1 Historical Developments and Future Perspectives …
49
Fig. 1.28 The time evolution of the decay for one (circles) out of twelve registered 50-photon
excitations. The solid line is theory fit. The error bars show the standard deviations related to the
numbers of counts per channel. (From [111])
these facilities promise to increase the count rate by several orders of magnitude and
to allow for NRS pump-probe experiments on the femto-second time scale.
With XFEL radiation, a time spectrum of Nuclear Resonance Scattering can
be recorded in a single-shot of a laser pulse. In the first NRS experiment at an
XFEL [111], the most probable number of nuclear resonance photons in a single shot
was ∼ 8, the mean number of photons 16, and the maximum number of recorded photons 68. Single-pulse time spectra recorded after a shot with highest photon numbers
provide already sufficient statistical accuracy for single-shot hyperfine spectroscopy.
Figure 1.28 shows the time spectrum measured for a single pulse of XFEL radiation with 50 detected photons. The data were fit with the theory using the nuclear
magnetic hyperfine splitting as adjustable parameter. The obtained value of the splitting 513(30) neV is in the excellent agreement with the previously reported value of
512.6 neV [135]. The proper determination of the hyperfine parameters is the first
demonstration of hyperfine spectroscopy with a single shot of the XFEL radiation.
This result opens access to nuclear resonance pump-probe experiments with femtosecond time resolution at the presently available XFEL facilities and with APD
detectors. Indeed, coherent nuclear scattering relies on the phasing of scattering over
a nuclear ensemble and occurs only if the phasing/position of atoms are well defined,
relative to each other at the excitation and emission time. This condition is valid for
solids, but not for liquids. This allows one to use nuclear forward scattering to study,
for example, heat transfer or melting (or even magnetic order) on ∼ femto-second
time scales, only limited by the XFEL pulse length.
Consider the scheme shown in Figure 1.13, where a resonant sample is placed
just after the double-crystal monochromator (HHLM). Because the sample is located
before high-resolution optics, it is still illuminated by the short pulses of the XFEL,
with ∼ fs pulse width. Suppose that the sample is melted by a short pulse of a pump
laser.
If the sample remains liquid at the arrival of the probe XFEL radiation, the measured spectrum will be the one prepared only by nuclear diffraction in the iron borate
crystal. On the contrary, if the sample cools down to a solid state before the arrival of
the probe XFEL radiation, the conditions for nuclear forward scattering are fulfilled,
and the time spectrum measured after the sample and the iron borate crystal will be
49
Fig. 1.28 The time evolution of the decay for one (circles) out of twelve registered 50-photon
excitations. The solid line is theory fit. The error bars show the standard deviations related to the
numbers of counts per channel. (From [111])
these facilities promise to increase the count rate by several orders of magnitude and
to allow for NRS pump-probe experiments on the femto-second time scale.
With XFEL radiation, a time spectrum of Nuclear Resonance Scattering can
be recorded in a single-shot of a laser pulse. In the first NRS experiment at an
XFEL [111], the most probable number of nuclear resonance photons in a single shot
was ∼ 8, the mean number of photons 16, and the maximum number of recorded photons 68. Single-pulse time spectra recorded after a shot with highest photon numbers
provide already sufficient statistical accuracy for single-shot hyperfine spectroscopy.
Figure 1.28 shows the time spectrum measured for a single pulse of XFEL radiation with 50 detected photons. The data were fit with the theory using the nuclear
magnetic hyperfine splitting as adjustable parameter. The obtained value of the splitting 513(30) neV is in the excellent agreement with the previously reported value of
512.6 neV [135]. The proper determination of the hyperfine parameters is the first
demonstration of hyperfine spectroscopy with a single shot of the XFEL radiation.
This result opens access to nuclear resonance pump-probe experiments with femtosecond time resolution at the presently available XFEL facilities and with APD
detectors. Indeed, coherent nuclear scattering relies on the phasing of scattering over
a nuclear ensemble and occurs only if the phasing/position of atoms are well defined,
relative to each other at the excitation and emission time. This condition is valid for
solids, but not for liquids. This allows one to use nuclear forward scattering to study,
for example, heat transfer or melting (or even magnetic order) on ∼ femto-second
time scales, only limited by the XFEL pulse length.
Consider the scheme shown in Figure 1.13, where a resonant sample is placed
just after the double-crystal monochromator (HHLM). Because the sample is located
before high-resolution optics, it is still illuminated by the short pulses of the XFEL,
with ∼ fs pulse width. Suppose that the sample is melted by a short pulse of a pump
laser.
If the sample remains liquid at the arrival of the probe XFEL radiation, the measured spectrum will be the one prepared only by nuclear diffraction in the iron borate
crystal. On the contrary, if the sample cools down to a solid state before the arrival of
the probe XFEL radiation, the conditions for nuclear forward scattering are fulfilled,
and the time spectrum measured after the sample and the iron borate crystal will be