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.     

Sensitized multiphoton dissociation of UF6 IN SF6-UF6 mixtures by a tea CO2 laser

UF₆ undergoes decomposition in the presence of SF₆ when mixtures of both are irradiated with a TEA CO₂ laser. The mechanism for UF₆ decomposition may involve vibrational energy transfer from excited SF₆ and laser absorption from the same laser pulse by excited UF₆ in its vibrational quasi-continuum     

Sub-picosecond laser ionization of free C60 and C70 at 248 nm

Delayed ionization is found to be absent for sub-picosecond laser excitation of free C₆₀ and C₇₀ at 248 nm. The autocorrelation trace obtained for C ₆₀ ⁺ in a laser time-of-flight (TOF) mass spectrometer using two time-delayed and collinear 248 nm ultrashort laser pulses has a width of 1.1 ps (715 fs for sech² pulses), in agreement with the laser pulse duration measurement in NO gas. Both above observations can be explained by direct ionization of C₆₀ via coherent two-photon absorption by the high intensity sub-picosecond 248 nm laser excitation avoiding the channel leading to delayed ionization.     

Laser-induced ablation of a steel sample in different ambient gases by use of collinear multiple laser pulses

The sensitivity of laser-induced breakdown spectroscopy of solid samples depends on the number of ablated and excited analytes. Laser ablation of solid samples can be enhanced by using collinear multiple laser pulses, for example double or triple pulses, rather than single laser pulses with the same total laser pulse energy. The ablation rates and the plasma conditions are affected by the ambient gas. In this study laser ablation was examined by varying the interpulse separation of the multiple pulses, within double and triple-pulse bursts, and the gas mass density at constant gas pressure. Different ambient gases and gas mixtures consisting of argon, oxygen, and nitrogen were used to study their effect on ablation rates. In a pure argon atmosphere (99.999% v/v Ar) the ablation burst number required to penetrate a steel plate of thickness 100 μm is reduced by a factor of approximately six by use of triple-pulse bursts with a symmetric interpulse separation of 15 μs rather than single pulses with the same total burst energy of 105 mJ. For double and single pulses the factors are 1.6 for Ar and 2.8 for synthetic air. Analyte lines are 4 to 8 times more intense if an argon atmosphere, rather than air, is used.     

Pressure and laser intensity effects on kinetic rates in binary reactive gas mixtures

The irradiation of binary reactive gaseous mixture by a CW CO₂ laser at changing pressure of absorber or transparent reactant have been performed at over-all pressures of several hundreds torrs with a maximum laser intensity of 680 W.cm⁻². A radial thermodiffusion effect leading both to a transient depletion of absorber concentration in the irradiation zone and a decrease of the rate of decomposition have been shown. The maximum yield of decomposition which appears at a given intensity and pressure of absorber when the pressure of reactant is changed is general for the binary gas mixtures tested ; it may be understood in terms of laser energy deposited in the mixture and distributed between the absorber and reactant partners. A rate decomposition dependent on the square of incident laser intensity and proceeding from this distribution is also exhibited.     

Surface modification of LDPE film by CO2 pulsed laser irradiation

The influence of the pulsed CO₂ laser irradiation on the surface structure of the LDPE film was investigated. Significant changes were observed on the surface of laser treated films as it was verified by the attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy and contact angle-measurement. Formation of polar functional groups onto the LDPE surfaces exhibited by the ATR-FTIR spectra was shown to be strongly dependent on the number of the CO₂ laser pulses. The intensity of the polar groups increased with increasing the number of pulses up to two and then slightly decreased at three laser pulses. This was also confirmed with the contact angle measurements in which the sample subjected to two laser pulses showed the highest wettability i.e. the lowest water drop contact angle. The concentration of peroxide groups formed on the surface of the laser treated films was determined quantitatively by UV spectroscopic method using iodide procedure. The latter results showed a similar trend with the results obtained using FTIR spectroscopy.     

Optical Nonlinearities of Phthalocyanines and Naphthalocyanines

The nonlinear absorption and refraction of soluble phthalocyanines and naphthalocyanines have been measured with z-scan technique using 8 ns, 532 nm laser pulses. The excited-state absorption cross-section and the excited-state refractive-index cross section have been estimated under the consideration of laser induced thermal effect. The excited state absorption induced n2 and Re χ⁽³⁾ values are in the range of 10^?12 esu. These observed data are enhanced by the concentrations of both metal substituted Pc's and Nc's, but this increment became very small for the metal free phthalocyanine. During the optical limiting experiments, a 25% linear transmitted solution of R₈PcH² can limit laser pulses to ≤ 120 μJ from incident energies as high as 2.0 mJ, and this limitation moved down to ≤ 50 μJ when replacing R₈PcCu from P₈PcH₂. A similar property has also been observed for the metal substituted naphthalocyanines. The results indicate that both the nonlinear refraction and optical limiting properties should be strongly affected by the metal induced triplet-state nonlinear absorption in both Pc's and Nc's. Moreover, the laser pulses created a self trapped filament inside the solution of this optical limiter, while emitting time-resolved, concentric rings on a projection screen, which could be induced by the thermal lensing effect. The various optical nonlinear properties observed from these materials should make them valuable as an optical limiter.     

Generation of shaped pulses for IR — laser chemistry

10 μm laser pulses with different temporal shape and a duration between 2 and 80 ns were produced in a TEA-CO₂ laser by the technique of a saturable absorber in an intracavity cell. At low absorber gas pressures (≤ 400 Pa) single longitudinal mode operation of the laser was obtained at a great number of CO₂ laser lines with 9 different selective absorbers. Stable mode locked operation has been achieved over the entire range of the CO₂ laser leading to pulses of 2 ns duration. Single peaks of the mode locked pulse train were sliced out by a fast CdTe electro-optical switch and amplified in a second CO₂ laser up to an energy of 2 J corresponding to an intensity of 1 GW. The spectral properties of the useful absorbers are presented systematically. As an application we show some quantitative results on the nonlinear intensity dependence of the IR-laser chemical reaction CF3B→^nhvCF₃ + Br in the transition range from case C to case B.     

Multiphoton ionization of magnesium and calcium atoms by short and intense laser pulses

Single and double ionization of magnesium and calcium atoms following Nd: YAG laser multiphoton excitation at 1064 and 532 nm have been studied by employing pulses of 35 ps and 200 ps duration at intensities of the order of 10¹⁰–2×10¹³ W/cm². The dependence of ion formation on the laser intensity has been measured and the slopes of the linear parts of the log-log plots and the ratios of saturation intensities for two pulse durations have been compared with the predictions of the scaling law. No evidence for a pure direct double ionization process has been obtained.     

Photofragmentation of C60

Photo-ionisation and -fragmentation ofC ₆₀ by 15 ns excimer laser pulses at 308 nm and 193 nm as well as 0.8 ps laser pulses at 193 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is ejection ofC _n,n>2, as opposed to successiveC ₂ evaporation. Studies of the relative intensities of metastable fragmentation processes compared with direct fragmentation provide new insight into the fragmentation mechanism and provide a thermometer for the internal energy ofC ₆₀ ⁺ prior to fragmentation. The proposed mechanism is in agreement with measurements of the fragment ion kinetic energies. The results are compared with molecular dynamics simulations.     

Picosecond time scale optical coherence experiments in mixed molecular crystals: pentacene in naphthalene

Picosecond time scale high-power pulse optical coherence measurements including photon echo and the stimulated photon echo are obtained with a mode-locked dye laser synchronously pumped by a double Q-switched and mode-locked Nd: YAG laser. Effects on coherence arising from excitation with gaussian laser pulses rather than square pulses are examined. Preliminary echo decay measurements are reported.     

Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-excited atomic fluorescence spectrometry

A diode laser is used for the selective excitation of ²³⁵U and ²³⁸U in a laser-induced plasma applying Nd:YAG laser pulses to UO₂ samples. The diode laser is rapidly scanned immediately following each laser sampling and the resonance atomic fluorescence spectrum for both isotopes is obtained on a pulse-to-pulse basis. Time-integrated measurements, with the diode laser fixed at either isotope, were also made. Optimum signal-to-noise was obtained at a distance of 0.8 cm from the sample surface, a pressure of 0.9 mbar and a Nd:YAG laser pulse energy of 0.5 mJ (880 MW cm⁻²). Three samples with 0.204, 0.407 and 0.714% ²³⁵U were measured. For example, for the UO₂ pellet with the natural uranium isotopic composition (99.281% ²³⁸U and 0.714% ²³⁵U), the accuracy and precision were 7% and 5% (460 shots), respectively, limited by the continuum emission background from the laser-induced plasma.     

Excimer Laser Crystallization of SnO2:Sb Sol-Gel Films

Instead of classical or rapid thermal annealing, KrF excimer laser irradiation has been successfully applied to crystallize dried SnO₂:Sb films elaborated by a sol-gel process. The penetration of the crystallization front below the film surface, as imaged by transmission electron microscopy, is controlled by the laser fluence and the number of pulses and can thus be confined in the film itself without affecting sensitive substrates. All films laser irradiated at fluences higher than 40 mJ/cm² become conductive. At constant laser fluence, the electrical sheet resistance goes through a minimum with increasing number of pulses. The consequence of film's densification and morphology on electrical properties is discussed.     

The photolysis of benzyltin compounds investigated by 1H CIDNP

The photochemical decomposition of (PhCH₂)₃SnMe and (PhCH₂)₃SnCl has been investigated by steady-state and time-resolved chemically induced nuclear polarization (CIDNP) ¹H spectroscopy of the methylene protons using 308 nm pulses of an excimer laser and a 250 MHz NMR spectrometer. From the sign of the polarization it is concluded that (PhCH₂)₃SnMe reacts like other comparable tin compounds via triplet radical pairs; the CIDNP effects are not influenced by the solvent. In contrast the CIDNP effects of (PhCH₂)₃SnCl are solvent dependent: in C₆D₆ the polarization is accounted for by a singlet radical pair precursor, while in CDCl₃ both singlet and triplet radical pairs are formed. The formation of singlet radical pairs during the reaction of (PhCH₂)₃SnCl in C₆D₆ may be interpreted as evidence for stannylene formation.     

13C separation by IRMPD of CF2HCL molecules with a powerful CO2-TEA laser

The single pulse separation parameters in IRMPD of CF₂HCL molecules have been obtained in relation with the laser fluence, the number of laser pulses, and the gas pressure. The measurements have been carried out using a fixed laser line (9P22) and an ordinary shape of the laser pulses.     

The Effects of Octanol on the Late Photointermediates of Rhodopsin

Membrane suspensions of unperturbed rhodopsin and rhodopsin perturbed with 2.5 niM octanol were photo-lyzed with 477 nm laser pulses at 20^OC and 35°C. Changes in absorbance were monitored at times ranging from 1 u-s to 80 ms after excitation. The data were analyzed using singular value decomposition, global exponential fitting and kinetic modeling. A recently proposed model involving the photointermediate Meta-I₃₈₀ (T. E. Thorgeirsson, J. W. Lewis, S. E. Wallace-Williams, and D. S. Kliger, Biochemistry 32,13861–13872, 1993) fits data for samples with and without octanol. Comparison of the microscopic rates shows this alcohol accelerates the formation of Meta-II via Meta-I₃₈₀. Activation and equilibrium thermodynamic parameters obtained from Ar-rhenius plots suggest that octanol reduces the entropy increase in forming both Meta-I_3g0 and Meta-II. It also lowers the enthalpy of Meta-1, _SI, relative to Lumi and of Meta-II relative to Meta-I₄₈₀. To help determine whether octanol affects the protein directly or indirectly through the lipid bilayer, similar experiments were conducted using rhodopsin solubilized in 0.13% dodecyl maltoside with and without octanol. Spectral shifts in the presence of octanol suggest that a direct protein interaction exists in addition to previously reported effects dependent on membrane free volume.     

Laser intensity effects in the IR multiple-photon absorption of OsO4

The visible luminescence resulting from multiple-photon absorption of CO₂ laser radiation in OsO₄ depends upon the laser intensity as well as fluence. The use of single-mode laser pulses, shaped by electro-optic crystal switching, has enabled this intensity dependence to be determined quantitatively.     

29Si and 30Si enrichment by IR MPD of Si2F6

The infrared multiple-photon decomposition of Si₂F₆ has been studied as a function of irradiation wavenumber using a collimated pulsed beam from a CO₂ TEA laser. The main decomposition product SiF₄ was found to be enriched with ²⁹Si and ³⁰Si. For example, SiF₄ contained 6% ²⁹Si and 46% ³⁰Si in the irradiation of 2 Torr natural Si₂F₆ with the laser radiation at 949.43 cm⁻¹ and 0.36 J cm⁻².     

Isotope selective infrared laser-induced unimolecular dissociation of dimeric bis (1,1,1.5,5,5-hexafluoropentane-2,4-dionato) dioxouranium (vi)

The uranium isotope selective gas phase decomposition of [UO₂ (hfacac)₂]₂ has been measured following absorption of infrared laser radiation by either of two strong absorption features near 10.6 μm These results show that isotopic selectivity can be attained with high yield at Ion laser fluence.     

Infrared multiphoton excitation of small molecules

The collisionless infrared excitation by short CO₂ laser pulses of the molecules SO₂, OCS, NO₂, NH₃ and DN₃ is compared with that of larger molecules. The average number of photons absorbed per molecule and the fraction of molecules dissociated depends predominantly on the laser intensity, while for larger molecules with higher densities of vibrational states the excitation is primarily determined by the laser fluence.