Journal of Luminescence 172 (2016) 243–248

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Full Length Article

Excitation process of Ce3 þ and Eu2 þ ions doped in SrGa2S4 crystals
under the condition of multiplication of electronic excitations
Mamoru Kitaura a,n, Senku Tanaka b, Minoru Itoh c, Akimasa Ohnishi a, Hiroko Kominami d,Kazuhiko Hara daDepartment of Physics, Faculty of Science, Yamagata University, Yamagata 990-8560, Japan
Department of Electric and Electronic Engineering, Faculty of Science and Engineering, Kinki University, Higashiosaka 577-8502, Japanc
Department of Electrical and Electronic Engineering, Faculty of Engineering, Shinshu University, Nagano 380-8553, Japand
Research Institutes of Electronics, Shizuoka University, Hamamatsu 432-8011, Japanb

art ic l e i nf o

a b s t r a c t

Article history:
Received 4 October 2015
Received in revised form
28 November 2015
Accepted 9 December 2015
Available online 18 December 2015

Excitation spectra for 5d–4f photoluminescence (PL) bands of Ce3 þ and Eu2 þ ions doped in SrGa2S4
crystals have been measured at room temperature in the energy region up to 25 eV. These spectra allow
us to investigate the excitation process of luminescent ions under host excitation with vacuum ultraviolet(VUV) photons. A gentle increase in intensity is commonly observed at around 16.5 eV in the excitation
spectra. Optical constant spectra were obtained from measurements of reﬂectivity and absorption edge
spectra of undoped SrGa2S4 crystals. These data were compared with the results of a relativistic DV-Xα
calculation, to clarify the electronic structure of SrGa2S4. Ultraviolet photoelectron spectroscopy (UPS)
has been carried out under the excitation with photons in the 12–20 eV range. The detailed analysis of
UPS spectra reveals that the multiplication of electronic excitations (MEEs) occurs above 16.5 eV. The
excitation process of Ce3 þ and Eu2 þ ions is discussed together with the processes of MEEs.
& 2015 Elsevier B.V. All rights reserved.

Keywords:
MEEs
Excitation spectra
UPS spectra
Synchrotron radiation
Ce3 þ ion
Eu2 þ ion
SrGa2S4

1. Introduction
Multiplication of electronic excitations (MEEs) in wide-gap
ionic crystals takes place when the energy of an excitation photon exceeds several times of their band-gap energies. The processes of MEEs have been investigated by photoluminescence (PL)and photoelectron spectroscopy with vacuum ultraviolet (VUV)
photons [1–4]. The primary process of MEEs by VUV photons is
explained on the basis of an inelastic scattering of hot photoelectrons with valence electrons. On the other hand, the excitation
processes of luminescent ions doped in the wide-gap materials
have also been studied in the VUV region [5–9]. Lushchick et al. [5]have measured VUV excitation spectra for Mn2 þ intra-3d PL bandof ZnS doped with Mn2 þ ions of high-concentration, and observed
a rapid increase in PL intensity at the photon energy lower than
the threshold for MEEs in host ZnS. This phenomenon has been
attributed to direct (elastic) impact excitation of hot photoelectrons with Mn2 þ 3d electrons. This type of excitation has beenfound in VUV excitation spectra for Tl þ 6p–6s PL bands of KCl and
KBr doped with Tl þ ions [4,7], and for Er3 þ 5d–4f PL band of LiYF4n

Corresponding author.
E-mail address: Kitaura@sci.kj.yamagata-u.ac.jp (M. Kitaura).

http://dx.doi.org/10.1016/j.jlumin.2015.12.007
0022-2313/& 2015 Elsevier B.V. All rights reserved.

doped with Er3 þ ions [8]. In contrast, excitation spectra for Ag þ
4s–3d PL band of NaCl doped with Ag þ ions [9] and for Gd3 þ
intra-4f PL band of Gd3Al2Ga3O12 [10] did not show any evidence
on such direct impact excitation. As mentioned here, the excitation
processes of luminescent ions in the VUV region are sensitive tothe electronic structures of host materials and luminescent ions,the concentration of luminescent ions and so on. In addition, theseprocesses are expected to be complex, because they take placethrough a number of competing processes. Hence, they are still thesubject of considerable discussion, and thus are worthwhile
studying to ﬁnd a collective view on these processes. The information would be useful in applications to luminescent devices inwhich hot conduction electrons play an important role for theexcitation of luminescent ions.
Strontium thiogallate (SrGa2S4) activated using rare earth ions
are known as a bright inorganic phosphor for solid-state lightening and information display [11,12]. The emission spectra
exhibit prominent bands at 2.79 and 2.33 eV for Ce3 þ - and Eu2 þ doped SrGa2S4, respectively. Since these bands are efﬁciently
excited with photons in the absorption due to 4 f–5d transitions of
Ce3 þ and Eu2 þ ions, they have been assigned to the 5d–4f transitions [13–18]. The onsets of Ce3 þ and Eu2 þ 4f–5d absorption
locate at around 2.8 and 2.5 eV, respectively. Many experimental
data on PL properties have been accumulated so far; however,M. Kitaura et al. / Journal of Luminescence 172 (2016) 243–248

Crystals of undoped SrGa2S4, and Ce3 þ - and Eu2 þ -doped
SrGa2S4 were grown by a chemical vapor transport techniqueusing iodine as a transport agent [15]. Synthesized powders of
undoped SrGa2S4, and Ce3 þ - and Eu2 þ -doped SrGa2S4 wereinserted into quartz ampoules, which were sealed after evacuatingin vacuum. The ampoules were placed in an electric furnace with
two heating zone for two weeks. The temperatures of growth andsource zones were kept at 900 and 750 °C, respectively. The
average size of grown crystals was 2  2  0.1 mm3. The ab plane
was used in our experiment. Excitation and reﬂectivity spectra
were measured at the BL3B beamline of UVSOR in the Institute for
Molecular Science (IMS). Synchrotron radiation (SR) was monochromatized through a 2.5-m normal incidence monochromator.
The incident, reﬂected and transmitted photons were detectedusing a calibrated Si photodiode (IRD, XUV100) placed in thesample chamber. The intensities of reﬂected and transmitted
photons were divided by that of the incident photon, in order todetermine the reﬂectivity and absorption coefﬁcient at various
photon energies. PL was detected using the system consisting of
two quartz lenses, a color ﬁlter, a grating monochromator, and aphotomultiplier tube. Excitation spectra were corrected for theintensity distribution of the excitation light source and thereﬂection loss at the crystal surface.
UPS experiment was carried out at the BL8B beamline of
UVSOR. SrGa2S4 thin-ﬁlms were deposited on gold-coated quartz
substrates heated at 200 °C using electron beam sources. The
thickness was adjusted to be 50 nm using a quartz oscillator. Asdeposited thin-ﬁlms were annealed at 850 °C in H2SþAr gas
mixture for 30 min, in order to supply deﬁcient sulfur ions. The
thin-ﬁlms obtained are undoubtedly of polycrystalline, because
substrates are made of silica glass. From the X-ray diffraction
measurement, they were conﬁrmed to be SrGa2S4. In UPS experiment, the surfaces of samples were cleaned in vacuum of
3  10  8 Pa with an Ar þ ion gun. An appearance of the cleansurface was judged from the peak energy and spectral shape of the
Ga-3d peak. The energy distribution of photoelectrons emitted
from the sample surface was analyzed using a hemi-sphericalelectron analyzer of 75 mm mean radius. The plane grating

3. Results
Fig. 1(a) shows the spectra of reﬂectivity (blue line) andabsorption edge (red line) for an undoped SrGa2S4 crystal at room
temperature. The fundamental absorption edge locates at around
4.1 eV. The reﬂectivity shown here is the absolute value obtained
as follows. (i) The refractive index n below 4 eV is determined by
the spectroscopic ellipsometry. (ii) The absolute reﬂectivity spectrum is calculated by substituting the values of n into the formulaR(E)¼ (n(E)–1)2/(n(E)þ1)2. (iii) The measured spectrum is connected to the calculated spectrum by multiplication with a constant factor. A weak exciton-like peak is observed at 4.72 eV.
Assuming that the direct band-to-band transition occurs in thehigh-energy side of this peak, we roughly estimate the band-gap
energy to be 4.8 eV. The most prominent peak appears at around6.24 eV. As the photon energy is increased, the reﬂectivity is gradually decreased in the 15–20 eV range. Two weak peaks are seenat 21.7 and 22.7 eV.
Fig. 1(b) and (c) shows excitation spectra for 2.79 eV PL band of
Ce3 þ -doped SrGa2S4 and 2.33 eV PL band in Eu2 þ -doped SrGa2S4,
respectively. The concentrations of Ce3 þ and Eu2 þ ions were2
SrGa2S4

0.4

1

0.2
0

SrGa2S4:Ce

E0+Eg

2Eg

SrGa2S4:Eu

0

0

0

3+

×5

0

E0+Eg

α

2. Experimentmonochromator (PGM) was used to obtain monochromatic excitation light. The intensity of the incident light was monitored
using a gold mesh installed in front of the measurement chamber.
UPS spectra were measured at room temperature. The spectral
resolution of UPS spectra was 0.5 eV. The bias voltage of  15.0 V
was applied to samples, in order to avoid the inﬂuence on a weakelectric ﬁeld generated around the sample holder and the analyzer. All the UPS spectra were divided by the photoelectric yield of
the gold mesh. No signiﬁcant effect of surface charging wasobserved.

R

most of them concern the direct 4f–5d excitation, and there is few
data obtained under photoexcitation in the fundamental absorption region of host SrGa2S4. Furthermore, fundamental opticalproperties and electronic structure of SrGa2S4 have not been
investigated in detail. These data are of a great signiﬁcance inunderstanding of relaxation of hot photoelectrons created underhost excitation, and of the interaction of luminescent ionswith them.
In the present study, we have measured excitation spectra for
5d–4f PL bands of Ce3 þ - and Eu2 þ -doped SrGa2S4 crystals,
respectively, with use of synchrotron radiation. The reﬂectivity andabsorption spectra of SrGa2S4 crystals were also measured todetermine basic optical constant spectra by a Kramers–Kronig (KK)
analysis and to correct the inﬂuence of reﬂection loss at the crystal
surface on excitation spectra. The electronic structure calculationof SrGa2S4 was performed by a relativistic DV-Xα method. Theenergy level structure and partial density of states (PDOS) enable
better understanding of optical spectra. Furthermore, ultraviolet
photoelectron spectroscopy (UPS) spectra were measured at various excitation energies, to obtain the knowledge on the process of
MEEs in SrGa2S4. The excitation processes of Ce3 þ and Eu2 þ ions
by VUV photons are clariﬁed from comparisons of experimentresults and calculation. The relaxation processes of photoholes arealso brieﬂy discussed using a simpliﬁed energy band diagram.

INTENSITY (arb. units)

244

2Eg

5
10
15
20
PHOTON ENERGY (eV)

2+

×5

25

Fig. 1. (a): reﬂectivity (blue line) and absorption-edge (red line) spectra of an
undoped SrGa2S4 crystal. (b) and (c): excitation spectra (blue line) for 5d–4f PL
bands of Ce3 þ and Eu2 þ ions doped in SrGa2S4 crystals, respectively. The excitation
spectra in the 10–25 eV range (green line) were magniﬁed by a factor of 5. These
spectra were obtained at room temperature. Eg and E0 are the bad-gap energies andthe onsets of the 4f–5d absorption of Ce3 þ and Eu2 þ ions in SrGa2S4, respectively.
(For interpretation of the references to color in this ﬁgure legend, the reader isreferred to the web version of this article.)M. Kitaura et al. / Journal of Luminescence 172 (2016) 243–248

1.0 and 2.0 mol%, respectively. These data were measured at room
temperature. In Fig. 1(b), the intensity of 2.79 eV band is highunder photoexcitation in the energy region below 4.1 eV, theenergy of which corresponds to the fundamental absorption edge.
The intensity is drastically decreased with increasing photon
energy, and is kept low under photoexcitation in the fundamental
absorption region. A dip appears at 13.4 eV, where the reﬂectivity
spectrum also exhibits a peak. These results suggest that nonradiative decay of electron–hole pairs occurs near the crystal
surface under excitation with photons of high reﬂectivity. Magnifying the excitation spectrum in the 10–25 eV range, we notice a
gentle increase in intensity above 16.5 eV. Except the energy
region below 4.1 eV, the spectral feature mentioned here is almost
the same as the excitation spectrum for 2.33 eV band of Eu2 þ doped SrGa2S4 (Fig. 1(c)).
We have obtained the spectra of optical constants from thereﬂectivity spectrum of Fig. 1(a) by the KK analysis. The KK analysis requires the reﬂectivity over the whole energy range from0 to 1. Since the actual reﬂectivity measurement is restricted to aﬁnite energy range, the reﬂectivity was set to be constant below
1.5 eV, and was extrapolated using a function R(E)¼R(30 eV)  (30/
E)p above 30 eV, where p is an adjustable parameter. The parameter p was adjusted to be 2.57, so that the value of ε2 is zero at
the fundamental absorption edge. The spectra of the complex
dielectric constant ε ¼ ε1 þ ε2, effective number of electrons permolecule Neff, and electron energy loss function –Im(1/ε) are
shown in Fig. 2(a)–(c), respectively. The value of Neff is given by thefollowing equation [19]:Z E2m
E0 ϵ2 ðE0 ÞdE0 ;ð1ÞN eff ¼
2
N m h e2 0
where Nm is the number of molecules per unit volume, e and m are
the charge and mass of electrons, and h is the Planck constant. In
Fig. 2(a), the value of ε1 is the highest at 4.53 eV, and is negative in245

the 12.8–15.6 eV range. The value of ε2 reaches the maximum at
6.1 eV. In Fig. 2(b), the value of Neff is monotonically increased
with photon energy. In Fig. 2(c), the  Im(1/ε) spectrum exhibits a
broad band around 18 eV.
In order to obtain information on relaxation processes of hot
photoelectrons in host SrGa2S4, we measured UPS spectra of
SrGa2S4 thin-ﬁlms at room temperature under the excitation with
12.0–19.5 eV photons. The results are shown in Fig. 3. They mainly
consist of two parts: high- and low-energy ones. As the photon
energy is increased from 14 to 19 eV, high-energy part shifts tohigher kinetic energy side, and is gradually weakened. In contrast,
low-energy part stays at the kinetic energy smaller than 2.5 eV,
and is slightly strengthened. The high-energy part is due to thephotoelectron emission from the valence band. The low-energy
part results not only from the photoelectron emission from thevalence band, but also from the secondary electron emission. The
secondary electron emission is regarded as an indication of MEEs
in SrGa2S4.
The electron afﬁnity χ is also one of the band parameterscharacterizing material properties. This value is determined frommeasurements of UPS spectra, and is calculated using the following equation [20],χ ¼ hν  Etk  Eg ;

ð2Þwhere hν is the energy of exciting photons, and Ek is the highest
energy threshold for the kinetic energy of photoelectrons. The
difference between hν and Ekt, namely, ionization potential, iscalculated to be 5.0 eV from Fig. 3. Considering the value of Eg to
be 4.8 eV from the reﬂectivity spectrum of Fig. 1(a), the value of χ
is estimated to be 0.2 eV in SrGa2S4.
The photon-energy dependence of the peak intensities of highand low-energy parts in Fig. 3 is critical to clarify the secondary
electron emission in the process of MEEs. However, it is not easy todetermine the peak intensity of low-energy part, especially under
excitation below 16 eV, because it strongly overlaps with the highenergy part which is dominant in UPS spectra of Fig. 3. Thus, thecontribution of the high-energy part to the low-energy part has tot

INTENSITY (arb. units)

SrGa2S4hν=14 eV

0

15 eV

0

16 eV

0

17 eV

0
0
0
0
Fig. 2. (a): real part ε1 (black line) and imaginary part ε2 (red line) of the dielectric
constant derived from the KK analysis. (b): the effective number of electrons permolecule Neff. (c): electron energy loss function  Im(1/ε). (For interpretation of thereferences to color in this ﬁgure legend, the reader is referred to the web version ofthis article.)

18 eV
19 eV
5
10
15
KINETIC ENERGY (eV)Fig. 3. Ultraviolet photoelectron spectroscopy (UPS) spectra of a SrGa2S4 thin-ﬁlm
on gold coated quartz substrate, measured at room temperature under excitation
with VUV photons in the 14–19 eV range. The energy was referred to the top of thevalence band. Vertical bars indicate the highest energy thresholds (Ekt) for the
kinetic energy of photoelectrons.

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M. Kitaura et al. / Journal of Luminescence 172 (2016) 243–248

Fig. 4. Photon energy dependence of the intensity ratio of the high- and lowenergy parts IL/IH, which were determined from the analysis of UPS spectra. The redline was drawn for eye guides.

be evaluated from Fig. 3. To do so, the peak intensity ratio of thehigh- and low-energy parts IL/IH was obtained in the present
study. Fig. 4 represents the photon-energy dependence of IL/IH. We
can recognize that the value of IL/IH is almost constant in the lowenergy region, but exhibits a remarkable increase in the region
above 16.5 eV.

4. Electronic structure by DV-Xα calculation
No electronic structure calculation is available for SrGa2S4. We
have performed a molecular orbital (MO) calculation on thiscrystal by a relativistic DV-Xα method. The calculation procedures
have been described in literatures [21,22]. This method is one ofcluster approaches which are beneﬁcial not only to the analysis ofelectronic structures of crystalline solids but also to the predictionof various material properties. The crystal structure of SrGa2S4 is
orthorhombic, and the space group is Fddd [23]. In this calculation,a [Sr9Ga12S64]74  molecular ion was picked up from the SrGa2S4
crystal lattice, and was used as a model cluster. A Sr2 þ ion locatesat the center of this cluster. The point symmetry is D2. The basic
functions used were 1s–5p orbitals for Sr, 1s–4p for Ga, and 1s–3p
for S. The charge of the model cluster was canceled out by considering the Madelung potential generated by point charges
outside it.
The energy level diagram and partial density of states (PDOS)
for the [Sr9Ga12S64]74  cluster are shown in Fig. 5. The band-gap
energy Eg and valence band width ΔEV were determined to be
4.58 and 6.56 eV, respectively. Both are supposed to be reasonable,compared to the absorption edge spectrum (Fig. 1(a)) and UPS
spectra (Fig. 3), respectively. Seeing PDOS of each MO level, we
notice that the valence band is dominated by the S-3p state with
Ga-4s and -4p character. The bottom of the conduction band ismade up of Ga-4s and S-3p states. The PDOS of the Sr-4d state
locating at the higher position is large, compared with those of Ga4s and S-3p states. Core bands of Sr-4p, Ga-3d, and S-3s are seen
below the valence band. This feature is consistent with the literature on X-ray photoelectron spectroscopy (XPS) [24]. However,the calculated binding energies for core levels are apparently
underestimated, compared to the XPS peaks. This is because
orbital relaxation induced by the creation of core holes is not taken
into consideration in our calculation.

Fig. 5. Energy level diagram and partial density of states (PDOS) per molecule
calculated for the embedded [Sr9Ga12S64]74  cluster. Solid and broken lines in theenergy level diagram represent occupied and unoccupied states, respectively. Theenergy distribution curves of PDOS are shown for the atomic orbitals of Sr, Ga, and
S. The top of the VB was taken as zero energy.

5. Discussion
We ﬁrst discuss optical properties of SrGa2S4 by referring to theelectronic structure. In Fig. 1(a), a weak peak is observed at
4.72 eV, which is naturally assigned to the electronic transition
from the top of the valence band of S-3p state to the bottom of the
conduction band of Ga-4s state. This peak would be connected tothe creation of excitons, but it is not so sharp compared to thelowest absorption peaks in halide crystals. We suppose that theeffect of the electron–hole interaction, i.e., exciton effect, is weakin SrGa2S4. The excitons in this crystal may be thermally decomposed into an electron–hole pair at room temperature, because of
its small binding energy ( 0.08 eV). The most intense peak locatesat around 6.24 eV. As seen in Fig. 5, the PDOS of S-3p and Sr-4d
peaks are highest among valence and conduction band peaks,
respectively. It is thus reasonable to assign the 6.24 eV peak to theinterband transition from the valence band of S-3p state to thelower conduction band of Sr-4d state. The broad peak around
13 eV is likely assigned to the interband transition from thevalence band to the higher conduction band of Sr-5p state. Twoweak peaks appear at 21.72 and 22.74 eV, being nearly close to thesum of the band-gap energy and the Ga-3d core level energy. Theenergy separation between them is larger than the Ga-3d spin–
orbit splitting energy (  0.4 eV). From these facts, the two peaks
would be due to the superposition of Ga-3d core peaks at criticalpoints [25].
In Fig. 2(b), the value of Neff is 24 at 12.5 eV. This value is thesame as the number of valence electrons per SrGa2S4 molecule,calculated from the sum of 3p electrons of four S2  ions. From
Fig. 5, the sum of Eg and ΔEV is estimated to be 11.14 eV. The
photon of 12.5 eV has the energy enough to put up an electronfrom the bottom of the valence band to the lower conduction
band. All the valence electrons are thus exhausted under photoexcitation with photons above 12.5 eV.
As seen in Fig. 2(a), the value of ε1 is increased in the 13.5–
20 eV range, while the value of ε2 is decreased. This feature suggests that excited electrons of SrGa2S4 respond as if they are freeelectrons. Optical excitation in this range carries electrons to the S3s and -3p states in the conduction band. The resulting conduction
electrons will cause collective oscillations due to the effect of theelectric ﬁeld of photons, leading to the formation of bulk plasmons. The formation energy Ep of a bulk plasmon is generally

M. Kitaura et al. / Journal of Luminescence 172 (2016) 243–248

Fig. 6. Schematic model on relaxation processes of a hot photoelectron (a) and aphotohole (b) in a simpliﬁed energy band diagram. The energies of band-gap Eg andvalence band width ΔEV were based on the calculated energy level structure of
Fig. 5. Solid arrows indicate photoexcitation from the valence band (VB) to the
conduction band (CB). Broken arrows depict nonradiative Auger transitions.

given by the following equation [19];sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
2
N e h e2
;
Ep ¼
πm

ð3Þ

where Ne is the number of conduction electrons per unit volume.
From Eq. (3), we obtain Ep ¼16.4 eV. The value of Ep is within theprominent peak at 18 eV in Fig. 2(c). It is, therefore, likely toconnect the 18 eV peak to the bulk plasmon formation.
Let us discuss the process of MEEs in SrGa2S4 caused by VUVexcitation. In Fig. 4, one can see a remarkable increase of IL/IH at
around 16.5 eV. Optical excitation at around 16.5 eV leads to theelectronic transitions from the valence band, which enables tocreate hot photoelectrons with the kinetic energies larger than Eg.
In this case, the hot photoelectrons can kick up valence electrons
to the conduction band through inelastic scattering. Thus, theincrease of IL/IH is interpreted as an indication of MEEs, due toinelastic scattering of hot photoelectrons with valence electrons.
The threshold Eth for MEEs is estimated to be 16.5 eV, which ismore than twice of Eg. This result is qualitatively explained byusing a simpliﬁed energy band diagram with direct band-gap.
Fig. 6(a) and (b) shows a schematic model on relaxation processesof a hot photoelectron and a photohole, respectively. The energies
of Eg and ΔEV are referred to the calculated energy level structureof Fig. 5. When a photon with the energy of hν excites a valence
electron at the wave vector K, a hot photoelectron and a photohole
are both created at the wave vector K. The former has the kinetic
energy Eke, and the latter has the kinetic energy Ekh. When the
momentum-conservation law is satisﬁed in the process of impact
excitation, hot photoelectron with K would kick up a valence
electron with  K to the conduction band bottom at K ¼0 (Fig. 6(a)). The valence hole thus created at  K has the same kinetic
energy as the photohole at K. Since this impact process completes
in a very short time, hot photoelectrons do not have any time tointeract with phonons. Furthermore, the energy-conservation law
requires Eke ¼Ekh þEg. Since the photon energy hν required tocause MEEs is given by the sum of Eke, Ekh and Eg, we get
Eth ¼2Eg þ2Ekh. It is, therefore, reasonable to expect that the valueof Eth exceeds the twice of Eg. As described above, the simpliﬁed
energy band diagram is well adopted in a phenomenologicalexplanation of MEEs in SrGa2S4; however, it has applicable limits,
especially for the quantitative analysis of Eth. In that case, moreadvanced theoretical approach based on the multiple-parabolicbranch band model [26] would be worthy of note.

247

The condition ΔEV 4 Eg may imply the possibility that photoholes also participate in the process of MEEs [27]. As show in Fig. 6(b), the hot photohole with  K has a chance to kick up a K ¼0
valence electron situating at the top of the valence band. In suchan excitation process, however, there is no ﬁnal state for the K ¼0
valence electron. Since the momentum-conservation law is notsatisﬁed in the relaxation process of photoholes, they cannot relax
quickly through the Auger process. Consequently, photoholes willlose their kinetic energies through the interaction with phonons,
to relax to the valence band top. The energy loss resulting fromrelaxation of photoholes, so-called hole energy losses, has been
observed in wide-gap materials with the condition of ΔEV 4Eg[28]. Details of hole energy losses still remain invisible. Further
experimental and theoretical investigations are desired to solve
this fundamental problem.
Finally, we consider the excitation process of Ce3 þ and Eu2 þions by VUV photons. As shown in Fig. 1(b) and (c), intensities of
5d–4f PL bands of Ce3 þ and Eu2 þ ions are remarkably low in thefundamental absorption region, compared to those in the 4f–5d
absorption region. This fact is probably due to the difference inpenetration depth of excitation photons. UV–VUV photons cannot
penetrate toward the inside of crystal, because the absorption
coefﬁcient is much high in the fundamental absorption region. If
created electron–hole (e–h) pairs cannot migrate in a long distance, very small amounts of Ce3 þ and Eu2 þ ions will be excitednear the surface layers, which leads to the low efﬁciency of hostsensitized Ce3 þ and Eu2 þ PL.
A gentle increase in intensity is found around 16.5 eV in the
excitation spectra of Fig. 1(b) and (c). This is apparently connectedto the MEEs in SrGa2S4, because it agrees with the thresholdenergy for MEEs. The intensities at 22 eV are two times larger thanthose at 12 eV where a photon creates one e–h pair. This may
indicate that two e–h pairs are formed after relaxation of hot
photoelectrons, resulting in the twofold increase in intensity, andis compatible with our assignment that the process of MEEs iscaused by the inelastic scattering of hot photoelectrons withvalence electrons.
It is noted that the excitation spectra for 5d–4f PL bands of
Ce3 þ and Eu2 þ ions are almost the same with each other. Thissuggests that Ce3 þ and Eu2 þ ions are not directly excited by hot
photoelectrons, in contrast with s2 type metal ions in alkali halides[4,5] and Er3 þ ion in LiYF4 [8]. Fig. 1(b) and (c) shows Eth 4 E0 þ Eg,
where E0 is the lowest threshold energies for 4f–5d absorption of
Ce3 þ and Eu2 þ ions. This fact strongly supports that no direct
excitation of 4f electrons in Ce3 þ and Eu2 þ ions by hot photoelectrons occurs in SrGa2S4. Furthermore, no intrinsic PL band isfound in SrGa2S4, indicating that the nonradiative resonant energy
transfer of Dexter type is impossible for Ce3 þ and Eu2 þ PL bands[29]. We believe that Ce3 þ and Eu2 þ ions will be converted to
Ce4 þ and Eu3 þ ions, respectively, by capturing valence holes created under the MEE process. The resulting Ce4 þ and Eu3 þ ionsform the excited states of Ce3 þ and Eu2 þ ions through therecombination with conduction electrons. The excited 5d states of
Ce3 þ and Eu2 þ ions relax to the ground states, giving rise to 5d–4f
PL bands in these ions.

6. Summary
The excitation processes of Ce3 þ and Eu2 þ ions doped in
SrGa2S4 crystals have been investigated in the energy region up to
25 eV through the measurements of excitation spectra for 5d–4f PL
bands. A gentle increase in intensity was commonly observed ataround 16.5 eV in both excitation spectra. This feature wasattributed to the MEEs of SrGa2S4. No direct excitation process of
Ce3 þ and Eu2 þ ions by hot photoelectrons could be seen. The

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M. Kitaura et al. / Journal of Luminescence 172 (2016) 243–248

electronic structure of SrGa2S4 has also been studied through themeasurement of the reﬂectivity spectrum and the theoretical
calculation by a relativistic DV-Xα method. It was clariﬁed that thevalence band is mainly of S-2p states, and the conduction band iscomposed of Ga-3s and Sr-4d mixed states. The band-gap energy
was calculated to be 4.58 eV, being narrower than the valenceband by 1.98 eV. The process of MEEs has been investigated bymeasuring UPS spectra. It was suggested that the MEEs is causedby the inelastic scattering of hot photoelectrons with valence
electrons.
If the condition ΔEV 4Eg is really satisﬁed in SrGa2S4, it isexpected that photoholes also participate in the process of MEEs.
The present experiment, however, did not provide positive evidence for this expectation. The existence of photoholes in theprocess of MEEs is very interesting from the viewpoint of fundamentals in solid-state physics and material science. The relaxationof hot photocarriers created by VUV radiation should be deeply
investigated using time-resolved UPS technique combined with SR
and femtosecond laser.

Acknowledgments
The present study was partially supported by the Cooperative
Research Project of the Research Institute of Electronics, Shizuoka
University. This study was performed at the UVSOR facility under
the Joint Study Program of the Institute for Molecular Science,
Okazaki.

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