Two-photon absorption of dissociating TLI in strong laser fields

In a strong laser field (I=2 GW/cm²) TlI is dissociated by a two-photon process and the Tl fragment is detected state specifically. The yield of Tl 6p ² P _3/2 (Tl*) is measured as a function of the dissociation wavelength (480 nm–540 nm). If the dissociation wavelength is close to two-photon transitions of the Tl atom from 6p ² P _3/2 to np² P _1/2,3/2 or mf² F _5/2,7/2,n=10 ... 15,m=7 ... 12 dips in the yield are observed. These dips show a significant asymmetric broadening, compared to a free atomic transition and the observation is interpreted as an absorption process of the transient state of dissociating TlI during the laser pulse. By applying the Landau-Zener approximation for the potential crossing of dressed molecular states, we are able to describe the broadening and the power density dependence of the observations. Simulations show that the asymmetry is determined by the difference potential of the electronic states which are coupled by the laser field.     

Photoprocesses induced by short-wavelength excimer laser radiation in adsorbed halomethane molecules

Photolysis and desorption induced by ArF and KrCl laser irradiation of CH₂I₂, CH₃I, CCl₄, CHCl₃, CH₂Cl₂, CF₂Cl₂, and CHF₂Cl molecules adsorbed on fused silica were studied. One-photon fragmentation, as well as nonlinear dissociation and desorption processes, were observed. It was found that three-step fragmentation is a widespread phenomenon for molecules that experience resonant absorption of laser radiation. To explain the phenomenon, a mechanism based on stimulated emission was proposed. The nonlinear character of desorption is associated with redistribution of absorbed energy in adsorbed molecules and its transfer to the substrate.     

Resonance enhanced laser ionisation mass spectrometry of four aromatic molecules

Multiphoton ionisation and fragmentation of aniline, benzene, N,N-dimethyl and 2,4-dimethyl aniline has been studied by laser ionisation mass spectrometry under collision free conditions. All four molecules undergo efficient resonance-enhanced two-photon ionisation (R2PI) with relatively low laser intensities at λ = 266 nm producing the parent ion almost exclusively. At higher intensities, high order processes compete producing extensive fragmentation. At 266 nm, all the major fragment ions are produced by R3PI. For aniline excited at 294 nm, energetic considerations suggest R4P1 formation of fragments with differences in fragmentation between 266 and 294 nm reflecting the differing orders and energies above threshold of the competing processes. Comparison of R2P1 efficiencies in aniline and benzene shows that the cross sections for ionisation of the resonant intermediate ₁B² excited state in both molecules are approximately equal and independent of wavelength in the range 250–300 nm.     

Complete dynamics of a two-level quantum system in interaction with a radiation field

Using the general theory of systems of linear differential equations with periodic coefficients, we derive a complete set of solutions of the equations of motion of a two-level quantum system in interaction with a classical radiation field. The structure of the solutions is investigated and a method of obtaining approximate solutions is discussed. These solutions are compared quantitatively with those in the rotating-wave approximation in dependence of field amplitude, detuning from resonance, time of interaction between two-level system and field, and initial state of the two-level system. In an appendix it is shown that the semiclassical dynamics of a harmonically driven two-level system may be derived from an associated fully quantum-mechanical motion by an asymptotic limit.     

On the effect of a radiation field in modifying the intermolecular interaction between two chiral molecules

The change in the mutual energy of interaction between a pair of chiral molecules coupled via the exchange of a single virtual photon and in the presence of an electromagnetic field is calculated using nonrelativistic quantum electrodynamics. The particular viewpoint adopted is one that has an intuitive physical appeal and resembles a classical treatment. It involves the coupling of electric and magnetic dipole moments induced at each center by the incident radiation field to the resonant dipole-dipole interaction tensor. The energy shift is evaluated for fixed as well as random orientations of the molecular pair with respect to the direction of propagation of the field. A complete polarization analysis is carried out for the former situation by examining the effect of incident radiation that is linearly or circularly polarized and traveling in a direction that is parallel or perpendicular to the intermolecular distance vector. After tumble averaging, all polarization dependence of the energy shift vanishes. In both cases the interaction energy is directly proportional to the irradiance of the applied field, and is discriminatory, changing sign when one optically active species is replaced by its enantiomer. The asymptotic behavior of the energy shift at the limits of large and small separations is also studied.     

Droplet formation in a thin layer of a two-component solution under the thermal action of laser radiation

Droplet growth in thin layers of water-alcohol solutions under the effect of a concentration-induced capillary flow, which is governed by a laser electromagnetic radiation, is investigated. It is established that an increase in water concentration in a solution accelerates the droplet growth. In low-concentrated solutions, a droplet may reach a quasi-steady state, in which its diameter is no larger than the diameter of the light beam projection onto the layer. A rise in the radiation power above an optimal magnitude decelerates the droplet growth. In low-concentrated solutions, this deceleration results from a decrease in the flow that feeds the droplet, while in high-concentrated solutions, it is due to a droplet constriction and an increase in its curvature radius.     

Effect of cation absorption on ionization/dissociation of cycloketone molecules in a femtosecond laser field

The mass spectra of a series of cycloketone molecules, cyclopentanone (CPO), cyclohexanone (CHO), cycloheptanone (CHPO), and cyclooctanone (COO) are measured in a 788 or 394 nm laser field with 90 fs pulse duration and the intensity ranging from 5 x 10(13) W/cm(2) to 2 x 10(14) W/cm(2). At 788 nm, a dominated parent ion peak and some weak peaks from the fragment ions C(n)H(m)+ are observed for CPO and CHO (a ratio P(+)/T(+), the parent ion yield to the total ion yield, is 81.6% and 52.6%, respectively). But the extensive fragment ion peaks are observed with the greatly reduced parent ion peak for CHPO (P(+)/T(+) = 5.5%) and that are even hard to be identified for COO. These observations are interpreted explicitly in the frame of the significant resonant effect of their cation photoabsorption on ionization and dissociation of these molecules. The present work also suggests that a nonadiabatic ionization occurs with a nuclear rearrangement due to the H movement in these molecules during the ionization in an intense femtosecond laser field.     

Tunable fragmentation of organic molecules in laser ablation glow discharge time-of-flight mass spectrometry

A DC-pulsed glow discharge (GD) has distinct temporal regimes which are characterized by “softer” or “harder” ionization of analytes introduced into the discharge. It is thus possible to obtain both molecular weight and structural fragment information from the same spectra. In order to extend the capabilities of this technique a laser ablation (LA) sampling system was coupled to a DC-pulsed GD and to a time-of-flight (TOF) mass spectrometer (MS) for characterizing organic samples such as oleic acid, reserpine, two different peptides, and a polymer. Both hard and soft ionization regimes were studied. These LAGD-TOFMS results were compared to matrix-assisted laser desorption ionization (MALDI) spectra using the same compounds (i.e., analytes, concentration, and matrix). It was found that LAGD offers tunable ionization and provides a reduced matrix dependence. However, the sensitivity achieved by the prototype LAGD-TOFMS was significantly lower when compared with commercially available MALDI-TOFMS instrumentation. Since LAGD-TOFMS is rather new, some technical details to increase its sensitivity are discussed.     

Decomposition of pentaammineaquacobalt(III) perchlorate under laser radiation action

Photolytic decomposition of the complex [Co(NH₃)₅(H₂O)](ClO₄)₃ under the action of a laser with a wavelength of 355 nm, which is resonant in energy to the energy of the (¹ A _1g → ¹ T _2g ) d–d transition, was studied. Decomposition of the complex is accompanied by a release of ammonia with its subsequent oxidation to nitrogen oxides and by partial cobalt reduction with the formation of the mixed cobalt(II, III) oxide Co₃O₄.     

Surfactant-mediated matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of small molecules

A variety of surfactants have been tested as matrix-ion suppressors for the analysis of small molecules by matrix-assisted laser desorption/ionization time-of flight mass spectrometry. Their addition to the common matrix alpha-cyano-4-hydroxycinnamic acid (CHCA) greatly reduces the presence of matrix-related ions when added at the appropriate mole ratio of CHCA/surfactant, while still allowing the analyte signal to be observed. A range of cationic quaternary ammonium surfactants, as well as a neutral and anionic surfactant, was tested for the analysis of phenolics, phenolic acids, peptides and caffeine. It was found that the cationic surfactants, particularly cetyltrimethylammonium bromide (CTAB), were suitable for the analysis of acidic analytes. The anionic surfactant, sodium dodecyl sulfate, showed promise for peptide analysis. For trialanine, the detection limit was observed to be in the 100 femtomole range. The final matrix/surfactant mole ratio was a critical parameter for matrix ion suppression and resulting intensity of analyte signal. It was also found that the mass resolution of analytes was improved by 25-75%. Depth profiling of sample spots, by varying the number of laser shots, revealed that the surfactants tend to migrate toward the top of the droplet during crystallization, and that it is likely that the analyte is also enriched in this surface region. Here, higher analyte/surfactant concentration would reduce matrix-matrix interactions (known to be a source of matrix-derived ions).     

High-resolution mass spectrometry of large molecules in a linear time-of-flight mass spectrometer

This report describes a new high-resolution linear time-of-flight mass spectrometer that has been constructed at this institute. The instrument is used for investigations of both direct and matrix-assisted laser desorption/ionization of large molecules. A unique feature of this new instrument is the incorporation of a 10-cm postsource pulse-focusing region for enhancing the resolution of the detected ion signals. This technique can correct for both high initial ion translational energies and long durations of ion formation and is expected to be particularly well suited for laser desorption/ionization applications. Results of calculations are presented to illustrate the gains in mass resolution that may be expected for a variety of ion formation conditions. In addition, initial experimental results are presented that demonstrate the capability of this new instrument to produce high-resolution ion signals. Signals with mass resolutions as high as 2750 (full width at half maximum) have been obtained for both direct and matrix-assisted laser desorption/ionization signals.     

Matrix-assisted laser desorption/ionization of high-mass molecules by Fourier-transform mass spectrometry.

Following the first demonstrations of high-mass analysis using time-of-flight matrix-assisted laser desorption/ionization (MALDI) techniques by Hillenkamp, Tanaka and their co-workers, there have been significant efforts in a number of laboratories to adapt the new methodology to Fourier-transform mass spectrometry (FTMS). The motivation for this research is obvious. Namely, it would be desirable to couple the unparalleled high mass resolution of FTMS with the extended mass range provided by MALDI, particularly for analysis of polymers and biomolecules. Unfortunately, prior to the present work, attempts to mate FTMS and MALDI have met with limited success. The highest mass matrix-assisted laser-desorption-FTMS result previously obtained appears to be the unpublished low resolution spectrum of bovine insulin recently reported by Russell and co-workers. We, Campana and co-workers, and Hettich and Buchanan have had some success with MALDI-FTMS of biomolecules with masses lower than 3000 Da, including melittin, a variety of lower mass peptides, and oligonucleotides with masses lower than 1800 Da. Furthermore, with the single exception of Campana's report of obtaining mass resolution of 5000 for the molecular ion of melittin, such spectra have not displayed high resolution. Here, we report successful development of MALDI-FTMS, demonstrated with spectra obtained from a variety of high-mass polymer and biomolecule samples, using 355 nm radiation from an excimer-pumped dye laser for desorption/ionization and sinapinic acid as matrix. Some of these spectra are of much higher mass resolution than is possible with current time-of flight mass spectrometers.     

Spectroscopy of the polyatomic molecules,excited by an intense IR laser field

Using double IR resonance technique the transformation of the SF₆ linear spectrum while excited by fn intense laser field has been investigated. The saturation intensities for the transitions between excited vibrational states of the SF₆ have been measured. It has been found that the multiplephoton laser excitation is forming two molecular ensembles, one of these two is strongly excited along the ν₃ mode. It is shown that between excited vibrational states a fast collisional relaxation is taking place.     

Useful transformation of a radiation field

The paper deals with the characteristics of the radiation field, namely the flux density and current density. Two transformations of the general radiation field are presented which could considerably simplify the calculations in several situations.     

Characteristic values of a detector in a radiation field

Two characteristics, the detection crosssection and efficiency of the detector in a radiation field are discussed. Two particular forms of the radiation field, namely the monodirectional and isotropic, were also considered. The characteristic values for two scintillators in a photon beam and photon isotropic field are presented for illustration.     

The effects of laser field fluctuations on multiphoton excitation of a multilevel system

In this paper we analyze the effects of the fluctuations of laser amplitude on the multiphoton excitation of a sparse multilevel molecular system. It is found that contrary to the excitation by a laser with phase fluctuations, the stochastic contribution of the amplitude fluctuations is non-diagonal and scrambles the off-diagonal elements of the general molecular density matrix, resulting in temporal oscillations of these terms. The chaotic field model for the laser amplitude cannot force the applicability of the kinetic master equation for a general multilevel system. It is found that, for the two-level system driven by a chaotic field model laser, The stochastic interlevel rate dumps the oscillatory behaviour of the molecular populations, validating the master equation by T₁-type effects.     

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.