Exotic Behavior of Molecules and Clusters in
Intense Laser Light Fields
Kaoru Yamanouchi
Department of Chemistry, School of Science, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
e-mail: kaoru@chem.s.u-tokyo.ac.jp
Abstract. In order to investigate dynamics of molecules and clusters in intense laser fields, new
experimental approaches such as mass-resolved momentum imaging, tandem-type time-of-flight
spectrometry, pulsed gas electron diffraction, and coincidence momentum imaging have been
developed. From a series of studies using these newly developed techniques, it has been found
that mixing among the electronic structures of a molecule induced by intense laser fields plays a
crucial role in determining the fate of molecules after ionization.
INTRODUCTION
From recent pioneering studies, it has been revealed that molecules behave in a
very characteristic way in intense laser fields (10n~1016 W/cm2) [1], i.e. they
significant structural deformation and ionization proceeds almost simultaneously, and
the resulting multiply charged molecular ions decompose into atomic fragment ions
having a large momentum. The process of this abrupt bond fission generating
fragment ions with large kinetic energies is called Coulomb explosion. We focused
our attention on the observation that such fragment ions are preferentially ejected
along the laser polarization direction, and proposed a new method called massresolved momentum imaging (MRMI) [2-4] as a method to evaluate quantitatively the
extent of the anisotropy of the fragment ejection. For example, the observed MRMI
map for N2+ ions ejected from N2Z+ (z = 3 - 5) is shown in Fig. 1, from which the
existence of the three major explosion pathways contributing to the formation of N2+
was clarified.
By the MRMI method, the responses of small polyatomic molecules such as N2,
NO, CO2, NO2 and F^O to intense laser fields were investigated systematically [5-7],
and it was found that extensive deformation of their skeletal structure commonly
occurs. This characteristic structural deformation of molecules in intense laser fields
was interpreted as a result of the formation of so-called dressed states (Fig. 2) in which
molecular electronic states are mixed through strong couplings with intense laser
fields. The investigation of this type of mixed states of molecules and light fields is
considered to be a first step for the realization of control of molecular dynamics in
intense laser fields. Because the shape of the potential energy surfaces of the dressed
states is varied in response to the temporal change of the magnitudes of the light fields,
CP634, Science of Super strong Field Inter actions, edited by K. Nakajima and M. Deguchi
© 2002 American Institute of Physics 0-7354-0089-X/02/$ 19.00
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the nuclear dynamics on the surfaces drawn schematically as an entangled arrow
should be governed by the intensity of the light field as well as its wavelength.
iii
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FIGURE 1. MRMI 2D map and 3D plot for N2+ ions ejected from N2Z+ (z = 3 - 5). An arrow indicates
the direction of the laser polarization.
FIGURE 2. Dressed potential energy surface formed by intense laser fields.
TANDEM-TYPE TOF MEASUREMENTS
Only through the measurements of MRMI maps, it is still uncertain whether the
dressed state formation, determining the fate of the molecules in intense laser fields,
occurs either at a neutral stage, a single-charged stage, or a higher-charged stage. In
intense laser fields, in most cases, electrons within a molecule escape from the
molecular domain one after another through tunneling ionization, resulting in the
formation of multiply charged parent ions. Therefore, it is worthwhile to identify the
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ion stage at which the major light-induced coupling between molecular electronic
states occurs. For this purpose, we developed a tandem-type time-of-flight (TOP)
mass spectrometer (Fig. 3) [8,9], and made it possible to identify the fate-determining
charge stage by irradiating ions having specific charge and mass numbers with intense
short-pulsed laser fields. In the cases of benzene [8] and aniline [9] molecules, it was
found that the dressed-state formation occurring at the singly charged ion stage
governs their dynamics in intense laser fields. Furthermore, it was clarified that
molecular ions are excited instantaneously to the vibrationally highly excited states
through the resonant coupling between a pair of electronic states caused by the shortpulsed intense laser fields. From the investigation of aniline-(NH3)w (n - 1-4) clusters
using the tandem-type TOF spectrometer, the responses of the clusters to the light
fields were found to be largely dependent on the size of the clusters, i.e. the number of
the attached ammonia molecules.
Lmm
FIGURE 3. Schematic diagram of the tandem-type time-of-flight mass spectrometer.
PULSED GAS ELECTRON DIFFRACTION
It would be wonderful if we could pursue in real time the variation of the
geometrical structure of molecules interacting with intense laser fields as well as their
alignment process along the laser polarization direction [10]. In order to achieve such
a real-time probing, we developed a pulsed gas diffraction apparatus (Fig. 4) [11,12]
and attempted to probe molecular responses to intense laser fields directly through an
electron diffraction pattern. The short-pulsed electron beam packets were generated
by irradiating a photo-cathode of a pulsed electron gun with a short-pulsed laser light
and by accelerating the electrons generated through a photoelectric effect to 25 - 40
keV. This pulsed gas electron diffraction method enabled us to record an alignment
process of linear polyatomic molecules as a form of electron diffraction images. When
ultrashort laser pulses with sub-pico second temporal width are employed for the
generation of the electron packets, real time probing of chemical dynamics with ~ 1 ps
resolution could be achieved as a series of snap shots of electron diffraction images.
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FIGURE 4. Schematic diagram of the pulsed gas electron diffraction apparatus.
COINCIDENCE MOMENTUM IMAGING
We also developed a method called coincidence momentum imaging (CMI) to
identify respective events of Coulomb explosion of a single molecule in intense laser
fields [13,14]. By using an apparatus shown in Fig.5, which is equipped with a
position sensitive detector, we were able to measure momentum vectors of all the
fragment ions for respective events of two-body and three-body Coulomb explosion
by the double and triple coincidence measurements, respectively. From the CMI
measurements, correlations among the nuclear motions within a molecule before the
Coulomb explosion were clarified, and furthermore, in the case of the three-body
explosion, it became possible to identify separately two types of three-body explosion,
i.e. (i) a concerted type in which three atomic fragment ions are ejected almost
simultaneously and (ii) a sequential type in which the two-body fragmentation into a
diatomic molecular ion and an atomic ion proceeds first followed by the two-body
fragmentation of the diatomic molecular ion.
Coinicidence
Momentum
Image
FIGURE 5. Schematic diagram of the coincidence momentum imaging apparatus.
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PERSPECTIVE
The recent development of ultrashort pulsed laser technology has afforded us an
invaluable opportunity to investigate essential feature of light-matter interaction
through phenomena occurring in extremely intense light fields. As described in the
present article, when we introduced a new method to investigate molecules and
clusters in intense laser fields, we were always encountered by new phenomena, from
which we were able to get deeper understanding about the roles of light fields. The
sensitive dependence of molecular dynamics on the laser-field intensity as well as on
the wavelength of laser light suggests that photochemical processes could be
controlled by manipulating properly the properties of short-pulsed laser fields, such as
intensity, wavelength, pulse shape, and phase structure.
It is certain that
interdisciplinary cooperative efforts by researchers in the fields of chemistry, physics,
and laser engineering will stimulate further the research on strong-field matter
interaction at the forefront.
ACKNOWLEDGMENTS
The results of our studies introduced in the present article were obtained mainly
through the CREST project supported by 1ST (Japan Science and Technology
Corporation). I would like to thank my colleagues, Dr. K. Someda, Dr. A. Hishikawa,
Dr. K. Hoshina, Dr. A. Iwamae, Dr. M. Kono, Dr. S. Liu, Dr. T. Sako, Dr. Y. Fukuda,
Dr. R. Itakura, and Dr. A. Iwasaki, Dr. H. Hasegawa, Mr. I. Maruyama, Ms. J.
Watanabe for their efforts in making the project fruitful. Finally, I thank Dr. H.
Todokoro, Mr. T. Ohshima, and Mr. Y. Oze for their support in the construction of the
gas electron diffraction apparatus.
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