11 Oct. 2011

Laboratory for Laser Chemistry

Tomoyuki Yatsuhashi (Professor)


1. Current Research and Principal Research Interests

(1) Interaction between an Intense Ultrafast Laser and Molecule.  New research fields opened related to the interaction between molecules and high-intensity optical fields, where the laser intensity ranges from 1012 ∼ 1017 Wcm-2. Ionization is the most fundamental process that such an intense femtosecond laser induces. A large set of experiments and theoretical considerations have been reported in the cases of atoms and few-atomic molecules. In the study of molecular ionization, the origin of fragmentation is one of the central topics today. The fragmentation of ions can occur for molecules but not for atoms. Most large molecules show heavy fragmentation with 0.8 μm pulses even at a low-intensity regime (1013 Wcm-2). This makes the analysis of experimental results highly complicated and prevents us from comparing the results against theoretical predictions. An intact molecular ion formation is desirable not only for analytical application but also for fundamental studies. Recently, the importance of radical cation in the post-ionization process has increasingly recognized, and the arguments about fragmentation mechanism is still continuing. To avoid fragmentation, we can use shorter duration pulse and/or pulses at a suitable wavelength, which is off-resonant with the molecular cation radicals. Generally, a longer wavelength is better than a shorter one. The clear presentations of experimental results were reported using longer wavelength pulses. These findings are quite useful for analytical purposes. We have shown the ability of femtosecond laser ionization method for dioxin detection.
In an intensity region higher than 1016 Wcm-2, electrons are stripped from molecules 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 be analyzed by molecular dynamic simulation. The intact ion formation can be used for femtosecond laser mass spectrometry.

(2) Ultrafast chemical reaction dynamics probed by time-resolved dissociative ionization.  We have been interested in dynamics of nonradiative processes such as internal conversion, solvent relaxation, and chemical reactions. Our aim is to study not only basic phenomena of nonradiative processes (the dynamics of intramolecular charge-transfer excited state: enhanced internal conversion induced by hydrogen bonding interaction) but also the reactions of highly vibrationally excited state formed after an internal conversion. It is concluded that an internal conversion is not a useless photophysical process (many people have thought it as an only energy dissipation process) but a very interesting process that forms an important intermediate in multiphoton hot molecule chemistry. Transient emission and absorption spectroscopy was used in the course of these studies to investigate the photophysical processes and photochemical reactions; however, the application of these techniques are limited to some extent. New transient spectroscopy that can measure the dark (non-emissive) state is strongly needed for more understanding about the nonradiative processes and photochemical reactions.
    The member of Max-Planck-Institut für Qunatenoptik (Germany) has developed a technique to monitor the path of the molecule all the way along the potential energy surfaces from the primary excited state (Franck-Condon region) down to the ground state of the products or reactant. There are no dark states in this method because nonresonant multiphoton ionization with mass selective detection of the ion yields was used. A crucial feature is that many (ion) signals are observed, and the success is based on the fact that the fragmentation pattern is different for different locations on the potential energy surfaces. We are also studying the nonresonant ionization process with a high intense femtosecond laser (operated with the laser power between 1013 ∼ 1016 Wcm-2) in collaboration with physicists of Osaka university. Our aims are: (1) detection of a small amount of molecule, which is difficult to be detected by other techniques such as laser induced fluorescence (e.g. chlorinated compound such as dioxin); (2) investigation and application of highly energized and charged atoms formed by Coulomb explosion (X-ray generation etc.). On the other hand, the scientists of Max-Planck-Institut aimed to investigate the reaction dynamics. Although we and the members of Max-Planck-Institut made different approaches, both found similar and important features in such an ionization process. Phenomenologically it was clear that femtosecond intense laser pulses lead to enhanced formation of parent ions compared with nanosecond laser excitation. However, heavy fragmentation was observed in some molecules. We generalize that the non-resonance between the laser wavelength and the electronic energy levels of parent cation is a key factor in the formation of the parent ion during femtosecond laser excitation.
    We started the joint-project on both, methodological development and its application to several interesting problems of ultrafast chemical reaction dynamics through conical intersection (intramolecular charge transfer excited state, highly vibrationally excited state etc.) based on the findings described above. (collaboration with Dr. Fuß, Dr. Schmid, and Dr. Trushin in Max-Planck-Institut für Quantenoptik, Germany)

(3) Chemical reactions of vibrationally excited molecules: single- and multiphoton reactions of hot molecules.  The highly vibrationally excited state, namely a hot molecule, which has an equivalent vibrational temperature of 2000-4000 K, would be produced after a rapid internal conversion from an electronically excited state. We are studying the hot molecule chemistry of large molecules. In these cases, product formations by the two- or more photon process were observed. A single photon absorption would not be sufficient to induce a chemical reaction of such large molecules, which have many vibrational modes. It would be necessary to accumulate energy by a successive second photon absorption to overcome the activation energy of the chemical reactions as well as to fulfill the reaction rates higher than the collisional relaxation rates. In these cases, it would be reasonable to conclude that the hot molecules, which were created by the first photon, effectively absorbed the second photon because the hot molecules have a strong absorption at the laser wavelength as same as the parent molecules. The new reaction pathway would be expected for many molecules even though they hardly react by photolysis (single photon absorption), if hot molecules absorbed the second photon. The equivalent vibrational temperature with an internal energy of two photons is high enough to induce the reactions in the electronically ground state. The most important difference from the thermal reactions at the same temperature is that the products can be cooled down by collisional relaxation for the case of hot molecule. Therefore, different products would be expected. We have found several examples of new reaction pathways in photo-inert molecules. The multiphoton hot molecule chemistry could be the way to develop a new area of photochemical reactions.

(4) Anisotropic radiationless deactivation process induced by hydrogen-bonding interaction.  Aminoanthraquinones (AAQ) and aminofluorenones are characteristic molecules that exhibit strong intermolecular hydrogen-bonding interaction in the excited state, since the excited states have a strong intramolecular charge-transfer nature. The large electron density on the carbonyl oxygen of AAQ in the excited state strongly promotes an intermolecular hydrogen-bonding interaction with nonconjugated molecule such as an alcohol. An intermolecular hydrogen-bonding with the hydroxyl group of alcohol was revealed to be a dominant mode of radiationless deactivation to the ground state, and the electronic excited energy was supposed to dissipate through the hydrogen bond as vibrational energy. Recently, we have found that not only alcohol but also hydroperoxide quenched the fluorescence of AAQ. With the fluorescence-quenching the decomposition of hydroperoxide was observed. The decomposition of hydroperoxide could mean that dissipated energy through the intermolecular hydrogen bond was converted selectively to induce chemical reaction such as bond cleavage. Stern-Volmer analyses for the obtained products show that the decomposition was closely related to fluorescence-quenching process. From the viewpoints of energy dissipation, upon radiationless deactivation from the electronically excited state the excited energy has generally been accepted to dissipate randomly through the surrounding solvent molecules. The sensitized decomposition of hydroperoxide coupled with radiationless deactivation strongly suggests that the excited energy is not randomly but at least partly selectively transferred to a specific molecule surrounding the chromophore in the case. An investigation of the microscopic molecular mechanism of radiationless deactivation by intermolecular hydrogen-bonding interactions would clarify the problem of whether the energy dissipation is inherently isotropic or anisotropic. (collaboration with Prof. Haruo Inoue in Tokyo Metropolitan University)

2. Selected Publications

1) Molecular Mechanism of Radiationless Deactivation of Aminoanthraquinones through Intermolecular Hydrogen–Bonding Interaction with Alcohols and Hydroperoxides
Yatsuhashi, T.; Inoue, H.*, J. Phys. Chem. A 1997, 101 (44), 8166-8173.

2) A Hot Molecule as an Intermediate in Multiphoton Reaction: First Photoinduced Reaction of Biphenylene
Yatsuhashi, T.;* Akiho, T.; Nakashima, N.*, J. Am. Chem. Soc. 2001, 123 (41), 10137-10138.

3) Ultrafast Charge Transfer and Coherent Oscillations in 4-Piperidino-benzonitrile
Yatsuhashi, T.; Trushin, S. A.; Fuß,* W.; Rettig, W.; Schmid, W. E.; Zilberge, S., Chem. Phys. 2004, 296 (1), 1-12.

4) Femtosecond Laser Ionization of Organic Amines with Very Low Ionization Potentials: Relatively Small Suppressed Ionization Features
Yatsuhashi, T.;* Obayashi, T.; Tanaka, M.; Murakami, M.; Nakashima, N., J. Phys. Chem. A 2006, 110 (25), 7763-7771.

5) Anisotropic Bulletlike Emission of Terminal Ethynyl Fragment Ions: Ionization of Ethynylbenzene-d under Intense Femtosecond Laser Fields
Yatsuhashi, T.;* Murakami, M.; Nakashima, N., J. Chem. Phys. 2007, 126 (19), 194316 (10 pages).

6) High-Order Multiphoton Fluorescence of Organic Molecules in Solution by Intense Femtosecond Laser Pulses
Yatsuhashi, T.;* Ichikawa, S.; Shigematsu, Y.; Nakashima, N., J. Am. Chem. Soc. 2008, 130 (46), 15264-15265.

7) Ionization of Anthracene Followed by Fusion in the Solid Phase under Intense Nonresonant Femtosecond Laser Fields
Yatsuhashi, T.; Nakashima, N.*, J. Phys. Chem. C 2009, 113 (27), 11458-11463.

8) Formation and Fragmentation of Quadruply Charged Molecular Ions by Intense Femtosecond Laser Pulses
Yatsuhashi, T.;* Nakashima, N., J. Phys. Chem. A 2010, 114 (28), 7445-7452.

9) VUV Laser Chemistry - Formation of Hot Molecules and Their Reactions in the Gas Phase –
Yatsuhashi, T.;* Nakashima, N., Bull. Chem. Soc. Jpn. 2001, 74 (4), 579-593.