Ionization and dissociation of CH3I in intense laser field

The ionization-dissociation of methyl iodide in intense laser field has been studied using a reflection time-of-flight mass spectrometry (RTOF-MS), at a laser intensity of < or =6.6x10(14) W/cm(2), lambda=798 nm, and a pulse width of 180 fs. With the high resolution of RTOF-MS, the fragment ions with the same M/z but from different dissociation channels are resolved in the mass spectra, and the kinetic energy releases (KERs) of the fragment ions such as I(q+) (q=1-6), CH(m) (+) (m=0-3), C(2+), and C(3+) are measured. It is found that the KERs of the fragment ions are independent of the laser intensity. The fragments CH(3) (+) and I(+) with very low KERs (<1 eV for CH(3) (+) and <0.07 eV for I(+)) are assigned to be produced by the multiphoton dissociation of CH(3)I(+). For the fragments CH(3) (+) and I(+) from CH(3)I(2+), they are produced by the Coulomb explosion of CH(3)I(2+) with the interaction from the covalent force of the remaining valence electrons. The split of the KER of the fragments produced from CH(3)I(2+) dissociation is observed experimentally and explained with the energy split of I(+)((3)P(2)) and I(+)((3)P(0,1)). The dissociation CH(3)I(3+)-->CH(3) (+)+I(2+) is caused by Coulomb explosion. The valid charge distance R(c) between I(2+) and CH(3) (+), at which enhanced ionization of methyl iodide occurs, is obtained to be 3.7 A by the measurements of the KERs of the fragments CH(3) (+) and I(2+). For the CH(3)I(n+) (n> or =3), the KERs of the fragment ions CH(3) (p+) and I(q+) are attributed to the Coulomb repulsion between CH(3) (p+) and I(q+) from R(c) approximately 3.7 A. The dissociation of the fragment CH(3) (+) is also discussed. By the enhanced ionization mechanism and using the measured KER of I(q+), all the possible Coulomb explosion channels are identified. By comparing the abundance of fragment ions in mass spectrum, it is found that the asymmetric dissociation channels with more charges on iodine, q>p, are the dominant channels.     

Velocity map imaging of dissociative ionization and coulomb explosion of CH3I induced by a femtosecond laser

The dissociative ionization and the Coulomb explosion of CH3I irradiated by a 35 fs 800 nm laser with a laser intensity of 4 x 10(13) to 6 x 10(14) W/cm2 was studied. In a relatively weak laser field (about 10(13) W/cm2), the dissociative ionization of CH3I took place. The speed distributions of the CH3+ and I+ fragments were measured and fitted using multiple Gaussian functions. Different product channels were found for CH3+ and I+, respectively. In a strong laser field (about 10(14) W/cm2), the multiply ionized fragment ions of Iq+ (q 相似文献   

Wavelength-dependent Coulomb explosion in carbon disulphide (CS2) clusters: generation of energetic multiply charged carbon and sulphur ions

A gigawatt laser-induced Coulomb explosion has been observed in carbon disulphide (CS(2)) clusters generating energetic, multiply charged [C](m+) (m = 1-4) and [S](n+) (n = 1-6) atomic ions of carbon and sulphur. The Coulomb explosion shows wavelength dependence. Comparison of these results with our earlier work shows that the polarizability and dipole moment might help in energy absorption from the laser field but they are not mandatory conditions for this low-intensity Coulomb explosion. The results show that in a field of 10(9) W/cm(2), absorption of 266 and 355 nm laser radiation by CS(2) clusters leads to multiphoton dissociation/ionization whereas at 532 nm the whole cluster explodes generating multiply charged atomic ions.     

Oxygen-containing gas-phase diatomic trications and tetracations: ReO(z+), NbO(z+) and HfO(z+) (z=3, 4)

Three oxygen-containing gas-phase diatomic trications ReO(3+), NbO(3+) and HfO(3+) as well as the diatomic tetracation NbO(4+) have been observed by mass spectrometry at non-integer m/z values. These unusual triply charged molecular ion species, together with the corresponding diatomic dications ReO(2+), NbO(2+) and HfO(2+), were produced by energetic, high-current oxygen ((16)O(-)) ion beam sputtering of rhenium, niobium and hafnium metal samples, respectively, whose surfaces were dynamically oxidized by oxygen primary ion incorporation. In addition, NbO(z+) (z≤ 4) were generated by intense femtosecond laser excitation and photofragmentation (Coulomb explosion) of Nb(x)O(y) clusters and were detected through Time-of-Flight Mass Spectrometry (TOF). Our experimental results confirm previous reports on the detection of NbO(4+), NbO(3+), NbO(2+), HfO(3+) and HfO(2+) with Atom Probe mass spectrometry, whereas ReO(3+) and ReO(2+) apparently had not been observed before. In addition, these multiply charged molecular ions have been studied theoretically for the first time. Ab initio calculations of their electronic structures show that the diatomic trications ReO(3+), NbO(3+) and HfO(3+) are long-lived metastable gas-phase species, with bond lengths of 1.61 ?, 1.62 ? and 1.86 ?, respectively. They present large potential barriers with respect to dissociation of more than 2.7 eV. The corresponding diatomic dications are thermochemically stable molecules with very large dissociation energies (>3.5 eV). Our calculations predict the diatomic tetracation ReO(4+) to be a metastable ion species in the gas phase. We compute a potential barrier toward fragmentation of 0.6 eV; its formation requires a quadruple adiabatic ionization energy of 85.7 eV. Even though our calculations show that NbO(4+) is a weakly bound (dissociation barrier ～0.1 eV) metastable molecule, it is here identified via linear time-of-flight mass spectrometry.     

Dissociation of benzene dication [C6H6]2+: exploring the potential energy surface

The singlet potential energy surface for the dissociation of benzene dication has been explored, and its three major dissociation channels have been studied: C6H6(2+) --> C3H3(+) + C3H3(+), C4H3(+) + C2H3(+), and C5H3(+) + CH3(+). The calculated energetics suggest that the products will be formed with considerable translational energy because of the Coulomb repulsion between the charged fragments. The calculated energy release in the three channels shows a qualitative agreement with the experimentally observed kinetic energy release. The formation of certain intermediates is found to be common to the three dissociation channels.     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. IV. Coulomb explosion of molecular heteroclusters

In this paper we present a theoretical and computational study of the temporal dynamics and energetics of Coulomb explosion of (CD4)(n) and (CH4)(n) (n=55-4213) molecular heteroclusters in ultraintense (I=10(16)-10(19) W cm(-2)) laser fields, addressing the manifestation of electron dynamics, together with nuclear energetic and kinematic effects on the heterocluster Coulomb instability. The manifestations of the coupling between electron and nuclear dynamics were explored by molecular dynamics simulations for these heteroclusters coupled to Gaussian laser fields (pulse width tau=25 fs), elucidating outer ionization dynamics, nanoplasma screening effects (being significant for I< or =10(17) W cm(-2)), and the attainment of cluster vertical ionization (CVI) (at I=10(17) W cm(-2) for cluster radius R(0)< or =31 A). Nuclear kinematic effects on heterocluster Coulomb explosion are governed by the kinematic parameter eta=q(C)m(A)/q(A)m(C) for (CA(4))(n) clusters (A=H,D), where q(j) and m(j) (j=A,C) are the ionic charges and masses. Nonuniform heterocluster Coulomb explosion (eta >1) manifests an overrun effect of the light ions relative to the heavy ions, exhibiting the expansion of two spatially separated subclusters, with the light ions forming the outer subcluster at the outer edge of the spatial distribution. Important features of the energetics of heterocluster Coulomb explosion originate from energetic triggering effects of the driving of the light ions by the heavy ions (C(4+) for I=10(17)-10(18) W cm(-2) and C(6+) for I=10(19) W cm(-2)), as well as for kinematic effects. Based on the CVI assumption, scaling laws for the cluster size (radius R(0)) dependence of the energetics of uniform Coulomb explosion of heteroclusters (eta=1) were derived, with the size dependence of the average (E(j,av)) and maximal (E(j,M)) ion energies being E(j,av)=aR(0) (2) and E(j,M)=(5a/3)R(0) (2), as well as for the ion energy distributions P(E(j)) proportional to E(j) (1/2); E(j)< or =E(j,M). These results for uniform Coulomb explosion serve as benchmark reference data for the assessment of the effects of nonuniform explosion, where the CVI scaling law for the energetics still holds, with deviations of the a coefficient, which increase with increasing eta. Kinematic effects (for eta>1) result in an isotope effect, predicting the enhancement (by 9%-11%) of E(H,av) for Coulomb explosion of (C(4+)H(4) (+))(eta) (eta=3) relative to E(D,av) for Coulomb explosion of (C(4+)D(4) (+))(eta) (eta=1.5), with the isotope effect being determined by the ratio of the kinematic parameters for the pair of Coulomb exploding clusters. Kinematic effects for nonuniform explosion also result in a narrow isotope dependent energy distribution (of width DeltaE) of the light ions (with DeltaE/E(H,av) approximately 0.3 and DeltaE/E(D,av) approximately 0.4), with the distribution peaking at the high energy edge, in marked contrast with the uniform explosion case. Features of laser-heterocluster interactions were inferred from the analyses of the intensity dependent boundary radii (R(0))(I) and the corresponding average D+ ion energies (E(D,av))(I), which provide a measure for optimization of the cluster size at intensity I for the neutron yield from dd nuclear fusion driven by Coulomb explosion (NFDCE) of these heteroclusters. We infer on the advantage of deuterium containing heteronuclear clusters, e.g., (CD4)(n) in comparison to homonuclear clusters, e.g., (D2)(n/2), for dd NFDCE, where the highly charged heavy ions (e.g., C4+ or C6+) serve as energetic and kinematic triggers driving the D+ ions to a high (10-200 keV) energy domain.     

Unimolecular chemistry of doubly protonated zwitterionic clusters

Electrospray ionization and tandem mass spectrometry experiments have been used to study the fragmentation and electron-ion interactions of doubly charged zwitterionic clusters, [M(15) + 2H](2+) (where M = Glycine Betaine (GB), (CH(3))(3)N(+)CH(2)CO(2)(-), and Dimethylsulfonioacetate (DMSA), (CH(3))(2)S(+)CH(2)CO(2)(-)) which are close to the stability limit, i.e., the Coulomb repulsion of the charge within the cluster competes with attractive forces. The intercluster chemistry was studied using collision-induced dissociation (CID) and electron-induced dissociation (EID) in which the energy of the electrons has been varied from >0 to 30 eV. Experimental results suggest that the zwitterionic binding energy in the clusters follow the order GB > DMSA, which is consistent with theoretical calculations that highlight that the lower dipole moment of DMSA leads to a binding energy of DMSA that is 0.86 times smaller than that for GB. Multiply protonated clusters of both GB and DMSA dissociate through Coulomb explosion, which is in competition with neutral evaporation for DMSA. Electronic excitation of the cluster under EID conditions at higher electron energies >12 eV can lead to new intercluster reactions associated with bond cleavages where differences between the sulfur and nitrogen betaines are minor.     

Modeling the interactions between peptide functions and Sr2+: formamide-Sr2+ reactions in the gas phase

The interactions between formamide, which can be considered a prototype of a peptide function, and Sr(2+) have been investigated by combining nanoelectrospray ionization/mass spectrometry techniques and G96LYP DFT calculations. For Sr an extended LANL2DZ basis set was employed, together with a 6-311+G(3df,2p) basis set expansion for the remaining atoms of the system. The observed reactivity seems to be dominated by the Coulomb explosion process yielding [SrOH](+) + [HNCH](+), which are the most intense peaks in the MS/MS spectra. Nevertheless, additional peaks corresponding to the loss of HNC and CO indicate that the association of Sr(2+) to water or to ammonia leads to long-lived doubly charged species detectable in the timescale of these experimental techniques. The topology of the calculated potential energy surface permits us to establish the mechanisms behind these processes. Although the interaction between the neutral base and Sr(2+) is essentially electrostatic, the polarization triggered by the doubly charged metal ion results in the activation of several bonds, and favors different proton transfer mechanisms required for the formation of the [SrOH](+), [SrOH(2)](2+) and [SrNH(3)](2+) products.     

Explosive photodissociation of methane induced by ultrafast intense laser

A new type of molecular fragmentation induced by femtosecond intense laser at the intensity of 2 x 10(14) W/cm2 is reported. For the parent molecule of methane, ethylene, n-butane, and 1-butene, fluorescence from H (n = 3-->2), CH (A 2Delta, B 2Sigma-, and C 2Sigma+-->X 2Pi), or C2 (d 3Pi g-->a 3Pi u) is observed in the spectrum. It shows that the fragmentation is a universal property of neutral molecule in the intense laser field. Unlike breaking only one or two chemical bonds in conventional UV photodissociation, the fragmentation caused by the intense laser undergoes vigorous changes, breaking most of the bonds in the molecule, like an explosion. The fragments are neutral species and cannot be produced through Coulomb explosion of multiply charged ion. The laser power dependence of CH (A-->X) emission of methane on a log-log scale has a slope of 10 +/- 1. The fragmentation is thus explained as multiple channel dissociation of the superexcited state of parent molecule, which is created by multiphoton excitation.     

溴甲烷在强激光场中的电离和解离

利用自制的反射式飞行时间质谱仪(RTOF-MS)研究了多原子分子CH₃Br在强激光场中的电离解离. 得到了溴甲烷在强激光场中电离解离的飞行时间质谱, 基于RTOF-MS的高分辨率(M/ΔM>2000), 测量了分子库仑爆炸产生的系列碎片离子的动能释放(KER), 用多光子解离和库仑爆炸解释了实验结果. 与碘甲烷在强场中的实验结果对比发现: (1) 在相同的激光场强下, 碘甲烷电离解离的最高价碎片离子为I⁶⁺而溴甲烷为Br³⁺; (2) 溴甲烷质谱中存在母体离子的脱氢产物CH_mBr⁺ 和CH_mBr²⁺, 而对于碘甲烷, 没有检测到这些通道, C-I键首先断开; (3) 质谱中存在H⁷⁹Br⁺和H⁸¹Br⁺, 而碘甲烷的电离解离中不存在HI产物; (4) 溴甲烷库仑两体爆炸的有效电荷间距随着两碎片电荷乘积的增大而增大, 而对于碘甲烷此间距几乎不随电荷乘积变化; (5) CH_m⁺(m=0, 1, 2)的主要生成通道可能与碘甲烷不同, 不是来自CH₃⁺的顺序脱氢, 而是来自脱氢母体离子的直接解离.     

An experimental and theoretical investigation of gas-phase reactions of Ca2+ with glycine

The gas-phase reactions between Ca(2+) and glycine ([Ca(gly)](2+)) have been investigated through the use of mass spectrometry techniques and B3-LYP/cc-pWCVTZ density functional theory computations. The major peaks observed in the electrospray MS/MS spectrum of [Ca(gly)](2+) correspond to the formation of the [Ca,C,O(2),H](+), NH(2)CH(2) (+), CaOH(+), and NH(2)CH(2)CO(+) fragment ions, which are produced in Coulomb explosion processes. The computed potential energy surface (PES) shows that not only are these species the most stable product ions from a thermodynamic point of view, but they may be produced with barriers lower than for competing processes. Carbon monoxide is a secondary product, derived from the unimolecular decomposition of some of the primary ions formed in the Coulomb explosions. In contrast to what is found for the reactions of Ca(2+) with urea ([Ca(urea)](2+)), minimal unimolecular losses of neutral fragments are observed for the gas-phase fragmentation processes of [Ca(gly)](2+), which is readily explained in terms of the topological differences between their respective PESs.     

Coulomb effects in binding of heme in gas-phase ions of myoglobin

Coulomb effects in binding of heme in gas-phase holomyoglobin ions are studied. Positive and negative ions are formed from solution myoglobin with Fe(2+) (ferromyoglobin) and Fe(3+) (ferrimyoglobin). The energy that must be added to the resulting holomyoglobin ions to cause heme loss has been measured by triple-quadrupole tandem mass spectrometry. With negative ions, neutral heme is lost regardless of the charge state of Fe in solution. It is likely that the Fe(3+) is reduced to Fe(2+) in the negative electrospray process. With positive ions, predominantly neutral heme loss is observed with ions formed from ferromyoglobin in solution, and positive heme loss with ions formed from ferrimyoglobin in solution. The energies required to induce neutral heme loss are similar for positive and negative ions. The energies required to induce charged heme loss from positive holomyoglobin ions are significantly less. Coulomb repulsion between the charged heme and charged protein appears to lower the barrier for heme loss. These results are consistent with a simple model potential with a long-range Coulomb repulsion and short-range attraction between the heme and protein.     

The anomalous shape of the cross section for the formation of SF3+ fragment ions produced by electron impact on SF6 revisited

The partial ionization cross section for the formation of SF(3) (+) fragment ions following electron impact on SF(6) is known to have a pronounced structure in the cross section curve slightly above 40 eV. We used the mass-analyzed ion kinetic energy (MIKE) scan technique to demonstrate the presence of a channel contributing to the SF(3) (+) partial ionization cross section that we attribute to the Coulomb explosion of doubly charged metastable SF(4) (2+) ions into two singly charged ions SF(3) (+) and F(+), with a threshold energy of about 45.5 eV. Thus the observed unusual shape of the SF(3) (+) partial ionization cross section is the result of two contributions, (i) the direct formation of SF(3) (+) fragment ions via dissociative ionization of SF(6) with a threshold energy of 22 eV and (ii) the Coulomb explosion of metastable SF(4) (2+) ions with a threshold energy of about 45.5 eV. A detailed analysis of the MIKE spectrum reveals an average kinetic energy release of about 5 eV in the Coulomb explosion of the SF(4) (2+) ions with evidence of a second channel corresponding to an average kinetic energy release of about 1.1 eV.     

Energetics and mechanisms of C-H bond activation by a doubly charged metal ion: guided ion beam and theoretical studies of Ta2+ + CH4

A guided ion beam tandem mass spectrometer is used to study the kinetic-energy dependence of doubly charged atomic tantalum cations (Ta(2+)) reacting with CH4 and CD4. As for the analogous singly charged system, the dehydrogenation reaction to form TaCH2(2+) + H2 is exothermic. The charge-transfer reaction to form Ta(+) + CH4(+) and the charge-separation reaction to form TaH(+) + CH3(+) are also observed at low energies in exothermic processes, as is a secondary reaction of TaCH2(2+) to form TaCH3(+) + CH3(+). At higher energies, other doubly charged products, TaC(2+) and TaCH3(2+), are observed, although no formation of TaH(2+) was observed. Modeling of the endothermic cross sections provides 0 K bond dissociation energies (in electronvolts) of D0(Ta(2+)-C) = 5.42 +/- 0.19 and D0(Ta(2+)-CH3) = 3.40 +/- 0.16. These experimental bond energies are in poor agreement with density functional calculations at the B3LYP/HW+/6-311++G(3df,3p) level of theory. However, the Ta(2+)-C bond energy is in good agreement with calculations at the QCISD(T) level of theory, and the Ta(2+)-CH3 bond energy is in good agreement with density functional calculations at the BHLYP level of theory. Theoretical calculations reveal the geometric and electronic structures of all product ions and are used to map the potential energy surface, which describes the mechanism of the reaction and key intermediates. Both experimental and theoretical results suggest that TaH(+), TaCH2(2+), and TaCH3(2+) are formed through a H-Ta(2+)-CH3 intermediate.     

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+.     

Kinetic energy distribution of multiply charged ions in Coulomb explosion of Xe clusters

We report on the calculations of kinetic energy distribution (KED) functions of multiply charged, high-energy ions in Coulomb explosion (CE) of an assembly of elemental Xe(n) clusters (average size (n) = 200-2171) driven by ultra-intense, near-infrared, Gaussian laser fields (peak intensities 10(15) - 4?×?10(16) W cm(-2), pulse lengths 65-230 fs). In this cluster size and pulse parameter domain, outer ionization is incomplete∕vertical, incomplete∕nonvertical, or complete∕nonvertical, with CE occurring in the presence of nanoplasma electrons. The KEDs were obtained from double averaging of single-trajectory molecular dynamics simulation ion kinetic energies. The KEDs were doubly averaged over a log-normal cluster size distribution and over the laser intensity distribution of a spatial Gaussian beam, which constitutes either a two-dimensional (2D) or a three-dimensional (3D) profile, with the 3D profile (when the cluster beam radius is larger than the Rayleigh length) usually being experimentally realized. The general features of the doubly averaged KEDs manifest the smearing out of the structure corresponding to the distribution of ion charges, a marked increase of the KEDs at very low energies due to the contribution from the persistent nanoplasma, a distortion of the KEDs and of the average energies toward lower energy values, and the appearance of long low-intensity high-energy tails caused by the admixture of contributions from large clusters by size averaging. The doubly averaged simulation results account reasonably well (within 30%) for the experimental data for the cluster-size dependence of the CE energetics and for its dependence on the laser pulse parameters, as well as for the anisotropy in the angular distribution of the energies of the Xe(q+) ions. Possible applications of this computational study include a control of the ion kinetic energies by the choice of the laser intensity profile (2D∕3D) in the laser-cluster interaction volume.     

Cationic and anionic fragmentation of dichloromethane following inner-shell (Cl 1s) photoexcitation

The cationic and anionic fragmentation of dichloromethane (CH2Cl2) molecule have been investigated in the energy range of the Cl K shell by using synchrotron radiation, ion yield spectroscopy, and electron-ion coincidence spectroscopy. Total and partial ion-yield and mass spectra have been recorded as a function of the photon energy. We were able to identify several singly and multiply charged cationic fragments and the following anionic species: H-; C-; Cl-. The present results provide the first experimental report of negative ion formation from a molecule excited at the Cl 1s edge. In addition, our electron-ion coincidence data provide strong evidence of the preservation of molecular alignment for the photodissociation of CH2Cl2 after deep core-electron resonant excitation.     

Substituent effects on the gas-phase fragmentation reactions of sulfonium ion containing peptides

The multistage mass spectrometric (MS/MS and MS3) gas-phase fragmentation reactions of methionine side-chain sulfonium ion containing peptides formed by reaction with a series of para-substituted phenacyl bromide (XBr where X=CH2COC6H4R, and R=--COOH, --COOCH3, --H, --CH3 and --CH2CH3) alkylating reagents have been examined in a linear quadrupole ion trap mass spectrometer. MS/MS of the singly (M+) and multiply ([M++nH](n+1)+) charged precursor ions results in exclusive dissociation at the fixed charge containing side chain, independently of the amino acid composition and precursor ion charge state (i.e., proton mobility). However, loss of the methylphenacyl sulfide side-chain fragment as a neutral versus charged (protonated) species was observed to be highly dependent on the proton mobility of the precursor ion, and the identity of the phenacyl group para-substituent. Molecular orbital calculations were performed at the B3LYP/6-31+G** level of theory to calculate the theoretical proton affinities of the neutral side-chain fragments. The log of the ratio of neutral versus protonated side-chain fragment losses from the derivatized side chain were found to exhibit a linear dependence on the proton affinity of the side-chain fragmentation product, as well as the proton affinities of the peptide product ions. Finally, MS3 dissociation of the nominally identical neutral and protonated loss product ions formed by MS/MS of the [M++H]2+ and [M++2H]3+ precursor ions, respectively, from the peptide GAILM(X)GAILK revealed significant differences in the abundances of the resultant product ions. These results suggest that the protonated peptide product ions formed by gas-phase fragmentation of sulfonium ion containing precursors in an ion trap mass spectrometer do not necessarily undergo intramolecular proton 'scrambling' prior to their further dissociation, in contrast to that previously demonstrated for peptide ions introduced by external ionization sources.     

氯碘甲烷在A带的光解动力学

利用离子速度影像方法结合共振增强多光子电离(REMPI)技术研究了氯碘甲烷在A带的光解机理. 从266和277 nm的I*(5p 2P1/2)和I(5p 2P3/2)离子速度影像获得了碎片的平动能分布和角度分布. I和I*的平动能分布呈单高斯型, 可用软自由基近似来解释. I和I*是在排斥的势能面上直接解离产生的. 实验得到的各向异性参数β证实分子受激发后主要产生3Q0态, 并且3Q0和1Q1态之间存在非绝热转移. 波长越短, 这种非绝热转移越强. 在235 nm附近, Cl和Cl*各向同性的离子影像说明氯原子来自于CH2ICl的二次解离过程, 即CH2ICl先解离产生CH2Cl自由基, 自由基再解离产生氯原子.