from 0.15 nm to 2.2 nm with pulse durations from 70 fs to 500 fs. The intensity of these XUV and x-ray free electron lasers (XFELs) is currently as high
as 10 13 photons per pulse, which is several orders of magnitude larger than those
obtained using XUV HHG sources. A new XFEL facility is currently under development in Hamburg, Germany. This European project aims to achieve even higher
photon intensities in the wavelength range from 0.1 nm to 6.2 nm.
105
few hundreds of picoseconds to avoid non-linear processes and damage in the gain
medium. Single pass amplifiers can be used to increase the pulse intensity further.
After amplification, the amplified pulses are compressed back to durations of a few
tens of femtoseconds. The amplifier is usually pumped by a nanosecond laser operating around 525 nm.
Harmonics of the fundamental output of the Ti:S amplifier can be obtained by
frequency-doubling and sum-frequency generation processes in β-barium borate
(BBO) crystals generating pulses around 400 nm, 267 nm and 200 nm. Optical parametric amplifiers (OPAs) [53, 54] are employed to generate femtosecond pulses that
are tuneable throughout the UV to around 235 nm. In an OPA, the fundamental output of the Ti:S amplifier is focussed onto a Sapphire crystal to generate a white light
continuum, which is then mixed with the fundamental in a BBO crystal, providing
tuneable light in the range 1150 nm to 2630 nm; shorter wavelengths are generated
by subsequent sum-frequency mixing and higher order harmonic generation.
The duration of a laser pulse is limited to the cycle of the laser field, which
for 800 nm is a few femtoseconds but can be less than a hundred attoseconds in
the XUV [55]. Attosecond XUV pulses are produced by high harmonic generation
(HHG) in nonlinear gases. In principle, it is possible to use any gas, however the
ionization potential of the atom or molecule has to be high enough for the HHG
to compete with multiphoton ionization. The energy range and intensity profile of
the XUV spectrum depends on the gas; in Ar, the spectrum ranges from 10 eV
(125 nm) to around 100 eV (12.5 nm). An XUV monochromator or filters can be
used to select the photon energy range for the experiment. The efficiency of HHG
is usually relatively low (∼ 10 −6 ) yielding photon fluxes of around 10 8 photons per
pulse. XUV laser sources have sufficient photon energy to ionize the ground-state
of any molecule, but they also have sufficient photon energy to ionise inner valence
electrons, which promises to yield complementary information.
Free electron lasers (FELs) can produce very short laser pulses with high photon energy and high brilliance [56]. FELs work by generating high quality electron
pulses which are accelerated to relativistic energies and compressed to femtosecond
duration before being fed into an undulator. The periodic trajectory of the electrons
in the magnetic field of the undulator results in the emission of synchrotron radiation. Due to the interaction between the emitted radiation and the electrons the radiation is amplified coherently, thus, producing coherent electromagnetic radiation
with a mean photon energy that is controlled by the electron energy and the undulator parameters. There are several international FEL facilities in the world, which
include the European soft x-ray FEL Free-Electron Laser in Hamburg (FLASH) [57]
and the USA x-ray FEL (XFEL) the Linac Coherent Light Source (LCLS) [58]. The
FLASH facility generates laser pulses in the wavelength range 6.8 nm to 47 nm and
pulse durations from 10 fs to 70 fs. The LCLS facility operates in the wavelength
range from 0.15 nm to 2.2 nm with pulse durations from 70 fs to 500 fs. The intensity of these XUV and x-ray free electron lasers (XFELs) is currently as high
as 10 13 photons per pulse, which is several orders of magnitude larger than those
obtained using XUV HHG sources. A new XFEL facility is currently under development in Hamburg, Germany. This European project aims to achieve even higher
photon intensities in the wavelength range from 0.1 nm to 6.2 nm.