Sequence-specific fragmentation of benzyloxycarbonyl dipeptides by resonance-enhanced multiphoton ionization

The mechanism of fragment ion formation in resonance-enhanced multiphoton ionization (REMPI) of benzyloxycarbonyl (CBZ)-derivitized dipeptides is presented. At 266 nm, the entire multiphoton process can be thought of as a two-part scheme where ionization occurs by resonant two-photon ionization followed by photodissociation of the created ions. When the energy of two photons exceeds the molecular ionization energy by a significant amount, REMPI has the advantage of producing both parent ions and low appearance energy fragments in large amounts. For CBZ dipeptides, resonant two-photon ionization at 266 nm produces parent ions as well as A type sequence ions with high abundance. On the other hand, a three-photon process (resonant two-photon ionization followed by parent ion photodissociation) forms sequence-related ions which also involve complex fragmentations of the CBZ chromophore. These results are compared to mass spectra obtained by other ionization/dissociation methods and to REMPI mass spectra of related compounds. Factors related to molecular structure elucidation based upon REMPI mass spectra are discussed. Enhanced isomer distinction is demonstrated for CBZ-leu-ala-OCH₃ and CBZ-ile-ala-OCH₃ based upon REMPI fragmentation.     

Wavelength-dependent photofragmentation of a mixed-ligand cyclometalated platinum(II) coordination compound in a molecular beam

The photofragmentation of (3-Me-4',6'-dfppy)Pt(dpm) (dfppy = difluorophenylpyridinato; dpm = dipivaloylmethyl or 2,2,6,6,-tetramethyl-3,5-heptanedionato- O, O) in a molecular beam is reported. Time-of-flight mass spectra (TOF-MS) and resonance-enhanced multiphoton ionization (REMPI) data are presented and discussed. The dissociation patterns are strongly wavelength-dependent. With 355 nm excitation, the heaviest mass platinum-containing fragments are Pt(+) and diatomic PtC(+). The formation of PtC(+) is the result of an intramolecular rearrangement on the ligand. During irradiation with 410-500 nm light, the fragmentation pattern changes such that the parent ion and platinum-containing fragments of the parent are formed in abundant yield. The (3-Me-4',6'-dfppy) ligand remains intact and coordinated to platinum, but coordinated (dpm) successively breaks apart. A spin-forbidden charge-transfer absorption band centered at around 460 nm plays an important role in the gas-phase photoexcitation of the parent molecule; it is observed in the REMPI spectrum of the parent ion.     

242-260nm波长区CS2分子的多光子解离电离

在242－260nm波氏范围通过CS2分子的共振增强多光子电离（REMPI）获得了母体离子CS和碎片离子的分质量激发谱．在λ＜246.4nm区间，CS激发谱上呈现出来源于CS2双光子电离的弥散谱带,碎片离子激发谱的归属强烈提示多光子过程中有中性基电子态的CS和S（经由CS2的光解离）产生：（1）CS 的谱带主要来源于中性CS碎片经由单光子跃迁产生的（1＋1）共振增强电离，（2）除了部分S 的谱峰来自CS 的光解外，多数S 的锐谱峰来自中性S原子经由3p3（2D0）4p，3p3（4S0）np（n＝6，7，8）←3p43pJ（J＝2，1,0）双光子跃迁产生的（2＋1）共振增强电离．     

Multiphoton ionization and fragmentation study of acetone at XeCl excimer laser radiation

Multiphoton ionization and fragmentation of the acetone molecule at 308 nm laser radiation are studied using diffuse molecular beam and multiphoton ionization mass spectroscopy. It is found that the ion peaks of acetone do not appear at m/e = 58, but only at m/e = 15 and 43, corresponding respectively to the two neutral fragments CH₃ and CH₃CO generated by the photodissociation of acetone. The intensities of these ions are proportional to the 3.6th and 3.0th power of the laser intensity, respectively.     

A general purpose XUV laser spectrometer: some applications to N2, O2 and CO2

Coherent radiation is generated from 90 to 105 nm by frequency tripling the frequency-doubled output of a pulsed Nd: YG-pumped dye laser using a synchronized pulsed jet of various gases as the nonlinear medium. The XUV radiation is isolated by a pair of dichroic mirrors before interacting with a supersonically cooled gas target. Provisions have been made to measure the photoion yield as a function of wavelength using a time-of-flight mass spectrometer. In addition absorption by the target gas or fluorescence from the excited target and/or its fragments can be detected. Some applications are presented which illustrate the versatility of this XUV laser spectrometer involving 1+1 resonance-enhanced multiphoton ionization of N₂ and excitation of Rydberg states of O₂ and CO₂.     

Photodissociation study of 1,3-dibromopropane at 234 nm via an ion velocity imaging technique

The photodissociation of 1,3-dibromopropane has been studied at 234 nm using a 2D photofragment ion velocity imaging technique coupled with a [2 + 1] resonance-enhanced multiphoton ionization scheme. The velocity distributions for the Br (2P1/2) (denoted Br) and Br (2P3/2) (denoted Br) fragments are determined, and each can be fitted by a narrow single-peaked Gaussian curve, suggesting that bromine fragments are generated as a result of direct dissociation via repulsive potential energy surfaces. The recoil anisotropies were measured to be beta = 0.80 for Br and 1.31 for Br, and the product relative quantum yields at 234 nm is Phi234 nm(Br) = 0.21.     

Multiphoton ionization of VOCl3

VOCl₃ is studied by resonance-enhanced multiphoton ionization in the region 430 to 440 nm. Mass spectrometric detection reveals the production of both V⁺ and VO⁺ with the molecular ion having the larger intensity. The V⁺ ion current spectrum shows sharp atomic resonance peaks while the VO⁺ spectrum is structureless.     

丙酮在XeCl准分子激光辐射下的多光子电离和碎裂研究

我们用扩散分子束多光子电离光谱技术研究了丙酮在XeCl准分子激光辐射下的多光子电离和碎裂行为。实验表明丙酮没有产生母体离子峰(m/e=58), 只给出m/e=43和15两个峰, 分别对应于丙酮光解所产生的两个碎片CH_3 CO和CH_3。其离子信号(1)分别与激光强度(Ⅰ)的3和3.6次方成正比。分析表明离子是由中性碎片电离产生的。     

Photodissociation of spatially aligned acetaldehyde cations

Photofragment translational energy and angular distributions are reported for the photodissociation of acetaldehyde cations in the wavelength range 354-363 nm obtained using the DC slice ion imaging technique. Vibrationally selected parent ions were produced by 2+1 resonance-enhanced multiphoton ionization (REMPI) via the 3s<--n Rydberg transition, with photodissociation resulting from absorption of a fourth additional photon. Three product channels were observed: HCO+, CH3CO+, and CH4+. The angular distributions reveal that all product channels have a predominantly parallel recoil anisotropy although the lower beta2 parameter of CH3CO+ indicates the concomitant presence of a perpendicular component. Furthermore, the distinct angular distribution of the CH3CO+ fragments shows a large value of the higher order Legendre polynomial term, providing evidence that acetaldehyde cations are spatially aligned during the ionization process.     

Surface analysis by laser desorption of neutral molecules with detection by fourier-transform mass spectrometry

Molecules chemisorbed on a platinum single crystal are desorbed by a pulsed laser, ionized by an electron beam or multiphoton ionization, and detected by Fourier-transform mass spectrometry (F.t.m.s.). Laser desorption of ethylene, methanol, cyanogen, benzene and naphthalene is described. In all cases, molecular ions are the major peaks observed in the mass spectra, and the minor peaks correspond to known electron-impact fragments of the adsorbates. For the systems investigated thus far, the laser-desorption F.t. mass spectra are less complex and esier to interpret than the spectra produced by secondary-ion mass spectrometry, which are complicated by the rapid ion/-molecule reactions that can occur directly above the surface between adsorbates and substrate atoms. In the laser-desorption method, these complications are avoided because the ions are formed after the desorbed molecules have moved away from the surface and have expanded into the vacuum. The sensitivity of the laser-desorption F.t.m.s. method is tested. For carbon monoxide adsorbed on platinum, the detection limit is ca. 5 × 10^?6 monolayer per cm². For naphthalene, a single laser pulse at 248 nm produces abundant molecular ions even when the electron beam is turned off. The ions appear to be produced by resonance-enhanced multiphoton ionization rather than a thermal process. In these experiments, multiphoton ionization of naphthalene at 248 nm is about 35-fold more sensitive than electron ionization.     

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．     

Fragmentation patterns in multiphoton ionization: a statistical interpretation

The simplest statistical theory wherein the mass spectral fragmentation pattern is governed by the mean energy is derived and compared with the observed results for resonance-enhanced multiphoton ionization of benzene. By varying the mean energy absorbed it is possible to span the range of different patterns up to the dominance of C⁺₁ions above 37 eV/molecule.     

MPI/MS detection of SiF and SiF2 radicals produced from the reaction of F2 and NF3 with silicon

SiF and SiF₂ radicals, produced in the thermal reaction of F₂ and NF₃ with solid silicon, have been detected by multiphoton ionization mass spectrometry in the 438 and 321 nm wavelength regions, respectively. The reaction pathway to the production of SiF radicals is of minor significance compared with SiF₂ production over a wide range of temperatures and pressures. In both cases it was found that little or no fragmentation of the parent ion occurs upon ionization. MPI/MS is thus shown to be a useful diagnostic tool for studies of fluorine-silicon etching reactions.     

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.     

溴代烷烃在紫外波段的光解离过程

在飞行时间质谱仪中，采用波长为234 nm和267 nm的激光，研究溴代烷烃CHBr3、CH2Br2、C2H5Br及C2H4Br2的光离解过程.在UV激光的作用下，溴代烷烃分子主要发生的是吸收1个光子解离出Br原子，然后继续吸收光子发生Br原子的（2＋1）共振增强多光子电离的过程.其中由溴代烷烃分子解离得到的Br原子可能存在着两种布居：基态Br(2P03/2)及激发态Br*(2P01/2).研究解离得到的Br原子的分支比 N(Br*)/N(Br)，并给出测量结果：溴代烷烃分子解离得到的Br原子在267 nm激光作用下的分支比明显大于在234 nm激光作用下的结果.对此多光子过程的机理，也进行了分析讨论.     

离子速度成像方法研究C8H17Br分子的光解动力学

用离子速度成像方法, 研究了长链C8H17Br分子在234 nm激光下的光解过程. 通过2+1共振增强多光子电离探测了两种光解产物Br*(2P1/2)和Br(2P3/2), 得到了它们的相对量子产率. 从光解产物Br*(2P1/2)和Br(2P3/2)的速度图像得到了能量和角度分布. 并根据相对量子产率和角度分布, 计算了不同解离通道的比例. 实验发现C8H17Br分子解离过程中大部分能量都转化为内能, 该能量分配可以较好地用软反冲模型来解释, 并分析了这种能量分配跟烷基大小的关系.     

Ion-Velocity Map Imaging Study of Photodissociation Dynamics of Acetaldehyde

The photodissociation dynamics of acetaldehyde in the radical channel CH3＋HCO has been reinvestigated using time-sliced velocity map imaging technique in the photolysis wavelength range of 275-321 nm. The CH3 fragments have been probed via （2＋1） resonance-enhanced multiphoton ionization. Images are measured for CH3 formed in the ground and excited states （v2=0 and 1） of the umbrella vibrational mode. For acetaldehyde dissociation on T1 state after intersystem crossing from S1 state, the products are formed with high translational energy release and low internal excitation. The rotational and vibrational energy of both fragments increases with increasing photodissociation energy. The triplet barrier height is estimated at 3.8814-0.006 eV above the ground state of acetaldehyde.     

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

The distribution of neutral copper oxide clusters in the gas phase created by laser ablation is detected and characterized through time-of-flight mass spectroscopy (TOFMS). The neutral copper oxide clusters are ionized by two different approaches: Multiphoton absorption of 355 and 193 nm radiation; and single photon absorption of 118 nm radiation. Based on the observed cluster patterns as a function of experimental conditions (e.g., copper oxide or metal sample, ablation laser power, expansion gas, etc.) and on the width of the TOFMS features, one can uncover the true neutral cluster distribution of CumOn species following laser ablation of the sample. Ablation of a metal sample generates only small neutral CumOn clusters for m less, similar 4 and n approximately 1, 2. Ablation of copper oxide samples generates neutral clusters of the form CumOm (m < or = 4) and CumO(m-1) (m > 4). These clusters are directly detected without fragmentation using single photon, photoionization with 118 nm laser radiation. Using 355 and 193 nm multiphoton ionization, the observed cluster ions are mostly of the form Cu2mOm+ for 4 < or = m < or = 10 (193 nm ionization) and CumO1,2 (355 nm ionization) for copper oxide samples. Neutral cluster fragmentation due to multiphoton processes seems mainly to be of the form CumO(m,m-1) --> CumO(m/2,m/2+1). Neutral cluster growth mechanisms are discussed based on the cluster yield from different samples (e.g., Cu metal, CuO powder, and Cu2O powder).     

REMPI detection of CH3 in low-pressure flames

This study presents results on the detection of CH₃ in low-pressure methane-oxygen flames using resonance-enhanced multiphoton ionization (REMPI) techniques. Experiments in a discharge-flow reactor were performed for optimization purposes. Excitation wavelengths near 286 and 333 nm were examined; the former is not recommended for flame applications, although it may be preferable under flow reactor conditions.     

Ultrafast REMPI in benzene and the monohalobenzenes without the focal volume effect

We report on the photoionization and photofragmentation of benzene (C(6)H(6)) and of the monohalobenzenes C(6)H(5)-X (X = F, Cl, Br, I) under intense-field, single-molecule conditions. We focus 50-fs, 804-nm pulses from a Ti:sapphire laser source, and record ion mass spectra as a function of intensity in the range ～10(13) W/cm(2) to ～10(15) W/cm(2). We count ions that were created in the central, most intense part of the focal area; ions from other regions are rejected. For all targets, stable parent ions (C(6)H(5)X(+)) are observed. Our data is consistent with resonance-enhanced multiphoton ionization (REMPI) involving the neutral (1)ππ* excited state (primarily a phenyl excitation): all of our plots of parent ion yield versus intensity display a kink when this excitation saturates. From the intensity dependence of the ion yield we infer that both the HOMO and the HOMO-1 contribute to ionization in C(6)H(5)F and C(6)H(5)Cl. The proportion of phenyl (C(6)H(5)) fragments in the mass spectra increases in the order X = F, Cl, Br, I. We ascribe these substituent-dependent observations to the different lifetimes of the C(6)H(5)X (1)ππ* states. In X = I the heavy-atom effect leads to ultrafast intersystem crossing to a dissociative (3)nσ* state. This breaks the C-I bond in an early stage of the ultrashort pulse, which explains the abundance of fragments that we find in the iodobenzene mass spectrum. For the lighter X = F, Cl, and Br this dissociation is much slower, which explains the lesser degree of fragmentation observed for these three molecules.