Ionization and fragmentation of solid C60 by femtosecond laser ablation

Ionization and fragmentation of solid C(60) dispersed on a silicon plate are investigated by femtosecond laser ablation. Bimodal mass distribution with large fragment ions C(60-2n) (+) (0< or =n< or =11) and small fragment ions C(n) (+) (13< or =n< or =28), formation of dimer ion (C(60))(2) (+), and delayed ionization of C(60) have been observed as reported in gas phase experiments with nanosecond laser excitation. Metastable dissociation of small fragment ions C(n) (+) has been observed for the first time, which suggests different structures of fragment ions compared with those of well-studied carbon cluster ions. From these observations, strong coupling of laser energy to electronic degrees of freedom of solid C(60) has been revealed for femtosecond laser ablation as compared with excitation in the gas phase.     

Study on the carbon fragment anions produced by femtosecond laser ablation of solid C60

Production of the anions (negative ions) has been observed by femtosecond laser ablation (fsLA) of solid C(60) with a time-of-flight (TOF) mass spectrometer. In contrast to C(60)(+), production of C(60)(-) due to an electron capture is found very limited because of the small electron affinity of the C(60) molecule. Narrow TOF peaks of small carbon fragment anions C(n)(-) (n ≤ 23) suggest instantaneous production of the fragment anions through dissociative ionization of C(60). Production of the mono-hydrogenated carbon fragment anions C(n)H(-) has been observed and also the abrupt change in the yield of C(n)H(-) has been observed at n = 10, which is attributed to the structural change of the carbon fragments from a linear chain to a monocyclic ring. The results are found similar to those obtained for the carbon fragments produced by nanosecond laser ablation (nsLA) of solid C(60), which demonstrates that the thermalization in an ablation plasma washes away any difference in the nature of carbon fragments produced by fsLA and nsLA.     

N2+2离子在线偏振和圆偏振强激光场中的解离

基于N+离子的飞行时间质谱, 研究了N2+2离子在线偏振和圆偏振强飞秒激光场中(45 fs, 5×1015-1×1016 W·cm-2, 800 nm)的解离. 通过对N+离子质谱和平动能的分析发现, N2+2离子在线偏振光和圆偏振光作用下具有不同的解离方式. 在线偏振光下, N2分子在平衡核间距RE处发生次序双电离生成N2+2离子, N2+2离子解离所释放的能量能够用单光子跃迁模型来解释. 而在圆偏振光下, N2分子首先电离生成N+2离子, N+2离子在核间距增大到临界核间距RC(>RE)时, 进一步被电离从而发生解离, 此时解离所释放的能量可以用库仑推斥模型来解释.     

Theoretical analysis of correlation between ionization threshold fluence in IR‐MALDI and IR absorption spectrum of matrix molecules

To investigate the correlation between the wavelength dependence of ionization threshold fluence of target molecule in matrix‐assisted laser desorption/ionization by infrared (IR) laser and the IR absorption spectrum of matrix molecule, we have analyzed the IR absorption spectra of four matrix molecules using density functional theory and correlated ab initio molecular orbital method. The calculated IR absorption spectra of the isolated molecules showed more qualitative correlation with the wavelength dependence of ionization threshold fluence than those of the solid state structures. We can consider that a portion of matrix molecules lost the ordered crystal structure and that the transition to the diluted or isolated state occurred at the early process of IR laser irradiation. © 2012 Wiley Periodicals, Inc.     

Polarization dependent fragmentation of ions produced by laser desorption from nanopost arrays

Tailored silicon nanopost arrays (NAPA) enable controlled and resonant ion production in laser desorption ionization experiments and have been termed nanophotonic ion sources (Walker et al., J. Phys. Chem. C, 2010, 114, 4835-4840). As the post dimensions are comparable to or smaller than the laser wavelength, near-field effects and localized electromagnetic fields are present in their vicinity. In this contribution, we explore the desorption and ionization mechanism by studying how surface derivatization affects ion yields and fragmentation. We demonstrate that by increasing the laser fluence on derivatized NAPA with less polar surfaces that have decreased interaction energy between the structured silicon substrate and the adsorbate, the spectrum changes from exhibiting primarily molecular ions to showing a growing variety and abundance of fragments. The polarization angle of the laser beam had been shown to dramatically affect the ion yields of adsorbates. For the first time, we report that by rotating the plane of polarization of the desorption laser, the internal energy of the adsorbate can also be modulated resulting in polarization dependent fragmentation. This polarization effect also resulted in selective fragmentation of vitamin B(12). To explore the internal energy of NAPA generated ions, the effect of the post aspect ratios on the laser desorption thresholds and on the internal energy of a preformed ion was studied. Elevated surface temperatures and enhanced near fields in the vicinity of high aspect ratio posts are thought to contribute to desorption and ionization from NAPA. Comparison of the fluence dependence of the internal energies of ions produced from nanoporous silicon and NAPA substrates indicates that surface restructuring or transient melting by the desorption laser is a prerequisite for the former but not for the latter.     

Deep‐ultraviolet laser ablation electrospray ionization mass spectrometry

A 193‐nm wavelength deep ultraviolet laser was used for ambient laser ablation electrospray ionization mass spectrometry of biological samples. A pulsed ArF excimer laser was used to ablate solid samples, and the resulting plume of the desorbed material merged with charged electrospray droplets to form ions that were detected with a quadrupole time‐of‐flight mass spectrometer. Solutions containing peptide and protein standards up to 66‐kDa molecular weight were deposited on a metal target, dried, and analyzed. No fragmentation was observed from peptides and proteins as well as from the more easily fragmented vitamin B₁₂ molecule. The mass spectra contained peaks from multiply charged ions that were identical to conventional electrospray. Deep UV laser ablation of tissue allowed detection of lipids from untreated tissue. The mechanism of ionization is postulated to involve absorption of laser energy by a fraction of the analyte molecules that act as a sacrificial matrix or by residual water in the sample.     

Stepwise photoionization of 1,2- dimethoxybenzene vapor

Processes of stepwise ionization of 1,2-dimethoxybenzene vapor by radiation in the range 295-275 nm were studied by the techniques of total current spectroscopy, mass spectrometry, electronic absorption spectroscopy, and threshold electron spectroscopy. A two-step ionization process via the first electronically excited state of the molecule yielding the molecular ion was found to prevail at an average laser radiation intensity less than 10⁶ W/cm². Molecular ions possess a considerable (up to 1 eV) vibrational excitation energy. As the radiation intensity increases, the progressively stronger and deeper degradation takes place via dissociation of molecular and, probably, fragment ions due to absorption of at least one additional photon.     

Infrared absorption features of gaseous isopropyl carbocations.

C3H7+ ions were formed in the cell of a Fourier transform ion cyclotron resonance mass spectrometer and assayed by their multi-photon dissociation (MPD) behavior, triggered by the absorption of tunable IR radiation from a free-electron laser source providing a high fluence. The derived experimental IRMPD spectrum, which reflects the active vibrational modes of the ion, was compared with the IR spectra calculated for the optimized structures of the most-stable species on the C3H7+ potential energy surface, namely, a chiral iC3H7+ ion of C2 symmetry and an asymmetric corner-protonated cyclopropane, cC3H7+. The significant features in the IRMPD spectra of both the unlabeled and the perdeuterated ions obtained by ionization and fragmentation of isobutane or 2-chloro[D7]propane confirm the presence of the isopropyl cation, the ground-state isomer, whose IR spectroscopic features can thus be comparatively checked in the gas phase and in condensed superacid media. Details of the IRMPD features are suggested to result from the nearly barrierless interconversion of the two C2 enantiomers.     

Fragmentation dynamics of methane by few-cycle femtosecond laser pulses

The fragmentation pattern of CH4 was experimentally studied at an intensity of approximately 10(14) W/cm2 with laser durations varying from 8 to 110 fs. When the laser duration was 8 fs, only the primarily fragmental CH3+ ion was observed in addition to the parent CH4+ ion. When the laser duration was 30 fs, small fragmental CH2+ and H+ ions appeared. When the laser duration was 110 fs, some doubly charged ions were also observed in addition to the abundant singly charged ions. The large mass spectra difference demonstrated that the pulse duration had a strong effect on the fragmentation of the parent ion produced in the single ionization. The effect of laser intensity on the fragmentation of CH4+ was also studied for few-cycle femtosecond laser pulses. The results demonstrated that the first-return recollision between the rescattered electron and the parent ion played a significant role in the fragmentation dynamics of the parent ion. Depending on the ion-electron impact energy, the recollision excited the parent ion to a dissociated state or doubly charged state. The experimentally observed singly charged fragmental ions resulted from the recollision-induced dissociation of CH4+ or the Coulomb explosion of CH(4)2+.     

Fragmentation of leucine enkephalin as a function of laser fluence in a MALDI TOF-TOF

The effects of laser fluence on ion formation in MALDI were studied using a tandem TOF mass spectrometer with a Nd-YAG laser and alpha-cyano hydrocinnamic acid matrix. Leucine enkephalin ionization and fragmentation were followed as a function of laser fluence ranging from the threshold of ion formation to the maximum available, that is, about 280-930 mJ/mm2. The most notable finding was the appearance of immonium ions at fluence values close to threshold, increasing rapidly and then tapering in intensity with the appearance of typical backbone fragment ions. The data suggest the presence of two distinct environments for ion formation. One is associated with molecular desorption at low values of laser fluence that leads to extensive immonium ion formation. The second becomes dominant at higher fluences, is associated initially with backbone type fragments, but, at the highest values of fluence, progresses to immonium fragments. This second environment is suggestive of ion desorption from large pieces of material ablated from the surface. Arrhenius rate law considerations were used to estimate temperatures associated with the onset of these two processes.     

Irradiation effects in MALDI,ablation, ion production,and surface modifications. Part II. 2,5-dihydroxybenzoic acid monocrystals

Irradiation effects at low and high laser fluence on 2,5-dihydroxybenzoic acid large crystals were investigated. Contrary to what was observed for matrices as cinnamic acid derivatives, no chemical degradation of matrix is evidenced and continuous ablation as well as ion production resulted of extended irradiation in all the fluence range corresponding to classical matrix-assisted laser desorption /ionization. Ripples are formed on the base of the crater for a limited number of laser shots under moderate fluence. For extended irradiation, conical shape craters are formed with the axis of the crater oriented along the incident direction of the laser beam. A study of the craters showed that ablation through the ablated volume slowly varied with the laser fluence when a strong increase of ion production (matrix and analyte) was recorded. Ablation volume was found to vary non-linearly with the number of laser shots. On a same spot, the ablated volume and the ion production were measured as a function of the laser energy. With an increasing laser energy (or fluence), the ablated volume slowly increases when the ion production strongly increases. This gives evidence of a decoupling between ablation and ionization. Interaction of the plume with the incoming beam is thus probable.     

Analysis of pesticides by gas chromatography/multiphoton ionization/mass spectrometry using a femtosecond laser

Gas chromatography/multiphoton ionization/time-of-flight mass spectrometry (GC/MPI/TOFMS) was utilized for analysis of a standard mixture sample containing 49 pesticides and 4 real samples using the third-harmonic emission (267 nm) of a femtosecond Ti:sapphire laser (100 fs) as the ionization source. A sample of a standard mixture of n-alkane was also measured for calibration of the retention time indices of the pesticides. Two photons are required for the excitation of n-alkane due to an absorption band located in the far ultraviolet region (140 nm). The n-alkane molecule in the excited state was subsequently ionized either directly or by absorbing another photon because of a high ionization potential. Due to a large excess of energy, the molecular ion was decomposed and formed many fragment ions. Compared to n-alkanes, most of the pesticides were softly ionized by the femtosecond laser; one photon was used for excitation and another was used for the subsequent ionization. The pesticides with no conjugated double bond had a lower ionization efficiency. The present analytical instrument was applied to several samples prepared from a variety of vegetables and a single fruit after pretreatment with solid-phase extraction. Three pesticides were found in these samples, although some of them were not detected by conventional GC/EI/MS–MS due to insufficient sensitivity and selectivity.     

Gas chromatography/time‐of‐flight mass spectrometry of triacetone triperoxide based on femtosecond laser ionization

Triacetone triperoxide (TATP), which is used as an explosive in acts of terrorism, was measured by means of gas chromatography/multiphoton ionization/time‐of‐flight mass spectrometry using a deep‐ultraviolet (deep‐UV) femtosecond laser as an ionization source. The fragmentation process was investigated by changing the intensity of the laser at the center axis of a molecular beam. A molecular ion was observed using a femtosecond laser, and the ratio of the intensities of the molecular and fragment ions decreased as the intensity of the laser increased. These results suggest that TATP can be efficiently ionized using a deep‐UV, ultrashort optical pulse. Furthermore, fragmentation was accelerated by excess energy supplied through higher‐order multiphoton processes under a strong radiation field. The detection limits obtained using the molecular ion and two dominant fragment ions, C₂H₃O⁺ and CH, were determined to be 670, 83 and 150 pg, respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

飞秒脉冲作用下氯丙烯的多光子解离和电离动力学

The dissociation and photoionization dynamics of C₃H₅Cl were studied at 200, 400, and 800 nm with femtosecond laser pulses. The time-of-flight mass spectra, laser power index and photoelectron images were recorded. At short wavelength (200 nm), ionization of the parent molecule was found to be the dominant channel, while other ions were generated by the dissociation of C₃H₅Cl⁺. With the shift to long wavelength (e.g., 800 nm), fragment ions became dominant, and were generated through the multiphoton ionization of neutral fragments after the photodissociation of C₃H₅Cl. These results imply that photodissociation plays a significant role at long wavelength, because neutral fragments are supposed to be generated from the intermediate states reached by 800 nm photons. At 400 nm, the dissociation on the intermediate states is also critical, but is not as high as that at 800 nm. Taken together, our results demonstrate that the dissociation/ionization behaviors of allyl chloride are wavelength-dependent, and reveal the complex dynamics of allyl chloride at 200, 400 and 800 nm.     

Comparison of the effects of ionization mechanism, analyte concentration, and ion “cool-times” on the internal energies of peptide ions produced by electrospray and atmospheric pressure matrix-assisted laser desorption ionization

The propensities of a series of peptide ions produced by both electrospray and atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI) to fragment in an ion trap mass spectrometer under various conditions were studied in detail by measuring the extent of fragmentation of precursor ions by collision induced dissociation (CID) as a function of applied resonance excitation RF voltage. For the most basic peptides, the energy required to fragment MH+ ions generated by electrospray exceeded that required to fragment equivalent AP-MALDI ions under identical instrumental conditions; the reverse was observed for a peptide incorporating no basic residues, while peptides of intermediate basicity showed little difference between the ionization methods. This correlation between peptide basicity and the difference in the energy required to induce fragmentation of MH+ ions generated by AP-MALDI and electrospray is attributed primarily to a trend in the internal energies of the ions generated by AP-MALDI (the greater the difference in gas-phase basicities between the matrix and the analyte the greater the internal energy of the analyte ions produced). Furthermore the internal energies of ions produced by AP-MALDI, but not the equivalent ions formed by electrospray, were observed to decrease with decreasing analyte concentration. We attribute this finding to the cooling effect of endothermic dissociation of analyte ion/matrix molecule clusters following the matrix assisted laser desorption step. Time-resolved analyses (measurement of extent of fragmentation of precursor ions by CID as a function of pre-CID "cool times") revealed that cooling periods in excess of 250 ms were required to achieve internal energy equilibrium through cooling collisions with the helium buffer gas. Furthermore, these analyses demonstrated that, even after these extended cooling times, equivalent ions formed by the two ionization techniques showed different propensities to fragment. We conclude that the two different ionization techniques produce ion populations that may differ in their three-dimensional structure.     

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.     

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.     

纳秒强激光电离苯产生高价碳离子的波长效应

The photoionization of seeded benzene beam by 25 ns laser pulse at wavelengths of 266,355 and 1064 nm has been studied by the time-of-flight mass spectrometry. The observed mass spectra at 266 nm and 355 nm at intensities of 1010-1011 W/cm2 indicate a multiphoton ionization and dissociation（MPID）process,in which C+,C2Hx+,C3Hx+,C4Hx+ and C6H6+ are main products. While at 1064 nm laser of similar intensities,the domain ion is C4+ which is produced from Coulomb explosion. The longer wavelength facilities the energy absorption rate during inverse bremsstrahlung,which leads to the resulting wavelength dependence of the multicharged atomic ions.     

CS2 +多光子光解产生CS+的动力学研究

The dynamics on the multi-photon dissociation of CS2+ molecular ions to produce CS + ions has been investigated by measuring the CS + photofragment excitation（PHOFEX）spectrum in the wavelength range of 385～435 nm,where the CS2+ molecular ions were prepared purely by［3+1］multiphoton ionization of the neutral CS2molecules at 483.2 nm. With the ～60 ns delay,which is much more than the laser pulse width（～5 ns）,between ionization laser and dissociation laser,the threshold wavelength of dissociation laser to produce CS+ fragment ion from CS2+ molecular ions was obviously observed in the PHOFEX spectrum. The adiabatic appearance potential of the CS+ was determined to be（5.852 ± 0.005）eV above the X 2Σg,3/2（0,0,0）level of CS2+. The product branching ratios,（CS+/S+）,as measured from the PHOFEX spectra,increase from 0 to slightly larger than 1 in the wavenumber range of 47200～50400 cm-1 . The［1+1］dissociation mechanism to get to CS++S from CS2+ was discussed and preliminarily attributed to（i）CS2+（X 2Πg）→ CS2+（A2Πu）through one-photon excitation,（ii）CS2+（A2Πu）→ CS2+（X*）via internal conversion process due to the vibronic coupling between the A and X states,（iii）CS2+（X*）→ CS2+（B 2Σ+u）through the second photon excitation,and（iv）CS2+（B 2Σ+u）→CS +（X 2Σ+）+S（3P）,because of the potential curve crossing with the repulsive 4Σ- state and/or the 2Σ- state correlated with the second dissociation limit. However,when the dissociation laser overlaps the ionization laser in time scale in the laser-molecule interaction zone,the appearance threshold is not available in the PHOFEX spectrum. This fact shows that there are other mixed three-photon paths of［1+1+1'］,［1+1'+1'］,and［1+1'+1］to produce CS+ fragment ion from CS2+ molecular ions besides the above［1+1］dissociation mechanism,that is,CS2+（X 2Πg）→ CS2+（A 2Πu）through one-photon excitation［1］of dissociation laser,CS2+（A 2Πu）→CS2+（X*）via internal conversion process due to the vibronic coupling between the A and X states,CS2+（X*）→ CS2+（B 2Σ +u）through the second photon excitation by dissociation laser［1］or ionization laser［1'］,and third photon excitation by ionization laser［1'］or dissociation laser［1］to reach the adiabatic appearance potential to produce CS+ with the dissociation laser wavelengths longer than 423. 89 nm,at which the［1+1］dissociation mechanism to get to CS+ is unavailable.     

多光子电离伴随的碎片化过程的速率方程分析

提出一个描述多原子分子多光子激发、电离和离子再被激发过程的布局速率方程模型，并且得到电离效率和离子体系吸收的平均能量＜E＞的代数解．通过计算这两个量与光强的关系，分析了不同电离机制下，多原子分子多光子电离实验中伴随的碎片化过程的控制性问题．结果表明只有采用1＋1电离方案，可以通过控制电离激光的强度来实现“软电离”和“硬解离”．采用3＋1电离，即使在单离子条件下，离子的＜E＞已高达20－30eV，远超过离子的解离阈值，特别是增加光强时，＜E＞的增加速度比电离效率增加得快，因此一般条件下不能让分子有效电离而不使其离子解离．上面的分析可以圆满解释呋喃等分子在3＋1电离时观察不到母体离子这个实验事实．