Controlling the dissociative ionization of ethanol with 800 and 400 nm two-color femtosecond laser pulses

Ethanol molecules were irradiated with a pair of temporally overlapping ultrashort intense laser pulses (10(13)-10(14) Wcm(2)) with different colors of 400 and 800 nm, and the dissociative ionization processes have been investigated. The yield ratio of the C-O bond breaking with respect to the C-C bond breaking was varied in the range of 0.17-0.53 sensitively depending on the delay time between the two laser pulses, and the absolute value of the yield of the C-O bond breaking was found to be increased largely when the Fourier-transform limited 800 nm laser pulse overlaps the stretched 400 nm laser pulse, demonstrating an advantage of the two-color intense laser fields in controlling chemical bond breaking processes.     

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.     

Application of femtosecond laser mass spectrometry to the analysis of volatile organic compounds

Femtosecond (fs) lasers have high intensity and ultrashort pulse duration. Tunneling ionization occurs for molecules subject to such intense laser fields. We have studied the mass spectra of a variety of molecules irradiated by intense fs laser pulses. These molecules include some typical volatile organic compounds contained in human breath and in the atmosphere. The results demonstrate that all of these molecules can be ionized by intense fs laser pulses. Dominant parent ion and some characteristic ionic fragments are observed for each molecule. The degree of fragmentation can be controlled by adjusting the laser intensity. Moreover, saturation ionization can occur for each molecule by increasing the laser intensity. These features indicate that fs laser mass spectrometry can be a sensitive tool to identify and quantify volatile organic compounds in human breath.     

Use of a film modulator with single-walled carbon nanotubes for mode locking in a pulse titanium-sapphire laser at a wavelength of 810 nm

The feasibility of mode locking in a pulsed laser at a wavelength of 810 nm was shown when a carboxymethylcellulose film with single-walled carbon nanotubes was used as a saturable absorber. With the use of such a film modulator for passive mode locking in the titanium-sapphire laser, a train of continuous-wave ultrashort pulses was obtained. The energy and the duration of a single pulse in the train were 0.4 nJ and 600 fs, respectively.     

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.     

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.     

分子动力学模拟飞秒激光对C60光裂解反应机理的影响

基于半经典分子动力学方法模拟超快激光诱导的C60光裂解反应.选择能量为2.0eV,半峰全宽(FWHM)分别为40和500fs的激光作用于C60分子,调节脉冲强度使其发生裂解反应,比较长短脉宽飞秒激光对C60裂解反应机理的影响.通过分析产物分布、原子平衡指数、温度以及吸收能量(包括动能、势能和电子能量),证实飞秒激光脉冲下C60的光裂解主要由电子激发态控制,非热力学效应在该反应中起重要作用.激光场的作用时间和强度均影响光裂解反应过程,而电子吸收能量饱和后光场强度的作用则变得不明显.     

用双激光脉冲操纵N2分子取向

利用求解含时薛定谔方程的方法, 研究了双原子N2分子在激光强度为1.5×10¹⁴ W·cm^-2, 脉冲宽度为50 fs激光脉冲作用下的取向行为. 研究结果表明, 保持总激光强度不变, 将一束激光脉冲分成具有同样脉宽, 强度比为0.3636的两束激光脉冲, 当第一束脉冲产生的分子取向即将达到最大时, 加上有一定延迟时间的第二束脉冲能够使N2分子达到最大的取向程度.     

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.     

Role of photolysis frequency in enhanced selectivity and yield for controlled bond breaking in HOD

Quantum dynamical calculations on HOD subjected to different combinations of IR and UV pulses have been made to isolate field attributes which maximize selectivity and yield in the photodissociation of the desired O-H/O-D bond. Results from IR/UV pulse combinations which provide very high selectivity and/or yield are analyzed in detail by using population transfer, probability density flow, and flux variations to obtain microdynamic details favoring selectivity and yield. Results indicate that a 2727 cm (-1) 50 fs Gaussian IR pulse in conjunction with a 46,062 cm (-1) 50 fs Gaussian UV pulse with a time lag of approximately 90 fs between the IR and UV pulses gives 79.1% flux in the H-O + D channel and 6.6% flux in the H + O-D channel, whereas a 3706 cm (-1) 50 fs IR pulse in conjunction with a 51 090 cm (-1) UV pulse gives 9.2% flux in the H-O + D channel and 82.1% flux in the H + O-D channel. A 2727 cm (-1) 50 fs IR pulse in conjunction with a 40,062 cm (-1) 50 fs UV pulse provides the greatest selectivity among the sampled field profiles with a flux branching ratio of H-O + D/H + O-D approximately 487.8, and a 3706 cm (-1) 50 fs IR pulse in conjunction with a 45090 cm (-1) 50 fs UV pulse achieves a flux branching ratio of H + O-D/H-O + D approximately 1354.8.     

Dissociative ionization of methane by chirped pulses of intense laser light

Measurements have been made of optical field-induced ionization and fragmentation of methane molecules at laser intensities in the 10(16) W cm(-2) range using near transform limited pulses of 100 fs duration as well as with chirped pulses whose temporal profiles extend up to 1500 fs. Data is taken both in constant-intensity and constant-energy modes. The temporal profile of the chirped laser pulse is found to affect the morphology of the fragmentation pattern that is measured. Besides, the sign of the chirp also affects the yield of fragments like C2+, H+, and H2+ that originate from methane dications that are formed by optical field-induced double ionization.     

Chemoselective quantum control of carbonyl bonds in Grignard reactions using shaped laser pulses

Grignard reactants like methylmagnesium chloride are not selective with respect to different carbonyl bonds. We present a theoretical study where shaped laser pulses are utilized to prefer specific bonds in a mixture of more than one carbonyl reactant. A mixture of cyclohexanone and cyclopentanone has been chosen as a representative example. The light pulse is supposed to provide the activation energy and to adopt the function of a protecting group. The control aim is to stretch exclusively the C-O bond of one compound to the length required in the Grignard transition state. To guarantee an experimentally realizable bandwidth for the unshaped pulse, we use our recently developed optimal control theory algorithm, which allows the simultaneous optimization of the light field in the time and frequency domain. Highly selective picosecond control pulses could be optimized in the infrared regime suggesting that laser assisted chemoselectivity is possible to a large extent. To obtain control not only on the final product but also on the excitation mechanism, various initial conditions and frequency restrictions were investigated.     

Quantum dynamics of proton migration in H2O dications: H2+ formation on ultrafast timescales

Irradiation of isolated water molecules by few-cycle pulses of intense infrared laser light can give rise to ultrafast rearrangement resulting in formation of the H(2) (+) ion. Such unimolecular reactions occur on the potential energy surface of the H(2)O(2+) dication that is accessed when peak laser intensities in the 10(15) W cm(-2) range and pulse durations as short as 9-10 fs are used; ion yields of ~1.5% relative to the H(2)O(+) ion are measured. We also study such reactions by means of time-dependent wavepacket dynamics on an ab initio potential energy surface of the dication and show that a proton, generated from O-H bond rupture, migrates towards the H-atom, and forms vibrationally excited H(2)(+) in a well-defined spatial zone.     

Acetylene-vinylidene isomerization in ultrashort intense laser fields studied by triple ion-coincidence momentum imaging

The isomerization of acetylene via hydrogen migration in intense laser fields (8 x 10(14) W/cm2) has been investigated by coincidence momentum imaging of the three-body Coulomb explosion process, C2H2 (3+)-->H+ + C+ + CH+. When ultrashort (9 fs) laser pulses are used, the angle between the momenta of C+ and H+ fragments exhibits a sharp distribution peaked at a small angle ( approximately 20 degrees ), showing that the hydrogen atom remains near the original carbon site in the acetylene configuration. On the other hand, a significantly broad distribution extending to larger momentum angles ( approximately 120 degrees ) is observed when the pulse duration is increased to 35 fs, indicating that the ultrafast isomerization to vinylidene is induced in the longer laser pulse.     

Multiphoton ionization of molecules: A comparison between femtosecond and nanosecond laser pulse ionization efficiency

Multiphoton ionization mass spectra of nonvolatile molecules laser desorbed into a supersonic beam are recorded. It is shown by indirect measurements that the laser desorption of neutrals is not mass limited, but lead to the formation of neutrals with intesities large enough for intense signals. To investigate the efficiency of the multiphoton ionization process with varying laser pulse durations, simultaneous laser pulses of 500 fs and 5 ns or 100 fs and 5 ns have been applied to the neutral beam. The energies of both femtosecond and nanosecond laser pulses are held in a comparable magnitude, and thus produce, in the resulting ion intensity, very large differences up to 4 orders of magnitude. For larger evaporated molecules (> 500 u) the ionization efficiency from nanosecond laser pulses drops significantly in comparison to femtosecond laser pulse excitation. A variety of possible reasons for the different ionization and dissociation behavior in femtosecond and nanosecond laser pulse excitations are discussed in this paper. It is rationalized that even with very short laser pulses and large molecules the “ladder switching model” for ionization and fragmentation is valid.     

Quantum dynamics of h(2)(+) in intense laser fields on time-dependent potential energy surfaces

We have exploited the fully time-dependent Born-Oppenheimer approximation to develop time-dependent potential energy surfaces for the lowest two states of H(2)(+) in the presence of intense, time-varying, few-cycle laser fields of 2-8 fs duration. Quantum dynamics are explored on these field-dressed, time-dependent potentials. Our results show that the potential well in the lowest-energy state of H(2)(+) (i) collapses as the laser pulse reaches its peak amplitude and (ii) regains its form on the trailing edge of the pulse, and (iii) the trapped nuclear wavepacket has a higher probability of leaking out from the well in the case of longer laser pulses. The carrier envelope phase is found to have negligible effect on the nuclear dynamics.     

Pulse shaping effect on two-photon excitation efficiency of alpha-perylene crystals and perylene in chloroform solution

We demonstrated that the two-photon excitation efficiency of perylene in chloroform solution as well as that of crystalline perylene was dramatically increased by optimizing the shape of intense femtosecond laser pulses of a regenerative amplifier output. The efficiency was three times higher than for an unshaped single femtosecond pulse with the pulse width of shorter than 50 fs. The pulse shape optimized for the solution sample was a pulse train with a repetition frequency of about 340 cm(-1), and the pulse shape optimized for crystalline perylene was very similar. These results supported our previous findings on alpha-perylene crystals using weak femtosecond pulses from a mode-locked laser oscillator [T. Okada et al. J. Chem. Phys. 121, 6385 (2004)]. Furthermore, it was confirmed that the shaped pulse optimized for the liquid sample could also increase the two-photon excitation efficiency of alpha-perylene crystals and vice versa. We concluded that the mechanism for the increase in excitation efficiency of perylene in chloroform was almost the same as that for alpha-perylene crystal, and that the efficiency increased mainly through intramolecular dynamical processes. Processes involving intermolecular interactions and/or energy states delocalized over the crystal cannot play the major role.     

Control of molecular fragmentation using shaped femtosecond pulses

The possibility that chemical reactions may be controlled by tailored femtosecond laser pulses has inspired recent studies that take advantage of their short pulse duration, comparable to intramolecular dynamics, and high peak intensity to fragment and ionize molecules. In this article, we present an experimental quest to control the chemical reactions that take place when isolated molecules interact with shaped near-infrared laser pulses with peak intensities ranging from 1013 to 1016 W/cm2. Through the exhaustive evaluation of hundreds of thousands of experiments, we methodically evaluated the molecular response of 16 compounds, including isomers, to the tailored light fields, as monitored by time-of-flight mass spectrometry. Analysis of the experimental data, taking into account its statistical significance, leads us to uncover important trends regarding the interaction of isolated molecules with an intense laser field. Despite the energetics involved in fragmentation and ionization, the integrated second-harmonic generation of a given laser pulse (ISHG), which was recorded as an independent diagnostic parameter, was found to be linearly proportional to the total ion yield (IMS) generated by that pulse in all of our pulse shaping measurements. Order of magnitude laser control over the relative yields of different fragment ions was observed for most of the molecules studied; the fragmentation yields were found to vary monotonically with IMS and/or ISHG. When the extensive changes in fragmentation yields as a function of IMS were compared for different phase functions, we found essentially identical results. This observation implies that fragmentation depends on a parameter that is responsible for IMS and independent from the particular time-frequency structure of the shaped laser pulse. With additional experiments, we found that individual ion yields depend only on the average pulse duration, implying that coherence does not play a role in the observed changes in yield as a function of pulse shaping. These findings were consistently observed for all molecules studied (p-, m-, o-nitrotoluene, 2,4-dinitrotoluene, benzene, toluene, naphthalene, azulene, acetone, acetyl chloride, acetophenone, p-chrolobenzonitrile, N,N-dimethylformamide, dimethyl phosphate, 2-chloroethyl ethyl sulfide, and tricarbonyl-[eta5-1-methyl-2,4-cyclopentadien-1-yl]-manganese). The exception to our conclusion is that the yield of small singly-charged fragments resulting from a multiple ionization process in a subset of molecules, were found to be highly sensitive to the phase structure of the intense pulses. This coherent process plays a minimal role in photofragmentation; therefore, we consider it an exception rather than a rule. Changes in the fragmentation process are dependent on molecular structure, as evidenced in a number of isomers, therefore femtosecond laser fragmentation could provide a practical dimension to analytical chemistry techniques.     

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.