Laboratory for Laser Chemistry

http://www.sci.osaka-cu.ac.jp/chem/laser

Current Research and Principal Research Interests
Selected Publications

1. Current Research and Principal Research Interests
My current research focuses on high intensity laser chemistry, more specifically, ionizationand fragmentation, and Coulomb explosion of organic molecules induced by femtosecond laserpulses. One of the applications called as Femtosecond Laser Mass Spectrometry(FMLS) would
be a new method of trace detection of dioxins. Five photon absorption followed by thefluorescence has been found recently. Mechanism of femtosecond pulse induced protein
neucleation is started to study. Multiphoton reactions via hot molecules formed by internalconversion using a VUV laser is another subject of interest.
1.1. High-Intensity Laser Chemistry
New research fields have opened related to the interaction between molecules andhigh-intensity optical fields, where the laser intensity ranges from 1012~1016 Wcm-2. For large
molecules, an intact molecular ion with little fragmentation has been found to form in anintensity region around 1014 Wcm-2. The intact ion formation can be used for femtosecond laser
mass spectrometry. In an intensity region of 1015 Wcm-2 and more, electrons are stripped frommolecules by optical field ionization and the highly charged ions reach a Coulomb explosion.
Coulomb explosions of benzene and C60 have been demonstrated and the mechanism can beanalyzed by molecular dynamic simulation .
1.1.1 Intact Molecular Ion and Fragment Ion Formation of Organic Molecules on
Irradiation with an Intense Femtosecond Laser Pulse
Ionization and fragmentation of several tens of organic molecules have been examined in ahigh intensity femtosecond laser field. Phenomenologically, it is clear that femtosecond intenselaser pulses lead to enhanced formation of molecular ions compared with nanosecond laser
excitation, even though the molecules have no electronic absorption at the laser wavelengths.
The predominance of parent cations should be of great advantage in the femtosecond laser mass
spectroscopy (FLMS) of aromatic hydrocarbons.
We have succeeded in generalizing that the resonance between the laser wavelength andthe electronic energy levels of the molecular cations is a key factor in the formation of fragment
ions during femtosecond laser excitation, as shown in Fig.1.
The cation of 1,4-cyclohexadiene is resonant at the excitation wavelength of 800 nm, and thespectra show heavy fragmentation. While the cation of 1,3-cyclohexadiene is not in resonancewith the laser wavelength, parent ions are produced. Excitation pulses are obtained from ahigh-intensity Ti:sapphire femtosecond laser in the intensity region of 1014 Wcm-2 [Chem. Phys.
Lett. 2001.].

Figure 1. Mass spectra of
1,3-cyclohexadiene (1,3-CHD) and
1,4-cyclohexadiene (1,4-CHD) observed at
800 nm with a 130 fs pulse width at 0.7 ×1014 Wcm-2. The molecular ions aredenoted by M+, and the doubly charged
molecular ions by M2+. 1,3-CHD+ is not inresonance with the laser wavelength andresults in a high yield of molecular ion
formation, while 1,4-CHD+ is resonant andresults in fragmentation.

1.1.2 Molecular Ion Formation Affected by Various Pulse Parameters: Excitation
Wavelength, Pulsewidth, and Laser Polarization
The most effective parameter was found to be wavelength related to resonance with thecation absorption. 2,3-Dimethyl-1,3-butadiene cation with weak intensity of the fragment ionswere successfully detected by excitation at the non-resonance wavelength of 1.4 μm. On thecontrary, the resonance wavelength of 0.8 μm produced many fragments, as shown in Fig.2.
Shorter pulse < 30 fs and linear polarization were better to produce molecular ions at 0.8 μm.
The re-scattering by the electron seemed to be minor effect for the fragmentation.

Figure 2. TOF mass spectra of
2,3-dimethyl-1,3-butadiene, by a) 1.4-μmexcitation at an irradiation intensity of 1.6 ×
1014 Wcm-2 and b) 0.8 μm excitation at anirradiation intensity of 1.6 × 1014 Wcm-2.
The molecular ion is indicated by M+. The
asterisk (*) indicates H2O+ signal. c) The
absorption spectrum of DB at a
low-temperature matrix. The two shaded
Gaussian shapes indicate the spectra of theexcitation pulses.

1.1.3 Application of FLMS to Dioxins
More than 20 chlorinated compounds including dioxins have been irradiated to produce
the molecular ions. Some of them are highly toxic and may not be easily detected by theconventional multiphoton ionization method(REMPI). FLMS is viewed as an alternativemethod. Chlorinated dioxins have been irradiated and the molecular ions were observed withreasonably high efficiency with small amount of fragments, one of the examples is shown inFig. 3.

Figure 3. TOF spectra of 1,3,6trichrorodibrnzo-p-dioxin ionized by 130-fs
pulses. The molecular ion intensity, M+,
was the highest. M2+ is the doubly charged
molecular ion.

1.3 Coulomb Explosion of Organic Molecules on Irradiation with an Intense Femtosecond
Laser Pulse
Coulomb explosion has been found to start in diatomic molecules at a critical internuclear
distance larger than Re, while isotropic explosions involving mega-eV particles have been
observed in clusters. C60 and benzene can be classified as an intermediate case between two- tothree-atom molecules and large clusters. Our results show that the explosion is anisotropic andhas no clear critical structure. The irradiation intensity was 1-8 × 1016 Wcm-2 [J. Chem. Phys.
2000, 2002.]. In the case of perfluorobenzene, smooth explosion of the carbon atoms was foundto be largely disturbed, because of the heavy F atoms [Chem. Phys. Lett. 2005.].

1.4 Multiphoton Absorption, Ionization, and Neucleation of Protein in Solution by an
Intense Femtosecond Laser Pulse
High intensity chemistry in solution will be important and interesting. One of the targets is
to clarify the mechanism of the neucleation. Five photon excitation was found to occur forterphenyl with 1.4 μm femtosecond pulse excitation even in solution.
2. VUV Laser Chemistry-Formation of Hot Molecules and their Reactions in the Gas PhaseVUV photochemistry of organic molecules in the gas phase has been studied from a novel
point of view. The highly vibrationally excited state can be formed by efficient internalconversion from an electronically excited state on irradiation with a VUV laser. We call themolecule in the vibrationally excited state a "hot" molecule. The hot molecule has a narrowinternal energy distribution and a high equivalent vibrational temperature up to 6000 K.
Therefore, hot molecules have been good targets to study the specific reaction rate, collisionalrelaxation rate constants, and an intermediate of multiphoton reactions. We have studied
benzenes, alkylbenzenes, olefins, etc, and have written two reviews [J. Phys. Chem. 1989; Bull.
Chem. Soc. 2001.] .
Two-photon chemistry via hot molecules is interesting. We recently have succeeded information of be 1,3-hexadiene-5-yne as a dominant photoproduct of benzene instead of fulvene
for the case of two-photon hot molecule reaction. The scheme is illustrated in Fig. 4 [J.
Photochem. Photobiol. A 2005.] .

Figure 4. Two-photon reaction via hot benzene produce 1,3-hexadiene-5-yne as a dominantphotoproduct. The corresponding vibrational temperature of benzene with two-photon energy of
ArF laser light is evaluated to be as high as 5900 K.

2. Selected Publications
1."Ionization and Fragmentation of some Organic Molecules with Intense Femtosecond Laser
Pulses"，N. Nakashima, T. Yatsuhashi, M. Murakami, R. Mizoguchi, and Y, Shimada, inAdvanced in Multi-photon Processes and Spectroscopy, the forthcoming volume, edited by S. H.
Lin, A. A. Villaeys, and Y. Fujimura (World Scientific, Singapore), in press.
2."Ionization and Fragmentation of Anthracene with an Intense Femtosecond Laser Pulse at 1.4

μm", M. Murakami, R. Mizoguchi, Y. Shimada, T. Yatsuhashi, N. Nakashima,Chem. Phys. Lett.
403, 238-241 (2005).
3. "A Key Factor in Parent and Fragment Ion Formation on Irradiation with an Intense
Femtosecond Laser Pulse", H. Harada, S. Shimizu, T. Yatsuhashi, S. Sakabe, Y. Izawa, N.
Nakashima, Chem. Phys. Lett. 342, 563-570 (2001).
4."Anisotropic Coulomb Explosion of C60 Irradiated with a High-intensity Femtosecond LaserPulse", J. Kou, V. Zhakhovskii, S. Sakabe, K. Nishihara, S. Shimizu, S. Kawato, M. Hashida, K.
Shimuzu, S. Bulanov, Y. Izawa, Y. Kato, N. Nakashima, J. Chem. Phys. 112, 5012-5020 (2000).
5."Multiphoton Reduction of Eu3+ to Eu2+ in Methanol using Intense, Short Pulses from aTi:sapphire Laser", N. Nakashima, S. Nakamura, S. Sakabe, H. Schillinger, Y. Hamanaka, C.
Yamanaka, M. Kusaba, N. Ishihara, Y. Izawa, J. Phys. Chem. 103, 3910-3916 (1999).
6. "Role of Hot Molecules Formed by Internal Conversion in UV Single-Photon andMultiphoton Chemistry", (Feature Article) N. Nakashima, K. Yoshihara, J. Phys. Chem. 93,7763-7771 (1989).
7. "Laser Flash Photolysis of Benzene. VIII. Formation of Hot Benzene from the S2 State andits Collisional Deactivation", N. Nakashima, Keitaro Yoshihara, J. Chem. Phys. 79, 2727-2735(1983).
8. "Nanosecond Laser Photolysis of Benzene Monomer and Excimer", N. Nakashima, M.
Sumitani, I. Ohmine, K.Yoshihara, J. Chem. Phys. 72, 2226-2230 (1980).
9. "Picosecond Flash Spectroscopy of Solvent-Induced Intramolecular Electron Transfer in the
Excited 9, 9'-Bianthryl", N. Nakashima, M. Murakawa, N. Mataga, Bull. Chem. Soc. Jpn. 49,854-858 (1976).
10. "Picosecond Flash Photolysis and Transient Spectral Measurements over the Entire Visible,
Near Ultraviolet and Near Infrared Regions", N. Nakashima, N. Mataga, Chem. Phys. Lett. 35,487-492 (1975).