PHOTON ENERGY CONVERSION
OF IRFEMTOSECONDLASERPULSES INTO X-RAYPULSES
USING ELECTROLYTE AQUEOUS SOLUTIONS IN AIR
Koji Hatanaka*, Toshifumi Miura, Hiroshi Ono, and Hiroshi Fukumura*
Department of Chemistry, Graduate School of Science,
Tohoku University, Sendai 980-8578, Japan
Abstract. Hard x-ray pulses were generated from aqueous solutions of electrolyte
such as CsClaq and RbClaq by the irradiation of femtosecond infrared laser pulses in
air. X-ray photon energy extended to 60 keV with the laser intensity of 0.6 mJ/pulse.
Possible conversion mechanisms were discussed based on the results of x-ray emission
spectra on laser intensity and solute concentration. A development of x-ray
diffractometer utilizing a palm-top x-ray pulse source was also introduced.
INTRODUCTION
There is now a growing interest in interactions between intense laser fields
and matters since they involve unsolved questions in physics and chemistry such as xray pulse generation. Most of studies, however, have chosen metals and gases as
targets for the subject [1,2]. Reports on solutions, on the other hand, have been
limited to a few such as carbon fluoride [3], copper nitrate aqueous solution [4], and
others. We consider that a solution can be an appropriate target since mechanisms of
the x-ray generation can be discussed by changing solute compounds, their
combinations, and their concentrations, which is different from the case of metal
targets. For practical applications, solution targets can be circulated by a pump and
recycled, so that high stability is expected. Furthermore, x-ray pulse width may be
controllable by changing solute concentrations since the lifetime of high energy
electrons might be short in highly-concentrated solutions [5]. In this proceeding, xray pulse generation from electrolyte aqueous solutions irradiated by focused fs laser
pulses in air is described. Additionally, a development of x-ray diffractometer with a
palm-top x-ray pulse source is also introduced.
*e-mails:hatanaka@orgphys.chem.tohoku.ac.jp,fukumura@ orgphys.chem.tohoku.ac.jp
CP634, Science of Superstrong Field Interactions, edited by K. Nakajima and M. Deguchi
© 2002 American Institute of Physics 0-7354-0089-X/02/$ 19.00
260
X-RAY PULSE GENERATION FROM AQUEOUS SOLUTIONS
Experimental
Figure 1 shows a top view of the experimental setup [6-8]. Distilled water
or a highly concentrated alkali metal (Cs or Rb) chloride (> 98 %, Aldrich) aqueous
solution was circulated by a pump through a flat glass nozzle of which the inner gap
was about 100 jim. Those solutions are transparent to the excitation laser light at 775
nm. Refractive indexes of the solutions range from 1.333 to 1.383 depending on
solute concentration and alkali metal species. Femtosecond laser pulses (Clark MXR,
CPA2001, 775 nm, 130 fs, 1 kHz) were focused by an objective lens (Mitsutoyo, M
Plan Apo 10X, NA= 0.28) onto a solution jet surface from the nozzle with an incident
angle ~ 45 degrees to the jet surface (S polarization). Laser focus waist was
estimated to be ~ 17 |im from the side view of the plasma, which is shown in Figure 1
(b). Therefore, the laser power at the focus can be calculated to be ~ 2PW/cm2 when
the laser intensity is 0.6 mJ/cm2. X-ray emission spectra were measured with a high
purity Ge solid state detector (Ge SSD, EG & G Ortec, GLP-25440-S) with a 250-jimihick Be window. Signals from the Ge SSD were processed by a multichannel
analyzer (MCA/PC98B, Laboratory Equipment Corp.). All experiments were
performed under atmospheric pressure at 294 K.
fs laser pulses
OL\
solution jet
FIGURE 1. (a) A top view of experimental setup for x-ray pulse generation with
solution jets and x-ray emission spectroscopy. OL: an objective lens (NA = 0.28).
Pb: a lead plate (1 mm thick) with an aperture (1 mm diameter), (b) A visible side
image of laser focus.
261
Results and Discussions
Figure 2 shows x-ray emission spectra from jets of distilled water, a CsCl
aqueous solution (6.5 mol/dm3), and a RbCl aqueous solution (6.0 mol/dm3). The
excitation laser intensity was 0.58 mJ/pulse. The spectra were unconnected by x-ray
absorption of air, the Be window, and the Ge absorbing layer (300 nm). X-ray
emission intensities were normalized with their maxima. In the case of distilled water,
a broad spectrum was observed with a tail up to ~ 15 keV. The intensity degradation
in the lower energy region was due to the absorption effect. In the case of a CsCl
aqueous solution, a similar broad spectrum with a gentler slope was observed up to ~
40 keV. Sharp x-ray lines were also observed clearly which were assigned to Ka
(30.968 keV), K$ (34.960 keV), and L(i (4.619 keV, 4.935 keV) characteristic x-ray
lines of Cs. The observation of these high-energy K lines certifies that the
measurement condition is free from the pile-up effect, which is characteristic to solid
state detectors. Similarly, in the case of a RbCl aqueous solution, a broad spectrum
was observed with K characteristic x-ray lines of Rb (13.373 keV and 14.956 keV).
Characteristic x-ray lines of Cl (Ka = 2.6 keV, K$ = 2.8 keV) are out of the detection
range. The depression observed commonly in all the spectra at ~ 11 keV are due to
the Ge K absorption edge (11.1 keV). Energy conversion efficiency of laser pulse to
x-ray pulse in the range 3-60 keV, in the case of a CsCl solution (6.5 mol/dm3), was
calculated to be ~ 10"8 under the assumption that x-ray radiation was sphericallyhomogeneous.
—— CsCl aq (6.5moydni)
—— RbCl aq (6.0mol/dirf)
—— distilled water
0
20
40
photon energy/keV
FIGURE 2. X-ray emission spectra of distilled water and alkali metal (Cs or Rb)
chloride aqueous solution irradiated by focused femtosecond laser pulses (775 nm,.
130 fs, 0.58 mJ/pulse, 1 kHz). X-ray emission intensities are normalized by their
maxima.
262
Figure 3 shows laser intensity-dependent x-ray emission spectra of a CsCl
solution (6.5 mol/dm3) with a spectrum of distilled water irradiated by 0.58 mJ/pulse
laser pulses. The spectra here are corrected by considering the absorption effect of air,
the Be window, and the Ge absorbing layer. As the laser intensity increased, x-ray
emission intensity increased, the slopes of broad spectral components became less
steep, and the x-ray cut-off energy extended to higher energy region. The slopes can
be analyzed quantitatively by using an equation, 7X(£) = exp (-E/T) x const., where 7X,
£, and T represents x-ray intensity, x-ray energy, and the slope parameter, respectively.
10zh
\ %
CsCl aq (6.5 mol/dmO
^— 0.58 mJ/pulse
—— 0.28 mJ/pulse
—— O.lOmJ/pulse
» — distilled water
(0.58 mJ/pulse)
"~^-^_ ,
iou 60
20
40
photon energy, EI keV
FIGURE 3. X-ray emission spectra of CsCl aqueous solution (6.5 mol/dm3) with
different laser intensities and an x-ray emission spectrum of distilled water with 0.58
mJ/pulse laser intensity. The spectra are corrected by absorption effect of air, Be
input window of Ge SSD, and Ge absorbing layer.
8
(a) CsCl aq, 6.5 mol/dm3
%
•
•
•
0
water
.
p.4f T/ ,
laser intensity / mJ/pulse
0.53 mJ/pulse
t
:.:.-,- . , ;
•
(b)
4 - water
!
f'
0.8 "
•
n
0.45 ml/pulse
0
4
solute cone. / mol/dm3
8
FIGURE 4. Electron temperatures as functions of excitation laser intensity (a) and
solute concentration (b). Open circle represents distilled water irradiated by 0.53
mJ/pulse fs laser pulses.
263
Figure 4 shows the slope parameter, 7, as functions of laser intensity (a) and solute
concentration (b). The value of T increased gradually as the laser intensity increases.
When the laser intensity exceeds ~ 0.4 mJ/pulse, the increasing slope changes
suddenly to a steeper slope. On the other hand, the value of T saturates when the
solute concentration increases. Furthermore, when the solute concentration is ~
lmol/dm3, T is almost the same with distilled water though the irradiating laser
intensity is the same.
The initial ionization is induced mainly through tunneling ionization under 2
2
PW/cm irradiation condition from the theory by Keldysh [9]. After ionized,
conductive electrons are accelerated by the intense laser field through mechanisms
such as inverse bremsstrahlung, stimulated Raman scattering, ponderomotive potential
[10]. In this study, however, the ponderomotive potential of laser field is calculated
to be only 110 eV, which is much lower than the electron temperature obtained from
the slope in Figure 4. Thus we have to invoke another mechanisms such as inverse
bremsstrahlung and/or stimulated Raman scattering to explain the observed high
electron temperature. The sudden rise of T (Figure 4 (a)) can be due to stimulated
Raman scattering and/or multiple ionization of secondary electrons.
The
confirmation of the mechanism details is now under consideration.
Broad X-ray emission spectra can be a result of recombination between
electron and ionic species and/or bremsstrahlung. Comparing the X-ray emission
spectrum of CsCl aqueous solution with that of distilled water (Figure 3), we found
that X-ray intensity was higher in CsCl solution than in distilled water. This X-ray
intensity enhancement by adding electrolyte with high atomic number elements is
reasonable because recombination or scattering cross section is much larger in Cs+ or
Cl" than in H2O. On the other hand, such effect of electrolyte can be observable only
when the electrolyte concentration is more than 1 mol/dm3, which is much dense from
the conventional chemistry sense (The distance between Cs ions is about 1 nm when
the concentration is 1 mol/dm3.). Solute concentration may be effective to all the
mechanisms such as ionization, conductive electron acceleration, and scattering and
recombination for x-ray generation. Although it is difficult to clarify the mechanism
on solute concentration effect at present, this is the first demonstration of solute
concentration effect on x-ray pulse generation.
264
X-RAY DIFFRACTION WITH X-RAY PULSES
Experimental
Figure 5 (a) shows an experimental setup for x-ray diffraction with our x-ray
pulse source. IR femtosecond laser pulses (775 nm, 130 fs, 0.6 mJ/pulse, 1 kHz)
were focused fully by the same objective lens onto a circulated Fe2O3 tape target, then
x-ray pulses were generated. An x-ray emission spectrum from the Fe2O3 target was
measured by the Ge SSD, which is shown in Figure 5 (b) where two peaks are
observed clearly in addition to low-intensity broad x-ray. Those peaks are assigned
to Fe Ka (0.194 nm) and Kfo (0.176 nm) lines. X-ray output through aPb collimator
(the inner diameter = 0.8 mm) was used as a probe. A highly-oriented pyrolytic
graphite substrate (NT-MDT, ZYH, HOPG) was chosen as a sample for x-ray
diffraction. The lattice constant of the graphite plane (002) is 0.335 nm. From these
values, the diffraction angle (29) can be calculated to be about 33 degrees for Fe Ka.
Images of transmitted and diffracted x-ray from the HOPG were converted to visible
images by an x-ray image intensifier (Hamamatsu Photonics, K. K., V7739P) and
captured by a cooled CCD camera (Andor, DV434BV).
CCB
(a)
objective $&&$ (tii) LJ
1
2
wavelength / angstrom
3
FIGURE 5. (a) An experimental setup for x-ray diffraction with x-ray pulses from a
Fe2O3 tape target, (b) An x-ray emission spectrum from a Fe2O3 tape target irradiated
by focused femtosecond laser pulses (775 nm, 130 fs, 0.6 mJ/pulse, 1 kHz).
265
Results and Discussions
A result is shown in Figure 6 (a), where the accumulation time was only 2 min.
which corresponded to 1.2 x 105 shots. When the 29 is on the Bragg angle, diffracted
x-ray spots were clearly observed beside the transmitted x-ray. X-ray intensity
surface plots are shown in Figure 6 (b). Diffracted x-rays of Fe Ka and Fe K$ are
clearly resolved. There is little difference in signal to noise ratios between the two
accumulation times of 2 min. and 60 min. This means that time-resolved x-ray
diffraction can be performed even with commercial-base laser systems in conventional
laboratories. This result encourages us to proceed experiments of fs-laser-pump and
x-ray-pulse-probe, which will be performed soon especially under laser ablation
condition [11].
(b)
700
accumulation time
— 2 min.
— 60 min.
800
channel
FIGURE 6. (a) An x-ray image of transmission and diffraction,
patterns obtained by 2 min. and 60 min. accumulation times.
900
(b) Diffraction
ACKNOWLEDGMENTS
The present work was supported by a Grant-in-Aid from the Ministry of
Education, Science, and Culture of Japan (11355035 and 12750004). The authors are
thankful to Professor Y. Udagawa at IMRAM, Tohoku University for the use of the Ge
SSD, to Associate Professor T. Sekine at Department of Chemistry, Tohoku University
for the use of a high voltage supplier and an amplifier, and to Andor Technology for
the use of the cooled CCD camera. The authors are also thankful to Dr. Yosuke
Watanabe at IMR for the great contribution on the development of x-ray
diffractometer.
266
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