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.     

Theoretical study of the photodissociation of Li(2)+ in one-color intense laser fields

A theoretical treatment of the photodissociation of the molecular ion Li(2) (+) in one-color intense laser fields, using the time-dependent wave packet approach in a Floquet Born-Oppenheimer representation, is presented. Six electronic states 1,2?(2)Σ(g)(+), 1,2?(2)Σ(u)(+), 1?(2)Π(g), and 1?(2)Π(u) are of relevance in this simulation and have been included. The dependences of the fragmental dissociation probabilities and kinetic energy release (KER) spectra on pulse width, peak intensity, polarization angle, wavelength, and initial vibrational level are analyzed to interpret the influence of control parameters of the external field. Three main dissociation channels, 1?(2)Σ(g)(+) (m = -1), 2?(2)Σ(g)(+) (m = -2), and 2?(2)Σ(u)(+) (m = -3), are seen to dominate the dissociation processes under a wide variety of laser conditions and give rise to well separated groups of KER features. Different dissociation mechanisms for the involved Floquet channels are discussed.     

Explosive desorption and fragmentation of molecular ion from solid fullerene by intense nonresonant femtosecond laser pulses

Desorption of C 60 (+) and its dimer cation was investigated on irradiation with nonresonant femtosecond laser pulses at 1.4 mum. Ionization of solid C 60 revealed strikingly different features, such as the absence of multiply charged molecular ions, the emission of C (+) at low laser intensity, C 2 attachments, delayed ionization, and dimer cation formation, as compared with the gas phase experiments. The large kinetic energy distribution of ions found in this study indicated that the desorption process was mainly driven by an electrostatic mechanism rather than by thermal, photochemical, or volume expansion mechanisms. Singly charged C 60 emission by a Coulomb explosion due to the high density of C 60 (+) is suggested.     

Probing the ultrafast nuclear motion in CS2 2+ in intense laser fields

The temporal evolution of the nuclear wave packet of CS2 2+ formed in an intense laser field (60 fs, 0.13 PW/cm2) is traced in real time by the pump-and-probe technique combined with coincidence momentum imaging of the Coulomb explosion process, CS2 3+-->S+ + C+ + S+. The momentum correlations among the fragment ions obtained as a function of the pump-probe time delay between 133 fs to 3 ps reveal that the nuclear wave packet in CS2 2+ evolves not only along the anti-symmetric stretching coordinate to yield S+ and CS+ but also along the symmetric stretching coordinate leading to the simultaneous breaking of the two C-S bonds. The contribution from two different electronic states having bent and linear-type geometrical configurations is identified in the wave packet motion along the bending coordinate of CS2 2+.     

Detection of OH radical in laser induced photodissociation of tetrahydrofuran at 193 nm

On excitation at 193 nm, tetrahydrofuran (THF) generates OH as one of the photodissociation products. The nascent energy state distribution of the OH radical was measured employing laser induced fluorescence technique. It is observed that the OH radical is formed mostly in the ground vibrational level, with low rotational excitation (approximately 3%). The rotational distribution of OH (v"=0,J) is characterized by rotational temperature of 1250+/-140 K. Two spin-orbit states, 2Pi3/2 and 2Pi1/2 of OH are populated statistically. But, there is a preferential population in Lambda doublet levels. For all rotational numbers, the 2Pi+(A') levels are preferred to the 2Pi-(A") levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 17.4+/-2.2 kcal mol-1, giving an fT value of approximately 36%, and the remaining 61% of the available energy is distributed in the internal modes of the other photofragment, i.e., C4H7. The observed distribution of the available energy agrees well with a hybrid model of energy partitioning, predicting an exit barrier of approximately 16 kcal mol-1. Based on both ab initio molecular orbital calculations and experimental results, a plausible mechanism for OH formation is proposed. The mechanism involves three steps, the C-O bond cleavage of the ring, H atom migration to the O atom, and the C-OH bond scission, in sequence, to generate OH from the ground electronic state of THF. Besides this high energy reaction channel, other photodissociation channels of THF have been identified by detecting the stable products, using Fourier transform infrared and gas chromatography.     

CO2 laser induced isomerization and photodissociation of 1,2-dichloroethylene

Infrared photoisomerization of both trans- and cis-1,2-dichloroethylene molecules sensitized by SF₆ has been observed by using a CO₂ laser. The direct infrared photoisomerization has also been observed for the trans molecule. The reaction rate of the sensitized photoisomerization increases by 6 times as compared with the direct process. The dielectric breakdown induced by an intense laser pulse enhances a dissociative reaction.     

Optimal laser control of ultrafast photodissociation of I2- in water: mixed quantum/classical molecular dynamics simulation

A linearized optimal control method in combination with mixed quantum/classical molecular dynamics simulation is used for numerically investigating the possibility of controlling photodissociation wave packets of I(2)(-) in water. Optimal pulses are designed using an ensemble of photodissociation samples, aiming at the creation of localized dissociation wave packets. Numerical results clearly show the effectiveness of the control although the control achievement is reduced with an increase in the internuclear distance associated with a target region. We introduce effective optimal pulses that are designed using a statistically averaged effective dissociation potential, and show that they semiquantitatively reproduce the control achievements calculated by using optimal pulses. The control mechanisms are interpreted from the time- and frequency-resolved spectra of the effective optimal pulses.     

Hydrogen scrambling in ethane induced by intense laser fields: statistical analysis of coincidence events

Two-body Coulomb explosion processes of ethane (CH(3)CH(3)) and its isotopomers (CD(3)CD(3) and CH(3)CD(3)) induced by an intense laser field (800 nm, 1.0 × 10(14) W/cm(2)) with three different pulse durations (40 fs, 80 fs, and 120 fs) are investigated by a coincidence momentum imaging method. On the basis of statistical treatment of the coincidence data, the contributions from false coincidence events are estimated and the relative yields of the decomposition pathways are determined with sufficiently small uncertainties. The branching ratios of the two body decomposition pathways of CH(3)CD(3) from which triatomic hydrogen molecular ions (H(3)(+), H(2)D(+), HD(2)(+), D(3)(+)) are ejected show that protons and deuterons within CH(3)CD(3) are scrambled almost statistically prior to the ejection of a triatomic hydrogen molecular ion. The branching ratios were estimated by statistical Rice-Ramsperger-Kassel-Marcus calculations by assuming a transition state with a hindered-rotation of a diatomic hydrogen moiety. The hydrogen scrambling dynamics followed by the two body decomposition processes are discussed also by using the anisotropies in the ejection directions of the fragment ions and the kinetic energy distribution of the two body decomposition pathways.     

Ab initio studies of ultrafast x-ray scattering of the photodissociation of iodine

We computationally examine various aspects of the reaction dynamics of the photodissociation and recombination of molecular iodine. We use our recently proposed formalism to calculate time-dependent x-ray scattering signal changes from first principles. Different aspects of the dynamics of this prototypical reaction are studied, such as coherent and noncoherent processes, features of structural relaxation that are periodic in time versus nonperiodic dissociative processes, as well as small electron density changes caused by electronic excitation, all with respect to x-ray scattering. We can demonstrate that wide-angle x-ray scattering offers a possibility to study the changes in electron densities in nonperiodic systems, which render it a suitable technique for the investigation of chemical reactions from a structural dynamics point of view.     

Formation of H3O+ from alcohols and ethers induced by intense laser fields

The processes of H₃O⁺ production from alcohols (ethanol, 2‐propanol, 1‐propanol, 2‐butanol) and ethers (diethyl ether and ethyl methyl ether), and their deuterium‐substituted species, by intense laser fields (800 nm, 100 fs, ～1 × 10¹⁴ W/cm) were investigated through time‐of‐flight (TOF) mass spectrometry. H₃O⁺ formation was observed for all these compounds except for ethyl methyl ether. From the analysis of TOF signals of H_(3?n)D_nO⁺ (n = 0, 1, 2, and 3) that have expanding tails with increasing flight time, it has been confirmed that the reaction proceeds through metastable dissociation from the intermediate species C₂H_(5?m)D_mO⁺(m = 0–5). The common shape of the H_(3?n)D_nO⁺ signal profiles contains two major distributions in the time constant, i.e., fast and slow components of <50 ns and ～500 ns, respectively. The H_(3?n)D_nO⁺ branching ratio is interpreted to be the result of complete scrambling of four hydrogen atoms at the C? C site in C₂H₄‐OH⁺, and partial exchange (18–38%) of a hydrogen atom in the OH group with four other hydrogen atoms within 1 ns prior to H_(3?n)D_nO⁺ production. Ab initio calculations for the isomers and transition states of C₂H₅O⁺ were also performed, and the observed H_(3?n)D_nO⁺ production mechanism has been discussed. In addition, a stable isomer having a complex structure and two isomerization pathways were discovered to contribute to the H₃O⁺ formation process. Copyright © 2010 John Wiley & Sons, Ltd.     

The primary step in the ultrafast photodissociation of the methyl iodide dimer

This paper shows the results of combined experimental and theoretical work that have unravelled the mechanism of ultrafast ejection of a methyl group from a cluster, the methyl iodide dimer (CH(3)I)(2). Ab initio calculations have produced optimized geometries for the dimer and energy values and oscillator strengths for the excited states of the A band of (CH(3)I)(2). These calculations have allowed us to describe the blue shift that had been observed in the past in this band. This blue shift has been experimentally determined with higher precision than in all previously reported experiments, since it has been measured through its effect upon the kinetic energy release of the fragments using femtosecond velocity map imaging. Observations of the reaction branching ratio and of the angular nature of the fragment distribution indicate that two main changes occur in A-band absorption in the dimer with respect to the monomer: a substantial change in the relative absorption to different states of the band, and, more importantly, a more efficient non-adiabatic crossing between two of those states. Additionally, time resolved experiments have been performed on the system, obtaining snapshots of the dissociation process. The apparent retardation of more than 100 fs in the dissociation process of the dimer relative to the monomer has been assigned to a delay in the opening of the optical detection window associated with the resonant multiphoton ionization detection of the methyl fragment.     

Ejection of high-energy photoelectrons by intense laser light

The mechanism by which a high-energy photoelectron acquires its large drift momentum is analyzed. Ejection by circularly polarized light is found to be most efficient, in accord with experimental observations.     

Ultrafast hydrogen scrambling in methylacetylene and methyl-d3-acetylene ions induced by intense laser fields

Three-body Coulomb explosion processes of triply charged positive ions of methylacetylene (CH(3)-C≡C-H) and its isotopomer, methyl-d(3)-acetylene (CD(3)-C≡C-H), induced by an ultrashort intense laser field (790 nm, ～40 fs, 5.0 × 10(13) W cm(-2)) are investigated by the coincidence momentum imaging method. Two types of three-body decomposition processes accompanying the ejection of a proton are identified for methylacetylene, and six types of three-body decomposition processes accompanying the ejection of a proton or a deuteron are identified for methyl-d(3)-acetylene. From the observed momentum vectors of all the three fragment ions for each decomposition pathway, the proton and deuteron distributions are constructed in the coordinate space, and the hydrogen migration processes are investigated. It was shown that the hydrogen migration proceeds more efficiently from the methyl group than from the methine group. In addition to the decomposition pathways accompanying the migration of one H (or D) atom, the decomposition pathways accompanying the migration of two light atoms (H/D exchange and 2D migration) are identified. Furthermore, the decomposition pathways ascribable to the migration of three light atoms (H/D exchange followed by D migration) are identified, showing the high intramolecular mobilities of H and D atoms within methylacetylene and methyl-d(3)-acetylene in an intense laser field, resulting in the H/D scrambling.     

Theory of three-dimensional alignment by intense laser pulses

We introduce a theoretical framework for study of three-dimensional alignment by moderately intense laser pulses and discuss it at an elementary level. Several features of formal interest are noted and clarified. Our approach is nonperturbative, treating the laser field within classical and the material system within quantum mechanics. The theory is implemented numerically using a basis set of rotational eigenstates, transforming the time-dependent Schrodinger equation to a set of coupled differential equations where all matrix elements are analytically soluble. The approach was applied over the past few years to explore different adiabatic and nonadiabatic three-dimensional alignment approaches in conjunction with experiments, but its formal details and numerical implementation were not reported in previous studies. Although we provide simple numerical examples to illustrate the content of the equations, our main goal is to complement previous reports through an introductory discussion of the underlying theory.     

Unveiling the nonadiabatic rotational excitation process in a symmetric-top molecule induced by two intense laser pulses

We experimentally investigate the nonadiabatic rotational excitation process of a symmetric-top molecule, benzene, in the electronic ground state irradiated by intense nonresonant ultrafast laser fields. The initial rotational-state distribution was restricted mostly to the five lowest levels with different nuclear spin modifications by an extensive adiabatic cooling with the rotational temperature well below 1 K, and distributions after the interaction with a femtosecond double-pulse pair (3-5 TW/cm(2) each with 160 fs duration) with time delays were probed in a quantum-state resolved manner by employing resonant enhanced multiphoton ionization via the S(1) ← S(0) 6(0) (1) vibronic transition. Populations of 10 rotational levels with J ranging from 0 to 4 and K from 0 to 3 were examined to show an oscillatory dependence on the time delay between the two pulses. Fourier analysis of the beat signals provides the coupling strengths between the constituent levels of the rotational wave packets created by the nonadiabatic excitation. These data are in good agreement with the results from quantum mechanical calculations, evidencing stepwise excitation pathways in the wave packet creation with ΔJ = 2 in the K = 0 stack while ΔJ = 1 and 2 in the K > 0 stacks.     

Vibrational response of nanorods to ultrafast laser induced heating: theoretical and experimental analysis

In this paper, we elucidate the vibrational response of cylindrical nanorods to ultrafast laser-induced heating. A theoretical analysis of the expected behavior is first presented. This analysis predicts that both extensional and breathing vibrational modes of the rods should be excited by laser-induced heating. Analytical formulas are derived assuming that the heating/expansion process is instantaneous, and that the lengths of the rods are much greater than their radii. These results show that the breathing mode dominates the mechanical deformation of the rod. However, because the frequency of the extensional mode is much lower than that of the breathing mode, the extensional mode will dominate the response for a real experiment (a finite-time heating/expansion process). The results of this model are compared to data from transient absorption experiments performed on gold nanorods with average lengths between 30 and 110 nm. The transient absorption traces show pronounced modulations with periods between 40 and 120 ps, which are only observed when the probe laser is tuned to the longitudinal plasmon band. The measured periods are in good agreement with the expected values for the extensional modes of the rods. For rods wider than 20 nm, the breathing mode can also be observed and, again, the measured periods are in good agreement with the theoretical calculations. The breathing mode is not observed for thinner rods (<20 nm width) because, in this case, the period is comparable to the time scale for lattice heating.     

On the limitations of adiabatic population transfer between molecular electronic states induced by intense femtosecond laser pulses

The possibility to perform a stimulated Raman adiabatic passage process in molecules on the ultrafast time scale is investigated theoretically. Motivated by recent experiments, the mid R:B<--mid R:X electronic transitions in molecular iodine are studied as a prototype example with the goal to selectively induce a population transfer employing two intense and time-delayed ultrashort laser pulses and different coupling schemes. For the purpose of interpretation, the coupled multilevel vibronic problem is reduced to a quasi-three-level system by averaging over the vibrational degree of freedom. It is shown that the vibrational dynamics becomes essential at high field intensities. Considering a 2-dimensional parameter space (intensity and delay time of the femtosecond laser pulses), a strong-field control landscape is constructed.     

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.