Open-loop and closed-loop control of dissociative ionization of ethanol in intense laser fields

The relative yield of the C-O bond breaking with respect to the C-C bond breaking in ethanol cation C2H5OH+ is maximized in intense laser fields (10(13)-10(15) Wcm2) by open-loop and closed-loop optimization procedures. In the open-loop optimization, a train of intense laser pulses are synthesized so that the temporal separation between the first and last pulses becomes 800 fs, and the number and width of the pulses within a train are systematically varied. When the duration of 800 fs is filled with laser fields by increasing the number of pulses or by stretching all pulses in a triple pulse train, the relative yield of the C-O bond breaking becomes significantly large. In the closed-loop optimization using a self-learning algorithm, the four dispersion coefficients or the phases of 128 frequency components of an intense laser pulse are adopted as optimized parameters. From these optimization experiments it is revealed that the yield ratio of the C-O bond breaking is maximized as far as the total duration of the intense laser field reaches as long as approximately 1 ps and that the intermittent disappearance of the laser field within a pulse does not affect the relative yields of the bond breaking pathways.     

Dissociative ionization of ethanol by 400 nm femtosecond laser pulses

The dissociative ionization of ethanol in short-pulsed laser fields at approximately 400 nm is investigated. The yield ratio of the C-O bond breaking with respect to the C-C bond breaking increases sharply as the temporal width increases from 60 to 400 fs, and the yield ratio is two to three times as large as that at 800 nm in the entire pulse-width range of 60-580 fs. The enhancement of the C-O bond breaking of singly charged ethanol at 400 nm and the bond elongation prior to the Coulomb explosion of doubly charged ethanol occurring in the relatively weak light field intensity of 10(12)-10(13) W cm(2) is interpreted by the efficient light-induced coupling among the electronic states at the shorter wavelength of 400 nm. From the double pulse experiment, in which ethanol is irradiated with a pair of short pulses (<80 fs), the most efficient coupling occurs at Deltat=160 fs that is much earlier than Deltat=250 at 800 nm, where Deltat denotes the temporal separation of the two pulses, indicating that the nonadiabatic field-induced potential crossings of singly charged ethanol occurs much earlier at 400 nm than at 800 nm.     

Two-body Coulomb explosion and hydrogen migration in methanol induced by intense 7 and 21 fs laser pulses

Two-body Coulomb explosion with the C-O bond breaking of methanol induced by intense laser pulses with the duration of Delta t=7 and 21 fs is investigated by the coincidence momentum imaging method. When Delta t=7 fs, the angular distribution of recoil vectors of the fragment ions for the direct C-O bond breaking pathway, CH(3)OH(2+)-->CH(3) (+)+OH(+), exhibits a peak deflected from the laser polarization direction by 30 degrees -45 degrees , and the corresponding angular distribution for the migration pathway, CH(2)OH(2) (+)-->CH(2) (+)+H(2)O(+), in which one hydrogen migrates from the carbon site to the oxygen site prior to the C-O bond breaking, exhibits almost the same profile. When the laser pulse duration is stretched to Delta t=21 fs, the angular distributions for the direct and migration pathways exhibit a broad peak along the laser polarization direction probably due to the dynamical alignment and/or the change in the double ionization mechanism; that is, from the nonsequential double ionization to the sequential double ionization. However, the extent of the anisotropy in the migration pathway is smaller than that in the direct pathway, exhibiting a substantial effect of hydrogen atom migration in the dissociative ionization of methanol interacting with the linearly polarized intense laser field.     

Instantaneous vibrational frequencies of diffusing and desorbing adsorbates: CO/Pt(111)

Electronic excitation of metal by intense laser pulses stimulates nuclear motions of adsorbates through nonadiabatic coupling, resulting in diffusion and desorption of adsorbates. These processes take place via precursor states: adsorbates whose vibrational modes with respect to substrate are highly excited. This paper reports the dynamics of precursor states of CO on Pt(111) probed by use of infrared-visible sum frequency generation with phase-sensitive detection, which allows us to obtain the second-order nonlinear susceptibility and thus the vibrational response function. Without pump pulses at 400 nm, the inverse Fourier transformation of the vibrational response function reveals a free induction decay of vibrational polarization of C-O stretching created by a short infrared pulse. The free induction decay is perturbed when an intense 400-nm pump pulse following the infrared pulse is irradiated, because diffusion and desorption of CO are induced by the pump pulse. The time evolution of instantaneous C-O stretching frequency retrieved from the perturbed free induction decay shows a redshift followed by a rapid reverse shift when the fluence of pump pulse is high enough to desorb CO. This indicates that the frustrated modes of CO is first substantially excited and then the parallel momentum of CO is converted to the normal one through mutual collisions, leading to substantial excitation of the external stretching mode of CO.     

Multiphoton near-infrared femtosecond laser pulse-induced DNA damage with and without the photosensitizer proflavine

The excitation of pBr322 supercoiled plasmid DNA with intense near-IR 810 nm fs laser pulses by a simultaneous multiphoton absorption mechanism results in single-strand breaks after treatment of the irradiated samples with Micrococcus luteus UV endonuclease. This enzyme cleaves DNA strands at sites of cyclobutane dimers that are formed by the simultaneous absorption of three (or more) 810 nm IR photons (pulse width approximately 140 fs, 76 MHz pulse repetition, average power output focused through 10x microscope objective is approximately 1.2 MW/cm2). Direct single-strand breaks (without treatment with M. luteus) were not observed under these conditions. However, in the presence of 6 microM of the intercalator proflavine (PF), both direct single- and double-strand breaks are observed under conditions where substantial fractions of undamaged supercoiled DNA molecules are still present. The fraction of direct double-strand breaks is 30 +/- 5% of all measurable strand cleavage events, is independent of dosage (up to 6.4 GJ/cm2) and is proportional to In, where I is the average power/area of the 810 nm fs laser pulses, and n = 3 +/- 1. The nicking of two DNA strands in the immediate vicinity of the excited PF molecules gives rise to this double-strand cleavage. In contrast, excitation of the same samples under low-power, single-photon absorption conditions (approximately 400-500 nm) gives rise predominantly to single-strand breaks, but some double-strand breaks are observed at the higher dosages. Thus, single-photon excitation with 400-500 nm light and multiphoton activation of PF by near-IR fs laser pulses produces different distributions of single- and double-strand breaks. These results suggest that DNA strand cleavage originates from unrelaxed, higher excited states when PF is excited by simultaneous IR multiphoton absorption processes.     

Stepwise photocleavage of two C-O bonds of 1,8-bis[(4-benzoylphenoxy)-methyl]naphthalene with three-step excitation using three-color, three-laser flash photolysis

Stepwise photocleavage of two naphthylmethyl-oxygen bonds of 1,8-bis[(4-benzoylphenoxy)methyl]naphthalene (1,8-(BPO-CH2)2Np, 1) was observed during three-color, three-laser flash photolysis at room temperature. The mechanism from 1 to the final product, acenaphthene (2), was clearly elucidated. The first (308 nm, 5 mJ pulse-1) XeCl laser excited 1 to the lowest triplet excited state 1(T1), in which the excited energy was localized in the naphthalene moiety, but the C-O bond cleavage did not occur. The second (430 nm, 7 mJ pulse-1) OPO laser excited 1(T1) to the higher triplet excited states 1(Tn) in which the excited energy is delocalized in the naphthalene moiety and C-O bonds, and one C-O bond cleavage occurred. The third (355 nm, 10 mJ pulse-1) YAG laser excited the carbon-centered radical in the ground state 1-(BPO-CH2)NpCH2*(D0) to its excited states 1-(BPO-CH2)NpCH2*(Dn), from which the second C-O bond cleavage occurred to give 2 as the final product. This is a successful example of stepwise cleavage of two equivalent C-O bonds in a molecule using three-color three-laser photolysis method.     

Fragmentation dynamics of fullerenes in intense femtosecond-laser fields: loss of small neutral fragments on a picosecond time scale

The fragmentation dynamics of C60 irradiated with intense femtosecond laser pulses is studied with one-color pump-probe spectroscopy. Small neutral fragments (C, C2, and C3) are formed by an 800-nm pump pulse which are then postionized by a delayed probe pulse. The respective ion signals detected by the time-of-flight mass spectrometry dramatically increase on a time scale of 10-20 ps.     

Structure induced by irradiation of femtosecond laser pulse in dyed polymeric materials

We investigated the structures induced by an irradiation of a near‐infrared (NIR) femtosecond laser pulse in dye‐doped polymeric materials {poly(methyl methacrylate) (PMMA), thermoplastic epoxy resin (Epoxy), and a block copolymer of methyl methacrylate and ethyl acrylate‐butyl acrylate [p(MMA/EA‐BA) block copolymer]}. Dyes used were classified into two types—type 1 with absorption at 400 nm and type 2 with no absorption at 400 nm. The 400‐nm wavelength corresponds to the two‐photon absorption region by the irradiated NIR laser pulse at 800 nm. Type 1 dye‐doped PMMA and p(MMA/EA‐BA) block copolymer showed a peculiar dye additive effect for the structures induced by the line irradiation of a NIR femtosecond laser pulse. On the contrary, dye‐doped Epoxy did not exhibit a dye additive effect. The different results among PMMA, p(MMA/EA‐BA) block copolymer, and Epoxy matrix polymers are supposed to be related to the difference of electron‐acceptor properties. The mechanism of this type 1 dye‐additive‐effect phenomenon for PMMA and p(MMA/EA‐BA) block copolymer is discussed on the basis of two‐photon absorption of type 1 dye at 400 nm by the irradiation of a femtosecond laser pulse with 800 nm wavelength and the dissipation of the absorbed energy to the polymer matrix among various transition processes. Dyes with a low‐fluorescence quantum yield favored the formation of thicker grating structures. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2800–2806, 2002     

Matrix‐assisted laser desorption/ionization by using femtosecond laser pulses in the near‐infrared wavelength regime

We observe a substantial matrix‐assisted laser desorption/ionization (MALDI) signal when irradiating femtosecond laser pulses in the near‐infrared spectral range centered around 800 nm and using standard MALDI matrices with absorption bands in the ultraviolet (UV) regime. The laser pulse energy dependence of this novel phenomenon is investigated in comparison with MALDI with near‐UV laser pulses. Our observations show that multiphoton absorption/ionization could be a major issue among the MALDI processes when the sample is irradiated with ultra‐short laser pulses. Copyright © 2009 John Wiley & Sons, Ltd.     

Near-field focused photoemission from polystyrene microspheres studied with photoemission electron microscopy

We use photoemission electron microscopy (PEEM) to image 3 μm diameter polystyrene spheres supported on a metal thin film illuminated by 400 nm (～3.1 eV) and 800 nm (～1.5 eV) femtosecond (fs) laser pulses. Intense photoemission is generated by microspheres even though polystyrene is an insulator and its ionization threshold is well above the photon energies employed. We observe intense photoemission from the far side (the side opposite the incident light) of the illuminated microsphere that is attributed to light focusing within the microsphere. For the case of p-polarized, 800 nm fs laser pulses, we observe photoemission exclusively from the far side of the microsphere and additionally resolve sub-50 nm hot spots in the supporting Pt∕Pd thin film that are located only within the focal region of the microsphere. We compare the PEEM images with finite difference time domain (FDTD) electrodynamic simulations to model our experimental results. The FDTD simulations predict light focusing in the microsphere and subsequent interaction with the supporting metal surface that is consistent with the experimental observations.     

Formation of molecular halide ions from alkyl-halide clusters irradiated by ps and fs laser pulses

We report on the interaction of alkyl-halide clusters with 35 ps and 20 fs laser pulses at λ = 266, 532, and 1064 nm and 400 and 800 nm, respectively. Particularly, we examine by means of time-of-flight mass spectrometry the intracluster photochemical processes, which give rise to the formation of molecular halogen ions. The efficiency of molecular halogen ion formation is found to depend strongly on the laser wavelength and pulse duration. The ionization/excitation schemes involve in both cases the multiphoton absorption by the clusters and the combined action of the laser and the intracluster electric field. Intracluster energy transfer processes seem to have a significant contribution to the molecular halogen ion formation in the ps domain, while in the fs region, this is probably facilitated by a rescattering process and/or by photon absorption. A physical mechanism for the interpretation of our experimental results is proposed.     

C-O bond cleavage of benzophenone substituted by 4-CH2OR (R = C6H5 and CH3) with stepwise two-photon excitation

The C-O bond cleavage from benzophenone substituted with 4-CH2OR (p-BPCH2OR, 1-3), such as p-phenoxymethylbenzophenone (1, R= C6H5) and p-methoxymethylbenzophenone (2, R= CH3), occurred by a stepwise two-photon excitation during two-color, two-laser flash photolysis. On the other hand, no C-O bond cleavage occurred from p-hydroxymethylbenzophenone (3, R = H). The first 355-nm laser excitation of 1-3 generates p-BPCH2OR in the lowest triplet excited state (T1) which has an absorption at 532 nm. When p-BPCH2OR(T1) is excited with the second 532-nm laser to p-BPCH2OR in the higher triplet excited state (T(n)), the C-O bond cleavage occurred within the laser flash duration of 5 ns. The quantum yields of the C-O bond cleavage during the second 532-nm laser irradiation were found to be 0.015 +/- 0.007 and 0.007 +/- 0.003 for 1 and 2, respectively. Although these values are low, the diminishing 1(T1) or 2(T1) was found to convert, in almost 100% yield, to phenoxyl (C6H5O*) and p-benzoylbenzyl (BPCH2*) radicals or methoxyl (CH3O*) and BPCH2* radicals, respectively. The T(n) excitation energy, the energy barrier along the potential surface between the T(n) states and product radicals, and delocalization of the T(n) state molecular orbital including BP and CH2OR (R = C6H5, CH3, H) moieties are important factors for the occurrence of the C-O bond cleavage. It is found that the C-O bond cleavage and production of free radicals, such as BPCH2*, C6H5O*, and CH3O*, can be performed by a stepwise two-photon excitation. The present study is an example in which the chemical reactions can be selectively initiated from the T(n) state but not from the S1 and T1 states.     

C60 in intense short pulse laser fields down to 9 fs: excitation on time scales below e-e and e-phonon coupling

The interaction of C60 fullerenes with 765-797 nm laser pulses as short as 9 fs at intensities of up to 3.7 x 10(14) W cm(-2) is investigated with photoion spectroscopy. The excitation time thus addressed lies well below the characteristic time scales for electron-electron and electron-phonon couplings. Thus, energy deposition into the system is separated from energy redistribution among the various electronic and nuclear degrees of freedom. Insight into fundamental photoinduced processes such as ionization and fragmentation is obtained from the analysis of the resulting mass spectra as a function of pulse duration, laser intensity, and time delay between pump and probe pulses, the latter revealing a memory effect for storing electronic energy in the system with a relaxation time of about 50 fs. Saturation intensities and relative abundances of (multiply charged) parent and fragment ions (C60(q+), q=1-6) are fingerprints for the ionization and fragmentation mechanisms. The observations indicate that for final charge states q>1 the well known C60 giant plasmon resonance is involved in creating ions and a significant amount of large fragments even with 9 fs pulses through a nonadiabatic multielectron dynamics. In contrast, for energetic reasons singly charged ions are generated by an essentially adiabatic single active electron mechanism and negligible fragmentation is found when 9 fs pulses are used. These findings promise to unravel a long standing puzzle in understanding C60 mass spectra generated by intense femtosecond laser pulses.     

Effect of laser parameters on ultrafast hydrogen migration in methanol studied by coincidence momentum imaging

The effect of intensity, duration, and polarization of ultrashort laser pulses (795 nm, 40-100 fs, and 0.15-1.5 × 10(15) W/cm(2)) on the hydrogen migration in methanol is systematically investigated using Coulomb explosion coincidence momentum imaging. The ratio of the ion yield obtained for the migration pathway CH(3)OH(2+) → CH(2)(+) + OH(2)(+) with respect to the sum of the yields obtained for the migration pathway and for the nonmigration pathway CH(3)OH(2+) → CH(3)(+) + OH(+) exhibits a small (10-20%) but clear dependence on laser pulse properties, that is, the ratio decreases as the laser peak intensity increases but increases when the pulse duration increases as well as when the laser polarization is changed from linear to circular.     

Laser induced and controlled chemical reaction of carbon monoxide and hydrogen

Bimolecular chemical reaction control of gaseous CO and H(2) at room temperature and atmospheric pressure, without any catalyst, using shaped femtosecond laser pulses is presented. High intensity laser radiation applied to a reaction cell facilitates non-resonant bond breakage and the formation of a range of ions, which can then react to form new products. Stable reaction products are measured after irradiation of a reaction cell, using time of flight mass spectroscopy. Bond formation of C-O, C-C, and C-H bonds is demonstrated as CO(2)(+), C(2)H(2)(+), CH(+), and CH(3)(+) were observed in the time of flight mass spectrum of the product gas, analyzed after irradiation. The formation of CO(2) is shown to be dependent on laser intensity, irradiation time, and on the presence of H(2) in the reaction cell. Using negatively chirped laser pulses more C-O bond formation takes place as compared to more C-C bond formation for unchirped pulses.     

Design of an infrared laser pulse to control the multiphoton dissociation of the Fe-CO bond in CO-heme compounds

Optimal control theory is used to design a laser pulse for the multiphoton dissociation of the Fe-CO bond in the CO-heme compounds. The study uses a hexacoordinated iron-porphyrin-imidazole-CO complex in its ground electronic state as a model for CO liganded to the heme group. The potential energy and dipole moment surfaces for the interaction of the CO ligand with the heme group are calculated using density functional theory. Optimal control theory, combined with a time-dependent quantum dynamical treatment of the laser-molecule interaction, is then used to design a laser pulse capable of efficiently dissociating the CO-heme complex model. The genetic algorithm method is used within the mathematical framework of optimal control theory to perform the optimization process. This method provides good control over the parameters of the laser pulse, allowing optimized pulses with simple time and frequency structures to be designed. The dependence of photodissociation yield on the choice of initial vibrational state and of initial laser field parameters is also investigated. The current work uses a reduced dimensionality model in which only the Fe-C and C-O stretching coordinates are explicitly taken into account in the time-dependent quantum dynamical calculations. The limitations arising from this are discussed in Sec. IV.     

Two-color two-photon excitation using femtosecond laser pulses

The use of two-color two-photon (2c2p) excitation easily extends the wavelength range of Ti:sapphire lasers to the UV, widening the scope of its applications especially in biological sciences. We report observation of 2c2p excitation fluorescence of p-terphenyl (PTP), 2-methyl-5-t-butyl-p-quaterphenyl (DMQ) and tryptophan upon excitation with 400 and 800 nm wavelengths using the second harmonic and fundamental wavelength of a mode-locked Ti:sapphire femtosecond laser. This excitation is energetically equivalent to a one-photon excitation wavelength at 266 nm. The fluorescence signal is observed only when both wavelengths are spatially and temporally overlapping. Adjustment of the relative delay of the two laser pulses renders a cross correlation curve which is in good agreement with the pulse width of our laser. The fluorescence signal is linearly dependent on the intensity of each of the two colors but quadratically on the total incident illumination power of both colors. In fluorescence microscopy, the use of a combination of intense IR and low-intensity blue light as a substitute for UV light for excitation can have numerous advantages. Additionally, the effect of differently polarized excitation photons relative to each other is demonstrated. This offers information about different transition symmetries and yields deeper insight into the two-photon excitation process.     

Ultrafast photofragmentation dynamics of molecular iodine driven with timed XUV and near-infrared light pulses

Photofragmentation dynamics of molecular iodine was studied as a response to the joint illumination with femtosecond 800 nm near-infrared and 13 nm extreme ultraviolet (XUV) pulses delivered by the free-electron laser facility FLASH. The interaction of the molecular target with two light pulses of different wavelengths but comparable pulse energy elucidates a complex intertwined electronic and nuclear dynamics. To follow distinct pathways out of a multitude of reaction channels, the recoil of created ionic fragments is analyzed. The delayed XUV pulse provides a way of following molecular photodissociation of I(2) with a characteristic time-constant of (55 ± 10) fs after the laser-induced formation of antibonding states. A preceding XUV pulse, on the other hand, preferably creates a 4d(-1) inner-shell vacancy followed by the fast Auger cascade with a revealed characteristic time constant τ(A2)=(23±11) fs for the second Auger decay transition. Some fraction of molecular cationic states undergoes subsequent Coulomb explosion, and the evolution of the launched molecular wave packet on the repulsive Coulomb potential was accessed by the laser-induced postionization. A further unexpected photofragmentation channel, which relies on the collective action of XUV and laser fields, is attributed to a laser-promoted charge transfer transition in the exploding molecule.     

Femtosecond laser time-of-flight mass spectrometry of labile molecular analytes: laser-desorbed nitro-aromatic molecules.

Femtosecond laser time-of-flight mass spectra of solid samples of trinitrobenzene (TNB), trinitrotoluene (TNT) and trinitrophenol (TNP) have been recorded. Desorption of the solid samples was enacted by the fourth harmonic output (266 nm) of a 5 ns Nd:YAG laser. Subsequent femtosecond post-ionisation of the plume of neutral molecules was achieved using 800 nm laser pulses of 80 fs duration. Mass spectra have been recorded for desorption laser intensities from 2-6 x 10(9) W cm(-2) with ionisation laser intensities between 2 x 10(14) and 6 x 10(15) W cm(-2). Femtosecond laser ionisation has been shown to be capable of generating precursor and characteristic high-mass fragment ions for labile nitro-aromatic molecules commonly used in high-explosive materials. This feature is critical in the future development of femtosecond laser-based analytical instruments that can be used for complex molecular identification and quantitative analysis of environmentally important labile molecules. Furthermore, a comparison of femtosecond post-ionisation mass spectra with standard 70 eV electron impact data has revealed similarities in the spectra and hence the fragmentation processes.     

Wolff Rearrangement of 2-diazo-1(2H)-naphthalenone induced by nonresonant two-photon absorption of NIR radiation

The irradiation of 2-diazo-1(2H)-naphthalenone (1), the common component of positive photoresists, with 800 nm pulses of ultrafast laser results in Wolff rearrangement via nonresonant two-photon absorbance of light. The 10% conversion of starting material resulting in the formation of methyl 1H-indene-3-carboxylate (2) was achieved after 11 min of irradiation of the methanol solution of 1 with an unfocused beam of a Ti:Sapphire laser operating at 1 kHz. The two-photon cross-section of the diazonaphthoquinone 1 at 800 nm was calculated to be sigma = 2.2 x 10-51 cm4 s photon-1 (0.2 GM).