Topics in Current Chemistry (2018) 376:28
1 3
challenging to researchers in establishing ultrafast spectroscopy as a standard analytical tool.
Despite these still-existing challenges, it is remarkable to recognize that multidimensional laser spectroscopy has evolved into and is still strongly evolving as
a highly active field of research that reflects an enormous amount of curiosity and
inventive genius of many of the contributing researchers. From the dozens of contributing labs that are active both in development of new methods as well as in testing of possible boundaries of existing ones, there still exists a continuous output of
novel scientific and technological insight. For instance, a highly active branch of
research is dedicated to combining methods from different frequency ranges. This
way, sophisticated combinations of UV–Vis/IR methods have been developed, and
it can be anticipated that other mixtures (i.e., UV–Vis/THz) will be demonstrated
shortly in the near future. As a side note, even a combination of UV–Vis and NMR
has been presented [112]. Moreover, it is also important to keep in mind that most of
the so-far-demonstrated methods exclusively rely on the detection of optical signals
derived from laser light [113–115]. However, other approaches can be envisioned
and are currently being tested such as detection of photocurrents [116–118]. Other
labs have even succeeded in combining ultrafast multidimensional optical spectroscopy with the advantages that detection of ultrafast photoelectrons provide, which
feature a severely shorter de-Broglie wavelength and, therefore, allow for extreme
spatial resolution down to the nanometer-length scale. Such ultrafast nanoscopy
[119] is not yet fully explored in its capabilities, but holds great promise regarding
possible insight into highly spatially confined processes in chemistry and material
science. In a very similar direction, significant efforts are currently being undertaken
to advance ultrafast multidimensional spectroscopy to the fewest possible contributing emitters, i.e., ideally single molecules [120]. Such methods might for instance
allow studying in detail and in a multidimensional way, how the environment around
single molecules influences the spectro-temporal characteristics of different entities.
Multidimensional spectroscopy has been restricted so far to conventional light
sources such as table top lasers. It will be interesting and important to see to what
extent it will become possible to transfer the corresponding methods and approaches
to even shorter wavelengths such as vacuum UV or X-ray ranges [121–123]. These
frequency ranges in combination with coherent light emission will, with sufficient
photon fluxes, possibly only be available in the near future from large-scale facilities such as the various free-electron lasers that have been set up in the last decades and new ones being currently assembled start operating. There exist current
efforts to also develop tabletop variants for these frequency ranges, but these sources
are far from being standard. In an attractive manner, such short wavelengths allow
the implementation of even attosecond pulses, making available the shortest currently reachable timescales that are relevant mostly for electronic motions. Many
different novel phenomena will be possible to be investigated with these methods
such as electronic spectroscopy from tightly bound core-near electrons, multi-photon ionization, charge migration after ionization, Auger-processes, auto-ionization,
or Coulomb explosions. This way, one might be able to access the ultimately fastest timescales that are relevant for chemistry, physics, and material science, possibly witnessing the impact of electronic movements and non-Born–Oppenheimer
16
Reprinted from the journal
1 3
challenging to researchers in establishing ultrafast spectroscopy as a standard analytical tool.
Despite these still-existing challenges, it is remarkable to recognize that multidimensional laser spectroscopy has evolved into and is still strongly evolving as
a highly active field of research that reflects an enormous amount of curiosity and
inventive genius of many of the contributing researchers. From the dozens of contributing labs that are active both in development of new methods as well as in testing of possible boundaries of existing ones, there still exists a continuous output of
novel scientific and technological insight. For instance, a highly active branch of
research is dedicated to combining methods from different frequency ranges. This
way, sophisticated combinations of UV–Vis/IR methods have been developed, and
it can be anticipated that other mixtures (i.e., UV–Vis/THz) will be demonstrated
shortly in the near future. As a side note, even a combination of UV–Vis and NMR
has been presented [112]. Moreover, it is also important to keep in mind that most of
the so-far-demonstrated methods exclusively rely on the detection of optical signals
derived from laser light [113–115]. However, other approaches can be envisioned
and are currently being tested such as detection of photocurrents [116–118]. Other
labs have even succeeded in combining ultrafast multidimensional optical spectroscopy with the advantages that detection of ultrafast photoelectrons provide, which
feature a severely shorter de-Broglie wavelength and, therefore, allow for extreme
spatial resolution down to the nanometer-length scale. Such ultrafast nanoscopy
[119] is not yet fully explored in its capabilities, but holds great promise regarding
possible insight into highly spatially confined processes in chemistry and material
science. In a very similar direction, significant efforts are currently being undertaken
to advance ultrafast multidimensional spectroscopy to the fewest possible contributing emitters, i.e., ideally single molecules [120]. Such methods might for instance
allow studying in detail and in a multidimensional way, how the environment around
single molecules influences the spectro-temporal characteristics of different entities.
Multidimensional spectroscopy has been restricted so far to conventional light
sources such as table top lasers. It will be interesting and important to see to what
extent it will become possible to transfer the corresponding methods and approaches
to even shorter wavelengths such as vacuum UV or X-ray ranges [121–123]. These
frequency ranges in combination with coherent light emission will, with sufficient
photon fluxes, possibly only be available in the near future from large-scale facilities such as the various free-electron lasers that have been set up in the last decades and new ones being currently assembled start operating. There exist current
efforts to also develop tabletop variants for these frequency ranges, but these sources
are far from being standard. In an attractive manner, such short wavelengths allow
the implementation of even attosecond pulses, making available the shortest currently reachable timescales that are relevant mostly for electronic motions. Many
different novel phenomena will be possible to be investigated with these methods
such as electronic spectroscopy from tightly bound core-near electrons, multi-photon ionization, charge migration after ionization, Auger-processes, auto-ionization,
or Coulomb explosions. This way, one might be able to access the ultimately fastest timescales that are relevant for chemistry, physics, and material science, possibly witnessing the impact of electronic movements and non-Born–Oppenheimer
16
Reprinted from the journal