Laser control of reactions of photoswitching functional molecules

Laser control schemes of reactions of photoswitching functional molecules are proposed based on the quantum mechanical wave-packet dynamics and the design of laser parameters. The appropriately designed quadratically chirped laser pulses can achieve nearly complete transitions of wave packet among electronic states. The laser parameters can be optimized by using the Zhu-Nakamura theory of nonadiabatic transition. This method is effective not only for the initial photoexcitation process but also for the pump and dump scheme in the middle of the overall photoswitching process. The effects of momentum of the wave packet crossing a conical intersection on the branching ratio of products have also been clarified. These control schemes mentioned above are successfully applied to the cyclohexadiene/hexatriene photoisomerization (ring-opening) process which is the reaction center of practical photoswitching molecules such as diarylethenes. The overall efficiency of the ring opening can be appreciably increased by using the appropriately designed laser pulses compared to that of the natural photoisomerization without any control schemes.     

Quantification of the intensity enhancements for the double-pulse laser-induced breakdown spectroscopy in the orthogonal beam geometry

Simple- and double-pulse laser-induced breakdown spectroscopy was studied on aluminum samples at atmospheric pressure in air. The double-pulse experiments were carried out in the orthogonal beam geometry in two different ways: the reheating scheme and the pre-ablation spark dual-pulse scheme. An ablation laser emitting at 532 nm was combined with a second laser operating at 1064 nm according to the orthogonal geometry. For both schemes, the influence of the delay between the two laser pulses was investigated. In particular, different optima of interpulse delays were determined, underlying the differences of physical mechanisms involved in both processes. The estimation of the plasma temperatures provided explanations on the signal increases for both schemes. Whatever the configuration developed in the orthogonal geometry, a correlation between the increases in emission lines intensities and their excitation energy levels was established in the double-pulse approach. Besides, the effect of laser energy for both pulses was studied so as to make comparisons of the different configurations at the same total laser energy.     

LIF, LEI, RIMS etc. — New promising techniques in elemental micro- and trace analysis

Summary The application of laser spectroscopy in spectrochemistry requires excitation and detection schemes different from the estabolished optical spectrochemical methods. The most promising techniques developed in laser spectrochemistry are described and the analytical figures of merits are discussed. The very low absolute detection limits of the laser methods are required if trace elements have to be measured in microsamples.     

Stationary molecular wave packets at nonequilibrium nuclear configurations

We study different schemes that allow laser controlled adiabatic manipulation of the bond in diatomic molecules by using sequences of nonresonant time-delayed chirped pulses. The schemes rely on adiabatic passage of the vibrational wave packet by laser-induced potential shaping from the ground electronic state to a laser-stabilized dissociative electronic state by two-photon absorption. The degree of control that is possible over the position (bond length) and width (bond spread) of the vibrational wave packet is compared for the different schemes. The dynamics is analyzed detailing the role of the different control knobs and the conditions that allow or break the adiabatic passage.     

Wavelength-modulated fabry-perot interferometry for quantitation of trace gas components

The change in refractive index of a gas sample caused by molecular absorption is monitored by Fabry-Perot interferometry in a continuous flow system. To reduce the background noise level from other absorption processes, wavelength modulation is incorporated into the excitation laser. An enhancement in the signal-to-noise ratio of up to 146 s is found for the present method relative to intensity-modulated schemes or Stark-modulated schemes.     

Control of Li2 wave packet dynamics by modification of the quantum mechanical amplitude of a single state

Sequences of pulses with different spectra are used to control rotational wave packet dynamics in Li(2) by exploiting quantum interference phenomena. Wave packet superpositions are excited in a two-step resonant Raman process by two different pulses. Interferences between individual states shared by both wave packets can be used to enhance or destroy specific components of a superposition by varying the time delay between the pulses and/or the relative phase within the pulses. Elimination of selected quantum beats is achieved by greater than 94% for each case. A simple, yet effective, method for generating different color phase-locked pairs of laser pulses in a liquid-crystal pulse shaper setup without the need for interferometric stabilization schemes is described. The ability to manipulate single states of a superposition is an important advancement for intuitive control schemes and provides a potential new approach for initialization schemes in the field of quantum information.     

Even-parity auto-ionizing states of iridium I observed by resonance laser ionization spectroscopy

Auto-ionizing states of neutral iridium were observed in the continuum structure near the first ionization limit using one-color and two-color two-step resonance laser ionization spectroscopy. The total angular momentum of 20 even-parity auto-ionizing states could be determined from a combined analysis of the two-color spectra obtained with ionization schemes using intermediate states with different total angular momentum. Double-resonant ionization schemes were evaluated by fluence-dependence measurements, and photo-ionization cross-sections for resonant ionization transitions were determined. We could also identify several high-lying members of ns, np and nd Rydberg series converging to the first ionization limit of the atom.     

Two-color multiphoton decomposition of 1-bromo 2-fluoroethane

Multiphoton decomposition of 1-bromo 2-fluoroethane (CH₂BrCH₂F, BFE) with a two-color CO₂ infrared laser is reported as a function of number of laser pulses. For single color irradiation at a wavelength chosen to excite BFE, the decomposition product, vinyl fluoride, which accumulates during the many-pulse MPD experiment, is found to deactivate BFE decomposition. When a second laser, tuned to a vinyl fluoride absorption is introduced, coincident with the first, the vinyl fluoride is found to activate further BFE decomposition. These results are accounted for in terms of a pressure-dependent model. Changes in model parameters, upon addition of the second laser, are shown to be consistent with interpretations in terms of collision-dependent reaction schemes. © 1994 John Wiley & Sons, Inc.     

Effect of low intensity laser interaction with human skin fibroblast cells using fiber-optic nano-probes

Over the past forty years, many efforts have been devoted to study low power laser light interactions with biological systems. Some of the investigations were performed in-vitro, on bulk cell populations. Our present work was undertaken to apply specially engineered fiber-optic based nano-probes for the precise delivery of laser light on to a single cell and to observe production of low power laser light induced reactive oxygen species (ROS). A normal human skin fibroblast (NHF) cell line was utilized in this investigation and the cells were irradiated under two different schemes of exposure: (1) an entire NHF cell population within a Petri dish using a fan beam methodology, and (2) through the precise delivery of laser energy on to a single NHF cell using fiber-optic nano-probe. Photobiostimulative studies were conducted through variation of laser intensity, exposure time, and the energy dose of exposure. Laser irradiation induced enhancement in the rate of cell proliferation was observed to be dependent on laser exposure parameters and the method of laser delivery. The total energy dose (fluence) had a greater influence on the enhancement in the rate of cellular proliferation than compared to laser intensity. The enhancement in the growth rate was observed to have a finite life-time of several days after the initial laser exposure. Fluorescent life-time imaging of ROS was performed during the nano-based single cell exposure method. The kinetics of ROS generation was found to depend strongly on the laser fluence and not on the laser intensity.     

Side-entry excitation and detection of square capillary array electrophoresis for DNA sequencing.

In high throughput DNA sequencing based on capillary electrophoresis, efficient coupling of the laser to each capillary is a challenge. Our group previously reported two multiple point irradiation schemes. The present work describes a more efficient excitation and detection method in which the laser light propagates through the capillary array without undergoing a serious reduction in power. An array of square capillaries (340 microns O.D. x 75 microns I.D.) was sandwiched between two fused-silica plates with an index-matching solution in between. The light was directed into the channel across the capillary array from the side. DNA sequences of PGEM/U from 24 capillaries were obtained even with a relatively low-power laser. The excitation scheme can be scaled up to hundreds of capillaries to achieve high-speed, high-throughput DNA sequencing, genetic typing and drug screening.     

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.     

On the zirconium oxide neutral cluster distribution in the gas phase: detection through 118 nm single photon, and 193 and 355 nm multiphoton, ionization

Zirconium oxide clusters are generated in the gas phase by laser ablation of the metal into a flow of ca. 5% O2/95% He at 100 psig and supersonic expansion into a vacuum chamber. Mass spectra of neutral gas phase zirconium oxide clusters are obtained through photoionization at three different laser wavelengths: 118, 193, and 355 nm. Ionization of the clusters with 118 nm laser radiation is through a single photon ionization mechanism, while ionization by 193 and 355 nm laser radiation is through a multiphoton (three or more photon) mechanism. Fragment ion features are observed in the mass spectra of ZrmOn+ for only the 193 nm and 355 nm ionization schemes. The true neutral ZrmOn cluster distribution is obtained only through 118 nm single photon ionization, as verified by mass spectral peak linewidths and calculations of the cluster binding energies, ionization energies, and fragmentation rates. The neutral cluster distribution consists mainly of the series ZrmO2m and ZrmO(2m+1) for m = 1,..., approximately 30.     

Fourier transform-Raman spectroscopy using pulsed lasers—II

This paper compares and contrasts two signal processing schemes, viz. the fast filter and the gated integrator. It is shown that the use of a gated integrator further increases the SNR advantage over CW excitation, and extends the use of pulsed excitation to allow the recording of Raman spectra samples at significantly higher temperatures. The use of a ratioing circuit to correct for laser pulse-to-pulse energy fluctuations is also demonstrated.     

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.     

Macrokinetics of laser-induced chemical reactions in gases

This paper deals with the problem of laser radiation interaction with chemical gas. The review of numerical simulation results on the basis of three different models is presented. The main modes of the system behavior and the conditions, with various processes (diffusion, thermoconductivity, thermal diffusion, heat effect of the reaction and others) contributing to their realization, have been established. Theorems about the convergence of finite difference schemes have been proved.     

PHTHALOCYANINE FLUORESCENCE IN TUMORS DURING PDT

Athymic nude mice with human tumors transplanted to one of the hind legs were given aluminium phthalocyanine disulfonate (AlPcS2) intraperitoneally. Twenty-four hours after the injection the mice were placed with the tumor in the sample position in a fluorescence spectrometer with modulated excitation. Exposure of the tumors to laser light at a fluence rate of 50-200 mW/cm2 led to a rapid transient reduction by up to 50% of the phthalocyanine fluorescence of the tumor. After the laser irradiation the fluence rate of the fluorescence increased almost up to the initial value within a few minutes. This finding should be taken into account when optimal fluence rates and dose fractionation schemes are sought for photodynamic therapy.     

Progress in circular dichroism laser mass spectrometry

Circular dichroism in ion yield has promising new potentials for chiral analysis. Our progress of its development is described here. Circular dichroism in ion yield is achieved by resonance-enhanced multiphoton ionization. The feasibility of circular dichroism spectroscopy and quantitative determination of circular dichroism by this method is demonstrated. Several excitation schemes have been applied using different types of lasers, which vary in wavelength and repetition rate. Progress to improve the statistical error and thus the lower limit of measurable circular dichroism is described. This is achieved by adding achiral compounds or racemic mixtures of chiral compounds to the sample gas as reference substances and ionizing them by the same laser pulse. Therefore, in the mass spectrum of every single laser pulse, ion signals of sample and reference species appear both being subject to the same kind of instrumental fluctuations (in particular of laser pulse energy). In another approach, a laser repetition rate of 200 Hz allowed averaging of large numbers of laser pulses.      

The stimulated raman scattering process for possible use in photoselective isotope enrichment

The process of stimulated Raman scattering (SRS) is discussed as a means of achieving sizeable transient populations in the first vibrationally excited state of one species out of an isotopic mixture. This may be useful for photoselective isotope enrichment schemes. The main advantage of SRS excitation is the possibility of using one fixed-wavelength laser for the study of various compounds; also there is an inherent tendency to selectivity in SRS since the more abundant species in an isotopic mixture will show a higher gain for SRS. Sample calculations are presented showing Q-switched solid-state lasers to be good sources for selective excitation of gaseous mixtures, while mode-locked laser pulses are required for liquid samples.     

Optimization of detection of atomic absorption lines in intracavity laser spectroscopy

An intracavity laser spectrometer equipped with a graphite furnace electrothermal atomizer and two alternative types of narrow atomic lines detection schemes (high resolution diffraction spectrograph with optical multichannel analyzer or a resonant detector based on a hollow-cathode lamp) is described. Such system was used to determine ultra-trace amounts of lithium and strontium in aqueous solutions. A significant reduction in the measurable absorbance was demonstrated for both elements. Careful optimization of the operating conditions of the detection systems and a comparison of their typical features and advantages were performed.     

Optimization of detection of atomic absorption lines in intracavity laser spectroscopy

An intracavity laser spectrometer equipped with a graphite furnace electrothermal atomizer and two alternative types of narrow atomic lines detection schemes (high resolution diffraction spectrograph with optical multichannel analyzer or a resonant detector based on a hollow-cathode lamp) is described. Such system was used to determine ultra-trace amounts of lithium and strontium in aqueous solutions. A significant reduction in the measurable absorbance was demonstrated for both elements. Careful optimization of the operating conditions of the detection systems and a comparison of their typical features and advantages were performed.