Supersonic Jet/Time-of-flight mass spectrometry of adenine using nanosecond and femtosecond lasers

Mass spectra and ionization efficiencies of adenine were measured with nanosecond (15 ns) and femtosecond (500 fs) laser pulses at identical energy levels. A molecular ion is clearly observed in both spectra, but the efficiency is improved 10-fold when a femtosecond laser is used, indicating the distinct advantage of ultrashort laser pulses for multiphoton ionization of nucleobases in supersonic jet spectrometry.     

Ultraviolet femtosecond laser ionization mass spectrometry

For this study, multiphoton ionization/mass spectrometry using an ultraviolet (UV) femtosecond laser was employed for the trace analysis of organic compounds. Some of the molecules, such as dioxins, contain several chlorine atoms and have short excited-state lifetimes due to a "heavy atom" effect. A UV femtosecond laser is, then, useful for efficient resonance excitation and subsequent ionization. A technique of multiphoton ionization using an extremely short laser pulse (e.g., <10 fs), referred to as "impulsive ionization," may have a potential for use in fragmentation-free ionization, thus providing information on molecular weight in mass spectrometry.     

Metal particles produced by laser ablation for ICP-MS measurements

Pulsed laser ablation (266 nm) was used to generate metal particles of Zn and Al alloys using femtosecond (150 fs) and nanosecond (4 ns) laser pulses with identical fluences of 50 J cm⁻². Characterization of particles and correlation with inductively coupled plasma mass spectrometer (ICP-MS) performance was investigated. Particles produced by nanosecond laser ablation were mainly primary particles with irregular shape and hard agglomerates (without internal voids). Particles produced by femtosecond laser ablation consisted of spherical primary particles and soft agglomerates formed from numerous small particles. Examination of the craters by white light interferometric microscopy showed that there is a rim of material surrounding the craters formed after nanosecond laser ablation. The determination of the crater volume by white light interferometric microscopy, considering the rim of material surrounding ablation craters, revealed that the volume ratio (fs/ns) of the craters on the selected samples was approximately 9 (Zn), 7 (NIST627 alloy) and 5 (NIST1711 alloy) times more ablated mass with femtosecond pulsed ablation compared to nanosecond pulsed ablation. In addition, an increase of Al concentration from 0 to 5% in Zn base alloys caused a large increase in the diameter of the particles, up to 65% while using nanosecond laser pulses. When the ablated particles were carried in argon into an ICP-MS, the Zn and Al signals intensities were greater by factors of ∼50 and ∼12 for fs versus ns ablation. Femtosecond pulsed ablation also reduced temporal fluctuations in the ⁶⁶Zn transient signal by a factor of 10 compared to nanosecond laser pulses.     

Molecular dynamics simulations of MALDI: laser fluence and pulse width dependence of plume characteristics and consequences for matrix and analyte ionization

Molecular dynamics simulations of matrix‐assisted laser desorption/ionization were carried out to investigate laser pulse width and fluence effects on primary and secondary ionization process. At the same fluence, short (35 or 350 ps) pulses lead to much higher initial pressures and ion concentrations than longer ones (3 ns), but these differences do not persist because the system relaxes toward local thermal equilibrium on a nanosecond timescale. Higher fluences accentuate the initial disparities, but downstream differences are not substantial. Axial velocities of ions and neutrals are found to span a wide range, and be fluence dependent. Total ion yield is only weakly dependent on pulse width, and consistent with experimental estimates. Secondary reactions of matrix cations with analyte neutrals are efficient even though analyte ions are ablated in clusters of matrix. Copyright © 2010 John Wiley & Sons, Ltd.     

Observation of Photoelectron Circular Dichroism Using a Nanosecond Laser

Photoelectron circular dichroism (PECD) is a fascinating phenomenon both from a fundamental science aspect but also due to its emerging role as a highly sensitive analytic tool for chiral recognition in the gas phase. PECD has been studied with single-photon as well as multi-photon ionization. The latter has been investigated in the short pulse limit with femtosecond laser pulses, where ionization can be thought of as an instantaneous process. In this contribution, we demonstrate that multi-photon PECD still can be observed when using an ultra-violet nanosecond pulse to ionize chiral showcase fenchone molecules. Compared to femtosecond ionization, the magnitude of PECD is similar, but the lifetime of intermediate molecular states imprints itself in the photoelectron spectra. Being able to use an industrial nanosecond laser to investigate PECD furthermore reduces the technical requirements to apply PECD in analytical chemistry.     

LIBS using dual- and ultra-short laser pulses

Pre-ablation dual-pulse LIBS enhancement data for copper, brass and steel using ns laser excitation are reported. Although large enhancements are observed for all samples, the magnitude of the enhancement is matrix dependent. Whereas all of the dual-pulse studies used ns laser excitation we see interesting effects when using ps and fs laser excitation for single-pulse LIBS. LIBS spectra of copper using 1.3 ps and 140 fs laser pulses show much lower background signals compared to ns pulse excitation. Also, the atomic emission decays much more rapidly with time. Because of relatively low backgrounds when using ps and fs pulses, non-gated detection of LIBS is shown to be very effective. The plasma dissipates quickly enough using ps and fs laser pulses, that high pulse rates, up to 1,000 Hz, are effective for increasing the LIBS signal, for a given measurement time. Finally, a simple near-collinear dual-pulse fiber-optic LIBS probe is shown to be useful for enhanced LIBS measurements.     

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.     

UV laser removal of varnish on tempera paints with nanosecond and femtosecond pulses

Two laser cleaning approaches based on ablation by ultraviolet laser pulses of femtosecond (fs) and nanosecond (ns) durations for the removal of shellac varnish from egg-yolk based tempera paints are investigated. Laser irradiation effects, induced on the varnish layer and on the underlying temperas by multiple pulses in the fs domain at 398 and 265 nm and single pulses in the ns domain at 213 nm, were examined following a spectroanalytical approach. By using optical microscopy, colorimetry and laser induced fluorescence it was found that irradiation of the varnished temperas with fs pulses changes the texture of the varnish surface and results in degradation of the underlying coloured paint. In contrast, operating with pulses of 15 ns at the highly absorbed wavelength of 213 nm, controlled micrometric layer removal of the varnish is possible without noticeable modification of the coloured temperas. These results widen the choice of laser conditions for painting restoration.     

LIBS using dual- and ultra-short laser pulses

Pre-ablation dual-pulse LIBS enhancement data for copper, brass and steel using ns laser excitation are reported. Although large enhancements are observed for all samples, the magnitude of the enhancement is matrix dependent. Whereas all of the dual-pulse studies used ns laser excitation we see interesting effects when using ps and fs laser excitation for single-pulse LIBS. LIBS spectra of copper using 1.3 ps and 140 fs laser pulses show much lower background signals compared to ns pulse excitation. Also, the atomic emission decays much more rapidly with time. Because of relatively low backgrounds when using ps and fs pulses, non-gated detection of LIBS is shown to be very effective. The plasma dissipates quickly enough using ps and fs laser pulses, that high pulse rates, up to 1000 Hz, are effective for increasing the LIBS signal, for a given measurement time. Finally, a simple near-collinear dual-pulse fiber-optic LIBS probe is shown to be useful for enhanced LIBS measurements. Received: 1 August 2000 / Revised: 2 November 2000 / Accepted: 8 November 2000     

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.     

Experimental and theoretical investigation of high-power laser ionization and dissociation of methane

The main purpose of this paper is to report the high-power laser ionization-dissociation of CH(4) at various femtosecond (fs) laser intensities (from 1 x 10(14) W/cm(2) to 2 x 10(15) W/cm(2)) with a laser pulse duration of 48 fs. The generalized molecular Keldysh theory has been applied to calculate the ionization yields for CH(4)+ and CH(4)++. Outside the influence of the fs intense laser, we propose to calculate the mass spectra due to the decomposition of CH(4)+ and CH(4)++, using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The agreement between the experimental mass spectra and calculated mass spectra seems to be reasonable.     

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.     

Time-of-flight mass spectroscopy of femtosecond and nanosecond laser ablated TeO2 crystals

Single-pulse femtosecond (fs) (pulse duration ～200 fs, wavelength 398 nm) and nanosecond (ns) (pulse duration 4 ns, wavelength 355 nm) laser ablation have been applied in combination with time-of-flight mass spectrometer (TOFMS) to analyze the elemental composition of the plasma plume of single-crystalline telluria (c-TeO₂, grown by the balance controlled Czochralski growth method). Due to the three-order difference of the peak intensities of the ns and fs-laser pulses, significant differences were observed regarding the laser-induced species in the plasma plume. Positive singly, doubly and triply charged Te ions (Te⁺, Te₂⁺, Te₃⁺) in the form of their isotopes were observed in case of both irradiations. In case of the ns-laser ablation the TeO⁺ formation was negligible compared to the fs case and there was no Te trimer (Te₃⁺) formation observed. It was found that the amplitude of Te ion signals strongly depended on the applied laser pulse energy. Singly charged oxygen ions (O⁺) are always present as a byproduct in both kinds of laser ablation.     

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.     

Control of branching ratios in the dissociative ionization of deuterium chloride

The dissociative ionization of deuterium chloride (DCl) has been investigated by employing femtosecond laser pulses at 805 nm. The product branching ratio D(+)/Cl(+) of the fragments D(+) and Cl(+) is strongly affected by the chirp alpha of the laser pulses. The ratio can be controlled by a factor of 3 ranging from D(+)/Cl(+) = 0.7 at alpha = -800 fs(2) to D(+)/Cl(+) = 1.9 at alpha = +150 fs(2). The observation can be rationalized by a model where negative chirp favors intra-electronic state excitation, and positive chirp favors inter-electronic state excitation in the dissociation of the molecular ion. Complementary experiments on hydrogen chloride (HCl) are discussed.     

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.     

Dynamic alignment of C2H4 investigated by using two linearly polarized femtosecond laser pulses

We have studied multielectron ionization and Coulomb explosion of C2H4 irradiated by 110 fs, 800 nm laser pulses at an intensity of approximately 10(15) W/cm2. Strong anisotropic angular distributions were observed for the atomic ions Cn+(n = 1-3). Based on the results of two crossed linearly polarized laser pulses, we conclude that such anisotropic angular distributions result from dynamic alignment, in which the rising edge of the laser pulses aligns the neutral C2H4 molecules along the laser polarization direction. The angular distribution of the exploding fragments, therefore, reflects the degree of the alignment of molecules before ionization. Using the same femtosecond laser with intensity below the ionization threshold, the alignment of C2H4 molecules was also observed.     

Excited-state relaxation of protonated adenine.

The excited state dynamics of protonated adenine in the gas phase were investigated by femtosecond pump-probe transient mass spectroscopy. Adenine was protonated in an electrospray ionization source and transferred to a Paul trap. Two femtosecond laser pulses at 266 nm and 800 nm excited the lowest electronic pipi* state and probed the excited-state dynamics by monitoring ion fragment formation. The measured excited state decay is monoexponential with a lifetime shorter than 161 fs. This agrees with a theoretical prediction of very fast internal conversion via a conical intersection with the ground state.     

Influence of polymer molecular weight on the chemical modifications induced by UV laser ablation

This paper investigates the influence of polymer molecular weight (M(W)) on the chemical modifications of poly(methyl methacrylate), PMMA, and polystyrene, PS, films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI), following irradiation at 248 nm (KrF excimer laser, 20 ns fwhm and hybrid excimer-dye laser, 500 fs fwhm) and at 308 nm (XeCl excimer laser, 30 ns fwhm). The changes of intensity and position of the polymer Raman bands upon irradiation provide information on cleavage of the polymer bonds. Degradation of PMMA, which is a weak absorbing system at 248 nm, occurs to a higher extent in the case of a larger M(W), giving rise to the creation of unsaturation centers and to degradation products. For highly absorbing PS, no degradation is observed upon irradiation with a KrF laser. Consistently irradiating doped PS at 308 nm, where the absorption is low, induces degradation of the polymer. Results provide direct support for the bulk photothermal model, according to which ejection requires a critical number of broken bonds. In the case of irradiation of doped PMMA with pulses of 248 nm and 500 fs, neither degradation nor dependence with polymer M(W) are observed, indicating that mechanisms involved in the femtosecond laser ablation differ from those operating in the case of nanosecond laser ablation. Participation of multiphoton/avalanche processes is proposed.     

Early stage emission spectroscopy study of metallic titanium plasma induced in air by femtosecond- and nanosecond-laser pulses

In this work the laser induced plasma obtained in air at atmospheric pressure by the interaction of a fs (femtosecond) or a ns (nanosecond) laser pulse with a metallic titanium target has been investigated by optical emission spectroscopy. The temporal evolution of plasma parameters such as electron number density and excitation temperature has been determined in order to highlight the processes involved when the emission spectra are acquired at short time delays from the ablating laser pulse. A survey of elementary processes implicated during plasma formation and expansion of ns- and fs-Laser Induced Plasma has been performed. Departures from equilibrium conditions are even discussed. The dynamic aspects corresponding to ns- and fs-LIP have been investigated by optical time of flight (TOF) and by fast emission imaging. The overall results have been used for clarifying the basic mechanisms occurring during plasma expansion due to either ns or fs laser source when experimental conditions usually used for laser-induced breakdown spectroscopy (LIBS) applications are employed.