In van der Veen et al., [Struct. Dyn. 2, 024302 (2015)], femtosecond and nanosecond electron energy loss spectroscopy of deep core-levels are demonstrated. These results pave the way to the investigation of materials and molecules with combined energy, time, and spatial resolution in a transmission electron microscope. Furthermore, the authors elucidate the role of the electron phonon coupling in the band-gap renormalization that takes place in graphite upon photo-excitation.

If today Prometheus were to ask condensed matter physicists “What would you want me to risk my liver for?” he would be answered “An element specific technique capable of correlating atomic motions with changes in the orbital occupancy, local spin states, and angular momenta.”

This is achievable today by ultrafast core-level spectroscopy available at modern X-ray Free Electron Lasers (XFEL) sources.1 However, a lab-based technique, such as femtosecond-Electron Energy Loss Spectroscopy (FEELS) delivers very similar information and is complementary to X-ray spectroscopy, as it allows the investigation of very tiny samples, such as nanostructures, thin films, and of samples consisting of very light elements.2–5 Furthermore, the very high spatial resolution reached by electron-based techniques can be combined with EELS in energy filtered electron microscopy and/or via scanning techniques,6–8 giving access to inhomogeneities and phase separation effects typically found near the critical point of phase transitions.

The challenge of these experiments resides in the inherent tendency of electrons to repel each other (the so-called space-charge effect), causing temporal broadening of the pulses, chromatic aberrations, and a loss in energy resolution.9,10 This is currently circumvented by operating the ultrafast transmission electron microscope (TEM) with one electron only per pulse, thus limiting the signal-to-noise ratio achievable in this kind of experiments.11,12

In the article “Ultrafast Core-Loss Spectroscopy in 4D Electron Microscopy,” 13 van der Veen et al. report on a detailed investigation of the photoinduced structural dynamics in graphite by means of femtosecond and nanosecond EELS. In particular, the achievement of ultrafast time-resolution at the Carbon K-edge absorption is a milestone in modern ultrafast technology.

The interplay between crystal distortions and the material's electronic structure is usually investigated by means of ultrafast transient optical spectroscopy14,15 and/or photoemission.16 However, real-time visualization of the nuclear dynamics of ions has been achieved via time-resolved X-ray or electron diffraction.17 The ability to observe both the nuclear and the electronic dynamics has only recently been possible via ultrafast X-ray absorption spectroscopy18 and ultrafast EELS,3,19,20 with the latter being limited to the low-loss plasmon region and the shallower core levels. Probing deeper core-levels offers the unique advantage of chemical selectivity, as opposed to lower-energy experiments such as optical spectroscopy that are sensitive to highly hybridized valence orbitals.21 

The authors demonstrate this capability on photoexcited graphite by probing the FEELS signal at the carbon K-edge. It is important to note as well that the machine used to perform these pioneering experiments is the first ever capable of achieving ultrafast EELS of core levels.2 Certainly, the technology has not reached its limits with this very first design, and ideas are already proposed to take the instrumentation to the next level.22 The key advance will be an increment in the signal-to-noise ratio, which would allow the observation by ultrafast EELS of more subtle effects excited by a smaller laser fluence.

In their experiment, van der Veen et al. excite a graphite thin film with a rather high laser fluence (tens of mJ/cm2). A rich variety of problems can be addressed in this regime, such as structural phase transitions23 and metal-insulator transitions,24 among others. A consequence of the high-fluence excitation is the observation, for the first time, of the Photon-Induced Near-Field Electron Microscopy effect (PINEM)25 at the carbon K-edge. Briefly, when an intense laser pulse strikes an object, transient electromagnetic fields can be induced on its surface. These fields can accelerate/decelerate electrons passing in their vicinity, resulting in weaker replicas of the elastic electron beam shifted in energy by multiple integers of the driving laser photons. The efficiency of this effect strongly depends on the excitation, shape, and dielectric properties of the surface and is therefore, rather insensitive to the bulk properties of the specimen under investigation. For this reason, the authors rightfully warrant attention in the interpretation of the first ps dynamics of the FEELS spectra. An open question, however, remains: Is the temporal evolution of the surface plasmonic field induced by the laser pulses completely insensitive to the material properties, for example, its metallicity or the electron-hole pairs recombination dynamics? So far, to the best of my knowledge, only systems characterized by a rather fast re-equilibration of their electronic structure have been investigated via PINEM: carbon nanotubes, silver nanowires, and a few other examples, including biological materials.7,25,26 Would the observed dynamics be different on the surface of a solid in which the electron-hole recombination time is very slow? In this respect, even the PINEM signal near time-zero may reveal dynamical properties of the system.

Besides the technological breakthrough that these results represent, they deliver great insight into the photoexcited dynamics of graphite. The evolution of photoexcited electron-hole pairs in graphite is a long-standing subject of debate.27–30 Hard to believe, a system whose electronic structure can be calculated with a pen and a sheet of paper, there are still open questions regarding the relaxation mechanism of light-induced electron-hole pairs27–35 and its impact on the out-of-equilibrium structural properties of the material. In particular, the ps dynamics of the transient optical absorption is still under debate. Originally, such a long-lasting transient optical signal was interpreted as resulting from band gap renormalization (BGR) caused by a long recombination time of the photoexcited electron-hole pairs.30 Later, ultrafast optical absorption experiments in the ultraviolet34 show that the recombination of the photoexcited carriers and the consequent BGR are much faster, lasting few hundreds of fs. An alternative scenario was proposed in which the recombination of the electron-hole pairs leaves the crystal structure out of equilibrium through the excitation of specific phonon modes, leading to a long-lasting dynamics of the optical constants.32,33 van der Veen et al. succeed in unraveling the precise microscopic mechanism of this effect, showing that the shifts of the electronic bands, responsible for the transient optical signal, are mainly due to a temperature-dependent electron-phonon coupling, combined with a band-shift due to the c-axis distortion, which is a consequence of the anharmonic couplings between the initially excited in-plane phonon modes and the out-of-plane vibrations.

The great news is that this is just the beginning. Further developments of the technique will allow reaching even deeper core levels, in the region of the transition metals (TM) L-edges, where most of the physics of TM oxides of current interest for microelectronics, spintronics, water splitting, bio-sensing, and solar energy conversion resides. Furthermore, the reported results open the path to extend to the time-domain other TEM-specificities such as element mapping, momentum-resolved EELS, and possibly even dichroic experiments with the newly discovered electron vortex beams.36 The development of these tools in parallel with the emerging new XFEL technology will offer a unique window of observation on virtually all types of materials: organic, inorganic, nanostructured, or bulk.

1.
T. J.
Penfold
,
C. J.
Milne
, and
M.
Chergui
,
Advances in Chemical Physics
, 2nd ed. edited by
Stuart A.
Rice
and
Aaron R.
Dinner
(
John Wiley & Sons, Inc.
,
2013
) Vol. 153.
2.
F.
Carbone
,
B.
Barwick
,
O. H.
Kwon
,
H. S.
Park
,
J. S.
Baskin
, and
A. H.
Zewail
, “
EELS femtosecond resolved in 4D ultrafast electron microscopy
,”
Chem. Phys. Lett.
468
(
4
),
107
111
(
2009
).
3.
F.
Carbone
,
O. H.
Kwon
, and
A. H.
Zewail
, “
Dynamics of chemical bonding mapped by energy-resolved 4D electron microscopy
,”
Science
325
(
5937
),
181
184
(
2009
).
4.
F.
Carbone
, “
Modern electron microscopy resolved in space, energy and time
,”
Eur. Phys. J. Appl. Phys.
54
(
03
),
33503
(
2011
).
5.
F.
Carbone
,
P.
Musumeci
,
O. J.
Luiten
, and
C.
Hebert
, “
A perspective on novel sources of ultrashort electron and x-ray pulses
,”
Chem. Phys.
392
(
1
),
1
9
(
2012
).
6.
J.
Cho
,
T. Y.
Hwang
, and
A. H.
Zewail
, “
Visualization of carrier dynamics in p (n)-type gaas by scanning ultrafast electron microscopy
,”
Proc. Natl. Acad. Sci. U.S.A.
111
(
6
),
2094
2099
(
2014
).
7.
A.
Yurtsever
,
R. M.
van der Veen
, and
A. H.
Zewail
, “
Subparticle ultrafast spectrum imaging in 4d electron microscopy
,”
Science
335
(
6064
),
59
64
(
2012
).
8.
V.
Ortalan
and
A. H.
Zewail
, “
4D scanning transmission ultrafast electron microscopy: Single-particle imaging and spectroscopy
,”
J. Am. Chem. Soc.
133
(
28
),
10732
10735
(
2011
).
9.
T.
LaGrange
,
M. R.
Armstrong
,
K.
Boyden
,
C. G.
Brown
,
G. H.
Campbell
,
J. D.
Colvin
,
W. J.
DeHope
,
A. M.
Frank
,
D. J.
Gibson
,
F. V.
Hartemann
 et al, “
Single-shot dynamic transmission electron microscopy
,”
Appl. Phys. Lett.
89
(
4
),
044105
(
2006
).
10.
D.
Shorokhov
and
A. H.
Zewail
, “
4D electron imaging: principles and perspectives
,”
Phys. Chem. Chem. Phys.
10
(
20
),
2879
2893
(
2008
).
11.
A. H.
Zewail
and
J. M.
Thomas
,
4D Electron Microscopy: Imaging in Space and Time
(
World Scientific
,
2009
).
12.
L.
Piazza
,
D. J.
Masiel
,
T.
LaGrange
,
B. W.
Reed
,
B.
Barwick
, and
F.
Carbone
, “
Design and implementation of a fs-resolved transmission electron microscope based on thermionic gun technology
,”
Chem. Phys.
423
,
79
84
(
2013
).
13.
R. M.
van der Veen
,
T. J.
Penfold
, and
A. H.
Zewail
, “
Ultrafast Core-Loss Spectroscopy in 4D Electron Microscopy
,”
Struct. Dyn.
2
,
024302
(
2015
).
14.
T. E.
Stevens
,
J.
Kuhl
, and
R.
Merlin
, “
Coherent phonon generation and the two stimulated Raman tensors
,”
Phys. Rev. B
65
(
14
),
144304
(
2002
).
15.
D. N.
Basov
,
R. D.
Averitt
,
D.
Van Der Marel
,
M.
Dressel
, and
K.
Haule
, “
Electrodynamics of correlated electron materials
,”
Rev. Mod. Phys.
83
(
2
),
471
(
2011
).
16.
G.
Moos
,
C.
Gahl
,
R.
Fasel
,
M.
Wolf
, and
T.
Hertel
, “
Anisotropy of quasiparticle lifetimes and the role of disorder in graphite from ultrafast time-resolved photoemission spectroscopy
,”
Phys. Rev. Lett.
87
(
26
),
267402
(
2001
).
17.
M.
Chergui
and
A. H.
Zewail
, “
Electron and x-ray methods of ultrafast structural dynamics: Advances and applications
,”
ChemPhysChem
10
(
1
),
28
43
(
2009
).
18.
Ch.
Bressler
,
C.
Milne
,
V.-T.
Pham
,
A.
ElNahhas
,
R. M.
Van der Veen
,
W.
Gawelda
,
S.
Johnson
,
P.
Beaud
,
D.
Grolimund
,
M.
Kaiser
 et al. “
Femtosecond Xanes study of the light-induced spin crossover dynamics in an iron (ii) complex
,”
Science
323
(
5913
),
489
492
(
2009
).
19.
F.
Carbone
,
P.
Baum
,
P.
Rudolf
, and
A. H.
Zewail
, “
Structural preablation dynamics of graphite observed by ultrafast electron crystallography
,”
Phys. Rev. Lett.
100
(
3
),
035501
(
2008
).
20.
L.
Piazza
,
C.
Ma
,
H. X.
Yang
,
A.
Mann
,
Y.
Zhu
,
J. Q.
Li
, and
F.
Carbone
, “
Ultrafast structural and electronic dynamics of the metallic phase in a layered manganite
,”
Struct. Dyn.
1
(
1
),
014501
(
2014
).
21.
F. M. F.
De Groot
,
J. C.
Fuggle
,
B. T.
Thole
, and
G. A.
Sawatzky
, “
2p x-ray absorption of 3d transition-metal compounds: An atomic multiplet description including the crystal field
,”
Phys. Rev. B
42
(
9
),
5459
(
1990
).
22.
R. K.
Li
and
P.
Musumeci
, “
Single-shot mev transmission electron microscopy with picosecond temporal resolution
,”
Phys. Rev. Appl.
2
(
2
),
024003
(
2014
).
23.
H. S.
Park
,
O. H.
Kwon
,
J. S.
Baskin
,
B.
Barwick
, and
A. H.
Zewail
, “
Direct observation of martensitic phase-transformation dynamics in iron by 4D single-pulse electron microscopy
,”
Nano Lett.
9
(
11
),
3954
3962
(
2009
).
24.
M. S.
Grinolds
,
V. A.
Lobastov
,
J.
Weissenrieder
, and
A. H.
Zewail
, “
Four-dimensional ultrafast electron microscopy of phase transitions
,”
Proc. Natl. Acad. Sci. U.S.A.
103
(
49
),
18427
18431
(
2006
).
25.
B.
Barwick
,
D. J.
Flannigan
, and
A. H.
Zewail
, “
Photon-induced near-field electron microscopy
,”
Nature
462
(
7275
),
902
906
(
2009
).
26.
D. J.
Flannigan
,
B.
Barwick
, and
A. H.
Zewail
, “
Biological imaging with 4D ultrafast electron microscopy
,”
Proc. Natl. Acad. Sci. U.S.A.
107
(
22
),
9933
9937
(
2010
).
27.
T.
Mishina
,
K.
Nitta
, and
Y.
Masumoto
, “
Coherent lattice vibration of interlayer shearing mode of graphite
,”
Phys. Rev. B
62
(
4
),
2908
(
2000
).
28.
T.
Kampfrath
,
L.
Perfetti
,
F.
Schapper
,
C.
Frischkorn
, and
M.
Wolf
, “
Strongly coupled optical phonons in the ultrafast dynamics of the electronic energy and current relaxation in graphite
,”
Phys. Rev. Lett.
95
(
18
),
187403
(
2005
).
29.
R.
Saito
,
A.
Jorio
,
A. G.
Souza Filho
,
G.
Dresselhaus
,
M. S.
Dresselhaus
, and
M. A.
Pimenta
, “
Probing phonon dispersion relations of graphite by double resonance Raman scattering
,”
Phys. Rev. Lett.
88
(
2
),
027401
(
2001
).
30.
M.
Breusing
,
C.
Ropers
, and
T.
Elsaesser
, “
Ultrafast carrier dynamics in graphite
,”
Phys. Rev. Lett.
102
(
8
),
086809
(
2009
).
31.
M.
Breusing
,
S.
Kuehn
,
T.
Winzer
,
E.
Malić
,
F.
Milde
,
N.
Severin
,
J. P.
l. Rabe
,
C.
Ropers
,
A.
Knorr
, and
T.
Elsaesser
, “
Ultrafast nonequilibrium carrier dynamics in a single graphene layer
,”
Phys. Rev. B
83
(
15
),
153410
(
2011
).
32.
F.
Carbone
,
G.
Aubock
,
A.
Cannizzo
,
F.
Van Mourik
,
R. R.
Nair
,
A. K.
Geim
,
K. S.
Novoselov
, and
M.
Chergui
, “
Femtosecond carrier dynamics in bulk graphite and graphene paper
,”
Chem. Phys. Lett.
504
(
1
),
37
40
(
2011
).
33.
F.
Carbone
, “
The interplay between structure and orbitals in the chemical bonding of graphite
,”
Chem. Phys. Lett.
496
(
4
),
291
295
(
2010
).
34.
S.
Pagliara
,
G.
Galimberti
,
S.
Mor
,
M.
Montagnese
,
G.
Ferrini
,
M. S.
Grandi
,
P.
Galinetto
, and
F.
Parmigiani
, “
Photoinduced π- π* band gap renormalization in graphite
,”
J. Am. Chem. Soc.
133
(
16
),
6318
6322
(
2011
).
35.
X. Q.
Yan
,
J.
Yao
,
Z. B.
Liu
,
X.
Zhao
,
X. D.
Chen
,
C.
Gao
,
W.
Xin
,
Y.
Chen
, and
J. G.
Tian
, “
Evolution of anisotropic-to-isotropic photoexcited carrier distribution in graphene
,”
Phys. Rev. B
90
(
13
),
134308
(
2014
).
36.
B. J.
McMorran
,
A.
Agrawal
,
I. M.
Anderson
,
A. A.
Herzing
,
H. J.
Lezec
,
J. J.
McClelland
, and
J.
Unguris
, “
Electron vortex beams with high quanta of orbital angular momentum
,”
Science
331
(
6014
),
192
195
(
2011
).