Tomoyuki Yatsuhashi

A key factor in parent and fragment ion formation on irradiation with an intense femtosecond laser pulse

Abstract Two pairs of organic molecules (2,3-dimethyl-1,3-butadiene, 2,5-dimethyl-2,4-hexadiene; 1,4-cyclohexadiene, 1,3-cyclohexadiene) were irradiated with a high-intensity 120-fs laser pulse in the intensity region of 10 14 W cm −2 . One molecule in each pair had no allowed electronic transition in the cation at the excitation wavelength of 800 nm, resulting in parent ion dominance. In contrast, the counter molecule, which had a similar structure but with absorption in the cation at the excitation wavelength, showed heavy fragmentation with a negligibly small signal of the parent ion. The previously reported observations are explainable on the basis of the proposed mechanism for fragmentation in femtosecond ionization.

Coherent oscillations in the charge-transfer system 4-dimethylamino-benzonitrile

Abstract 4-Dimethylamino-benzonitrile was excited in the gas phase at 270 nm into the S 2 (L a ) state and probed by femtosecond time-resolved photoionization at 2 μm. Coherent oscillations were detected in the transient ion signal. We claim that from S 2 , the molecule relaxes through a conical intersection and, going backwards along the charge-transfer (CT) reaction coordinate, enters into the S 1 (L b ) well, where it vibrates along the amino-group twist and inversion. Probably mainly the last slope stimulates these oscillations. We conclude that the conical intersection – and hence also the CT state – is displaced from the L b minimum in a direction containing the twist and inversion as components and that inversion is part of the nonadiabatic coupling vector.

The role of intersystem crossing in the deactivation of the singlet excited aminofluorenones

Solvent and substituent effects on the competition between internal conversion and triplet formation were studied systematically for aminofluorenones and their N-methylated derivatives. Intersystem crossing (ISC) was found to be the dominant process for the singlet excited 1-amino- and 1-methylaminofluorenone in all solvents. The short fluorescence decay time of these compounds does not originate from intramolecular hydrogen bonding induced internal conversion but it is due to the fast triplet formation. Rather slow (kISC⩽4.8 × 107 s−1) and solvent insensitive intersystem crossing characterizes the photophysical behavior of 2-, 3- and 4-aminofluorenones but their internal conversion rate strongly increases with solvent polarity. The change of the internal conversion rate constants with molecular structure and solvent can be rationalized in terms of the energy gap law.

Enhancement of anthracene fragmentation by circularly polarized intense femtosecond laser pulse

The authors compared circularly and linearly polarized lights in the ionization and fragmentation of anthracene, using 800 nm femtosecond laser pulses at intensities of 10(13)-10(15) W cm-2. Singly and doubly charged intact molecular ions as well as numerous fragment ions were observed in the mass spectra, which were investigated as a function of laser intensity and polarization. At comparable intensities above the saturation threshold for complete ionization, the fragmentation pathways are enhanced with a circularly polarized field compared to a linearly polarized field. Resonant excitation of the molecular cation through the 2Au<--2Bg transition is proposed to be the initial step to ion fragmentation. The circularly polarized field interacts with a larger fraction of the randomly oriented molecules than the linearly polarized field, and this is considered to be the reason for the enhanced fragmentation brought about by circularly polarized light.

Anisotropic bulletlike emission of terminal ethynyl fragment ions: Ionization of ethynylbenzene- d under intense femtosecond laser fields

The authors investigated Coulomb explosions of ethynylbenzenes under intense femtosecond laser fields. Deuteration on the edge of the triple bond gave information about specific fragment emissions and the contribution of hydrogen migration. Some fragments not resulting from migration were emitted in the direction of laser polarization. These were ethynyl fragment ions (D(+), CD(+), C(2)D(+), and C(3)D(+)). Although two bonds have to be cleaved to produce C(3)D(+), the rigid character of the triple bond was maintained in the Coulomb explosion process. In contrast, fragment ions, which are formed after single or double hydrogen migration, showed isotropic emissions with distinct kinetic energies. The character of the substituents has been found to hold even under strong laser light fields where violent fragmentation took place. The ethynyl parts were emitted like bullets from the molecular frame of ethynylbenzene despite the explosion into pieces of the main body of benzene ring.

Persistence of Iodines and Deformation of Molecular Structure in Highly Charged Diiodoacetylene: Anisotropic Carbon Ion Emission

Strong electric fields that are larger than those of valence electrons in molecules shake out many electrons from the molecules. The generation of highly charged molecular ions by femtosecond laser fields followed by a Coulomb explosion prepares charged atoms in close proximity. The ion dynamics have been investigated by covariance mapping, momentum imaging techniques, and simple consideration of kinetic energy releases. The interaction between ionic species is usually solved by a classical equation of motion under point-charge approximation. Supposing that two charges with different masses exist in close proximity, the light ion flies away while the heavy counterion remains near the original location to conserve momentum. If the heavy ion obstacles disturb the direction in which the precursor ion move, the light ions would undergo structural deformation that enables the light ions to fly away. Linear molecules exposed to intense laser fields would provide interesting information on this issue, especially when the terminal heavy atoms block the movement of lighter ions or ion clusters in the linear molecule. We demonstrate the mass effects on the Coulomb explosion processes of acetylene derivatives of similar geometry but with different electronic orbitals and weights of terminal atoms. We compare the Coulomb explosion dynamics between acetylene and diiodoacetylene (DIA, I C C I). Multiple ionization was carried out with a 40 fs pulse centered at 800 nm, and the ions were detected by a time-of-flight mass spectrometer. Figure 1 shows the time-of-flight mass spectrum of DIA. The principal ions were C2I2 w + (w = 1 4), C2I (x=1 2), C + (y = 1 3), C2 + , and I + (z = 1 6). The ion intensity depends strongly on the configuration between the direction of laser polarization and the ion flight axis. When the direction of laser polarization is parallel or perpendicular to the ion flight axis, this situation is referred to as the “parallel condition” or “orthogonal condition,” respectively. C2I2 w + , C2I x + , and C2 + were emitted isotropically because they have nearly zero kinetic energy, and thus all of the ions can pass through a narrow slit located on the extraction plate in the mass spectrometer. In contrast, C + and I + showed anisotropic emission, as the detection of fragment ions with certain kinetic energies was limited by the slit. I + was diminished under the orthogonal condition, whereas C + was detected in high abundance under the same condition as judged from the ion intensities shown in Figure 1. Because there is generally a high probability that electrons will be stripped from molecules along the molecular axis, energetic ions tend to be emitted along the laser polarization direction. As a result, the yield of energetic I + ejected along the laser polarization direction (molecular axis) was suppressed under the orthogonal condition. The predominant C + detection under the orthogonal condition in the case of DIA is unusual and needs ion alignment distortion before fragmentation can occur. In the case of acetylene, both H and C + were almost completely diminished under the orthogonal condition. We note several more differences in the ionization and fragmentation behaviors between DIA and acetylene: 1) the maximum charge state of the intact molecular ion; 2) the angular distribution of carbon ions; 3) the kinetic energy of carbon ions; 4) C2I x + and C2 + formation. We describe these differences in detail. The quadruply charged molecular ion of DIA was found in large abundance, while C2H2 2 + was the maximum charge state observed even at 4 10 W cm . Although molecular ions charged triply or more are generally unstable, a quadruply charged molecular ion was reported for ovalene (C32H14) for the first time using electron impact ionization. Ovalene is a large polycyclic aromatic hydrocarbon that has a fused structure of ten benzene rings, thus allowing the delocalization of multiple charges. We have found that the relative yield of Figure 1. Time-of-flight mass spectrum of DIA at 3.6 10 W cm . Laser polarization is orthogonal to the ion flight axis. Inset shows the spectrum taken under parallel condition otherwise identical conditions.

Reduction of Sm3+ to Sm2+ by an Intense Femtosecond Laser Pulse in Solution

Samarium 3+ ions in methanol were found to be reduced to the corresponding 2+ ions upon irradiation with intense femtosecond laser pulses. The reduction was observed at both pulses with central wavelengths of 403 nm converted from an 800 fs fundamental pulse and 800 nm with a duration of 43 fs. When the laser wavelength was tuned to the 4f-4f absorption at 403 nm corresponding to the (6)P(3/2) <-- (6)H(5/2) transition, the reduction occurred by multiphoton absorption, presumably due to reaching the deep charge transfer state. In the case of excitation by 800 nm pulses of the fundamental wavelength of the Ti:sapphire laser, the reduction is considered to occur via solvent ionization followed by electron capture by Sm(3+). The product Sm(2+) was detected by its fluorescence, which was observed for the first time in solution and showed a broad spectrum peak around 750 nm with a quantum yield of 0.050 in methanol in the presence of 15-crown-5-ether.

Dissociation and Multiply Charged Silicon Ejection in High Abundance from Hexamethyldisilane

Quadruply charged, neon-like silicon and helium-like carbon were generated by the exposure of hexamethyldisilane to intense femtosecond laser pulses. Dissociation of the silicon-silicon bond, the formation of highly charged silicons, as well as the saturation intensity of their formation were studied by mass spectroscopy. The production of these ions in high abundance, but also with lower laser intensity than theoretically expected for the element, was accomplished by using organosilicon compounds. Multiply charged silicon was generated at low laser intensity because stripping electrons from organosilicon compounds is much easier than from pure silicon due to the loose binding of electrons belonging to molecular orbitals. Femtosecond laser ionization is a valuable methodology for producing highly charged ions in high abundance and is useful in many fields of interest.

Ionization and Fragmentation of Alkylphenols by 0.8−1.5 μm Femtosecond Laser Pulses

Ionization and fragmentation were studied on alkylphenols with long alkyl chains (p-(C6H4)(OH)(C(n)H(2n+1)), n = 1,3,5,8,9) and, for reference, on alkylbenzenes ((C6H5)(C(n)H(2n+1)), n = 1,3,5,7,9) by intense femtosecond laser pulses, typically with 43 fs duration at 0.8 microm and 140 fs at 1.3 microm in an intensity range of 10(14) W cm(-2). The major products were the corresponding molecular and C7 fragment ions from the alkylphenols and alkylbenzenes. The molecular ion yields decreased from nearly 1 (n = 1) to 0.3-0.5 (n = 9) when the carbon number in the alkyl chain increased for both excitation wavelengths. Higher yields of the molecular ions were observed at a longer wavelength of 1.3 microm. The long wavelengths in the range of 1.3-1.5 microm were used to determine whether or not -OH absorption had any increase in fragment ions. No effect was observed by vibrational overtone excitation of the -OH group in this wavelength range. Direct dissociation by cation absorption is the most plausible explanation of the present fragmentation results. Other possible mechanisms were discussed, including a statistical model, an effect of electron rescattering, a multiactive electron model, and dissociation from the superexcited state. In the case of cyclohexane, nonresonant wavelength excitation with a pulse of 1.3 microm (150 fs) effectively suppressed fragmentation more than excitation by a resonant but short-duration pulse (0.8 microm, 15 fs).

Ionic Valence Change of Metal Ions in Solution by Femtosecond Laser Excitation Accompanied by White-Light Laser

Three lanthanide ions (Ln^(3+)), Ln = Eu, Sm, and Yb, and two transition metals, Fe^(3+) and Ag^+, were found to be reduced to the corresponding Ln^(2+), Fe^(2+), and Agn in methanol or aqueous solution upon irradiation with intense femtosecond laser pulses. The major excitation wavelength was 800 nm and single-photon-non-resonant with the electronic transitions of metal ion solutions. Laser pulses with wavelengths of 970, 1190, and 1930 nm were used for particular cases. Whenever the white-light laser was generated, the reductions were observed. The reduction mechanisms would be explained in terms of self-focusing, solvated electron formation followed by trapping the electron. The electron ejection under focused beam conditions in solution has been known to be accompanied by white-light laser. In the exceptional case of Fe^(3+) at 800 nm, two-photon excitation of the charge transfer state followed by the reduction would be operative. Fe^(2+) was detected even with an intensity lower than the threshold of the white-light laser generation.