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.     

Experimental studies of the dynamics of the bond-forming reactions of CF2(2+) with H2O using position-sensitive coincidence spectroscopy

The dynamics of the product channels forming OCF(+)+H(+)+HF and HCF(2) (+)+H(+)+O following the collisions of CF(2) (2+) with H(2)O have been investigated with a new position-sensitive coincidence experiment at a center-of-mass collision energy of 5.6 eV. The results show the formation of OCF(+) occurs via the formation of a doubly charged collision complex [H(2)O-CF(2)](2+) which subsequently undergoes a charge separating dissociation to form H(+) and HOCF(2) (+). The HOCF(2) (+) monocation subsequently fragments to form HF+OCF(+). The lifetimes of the collision complex and the HOCF(2) (+) ion are at least of the order of their rotational period. The kinetic energy release in this reaction indicates that it involves the ground state of CF(2) (2+) and forms the ground electronic states of OCF(+) and HF. The mechanism for forming HCF(2) (+) involves the direct and rapid abstraction of a hydride ion from H(2)O by CF(2) (2+). The resulting OH(+) ion subsequently fragments to H(+)+O, on a time scale at least comparable with its rotational period.     

The double photoionization of HCl: an ion-electron coincidence study

The double photoionization of HCl molecules by synchrotron radiation has been studied in the energy range between 30 and 50 eV. The HCl(2+) and Cl(2+) product ions have been detected by a photoelectron-photoion-coincidence technique, while the H(+)+Cl(+) formation, which follows the double ionization of HCl, has been studied by photoelectron-photoion-photoion coincidence. The photon energy threshold for the production of HCl(2+) ions has been found to be 35.4+/-0.6 eV, while for the dissociative channel leading to H(+)+Cl(+), it has been measured a threshold at 36.4+/-0.6 eV and a change in the slope of the cross-section energy dependence at 38.7+/-0.7 eV. The production of H+Cl(2+) occurs with a threshold photon energy of 42.8+/-1.1 eV. These results appear to be in a good agreement with previous data by different experimental techniques and recent theoretical calculations performed by our laboratory.     

Dissociative double photoionization of benzene molecules in the 26-33 eV energy range

A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the dissociative double ionization of benzene by UV synchrotron radiation. The threshold energy for the main dissociative processes, leading to CH(3)(+) + C(5)H(3)(+), C(2)H(3)(+) + C(4)H(3)(+) and C(2)H(2)(+) + C(4)H(4)(+) ion pairs were characterized by exploiting a photoelectron-photoion-photoion-coincidence technique, giving 27.8 ± 0.1, 29.5 ± 0.1, and 30.2 ± 0.1 eV, respectively. The first reaction also proceeds via the formation of a metastable C(6)H(6)(2+) dication. The translational kinetic energy of the ionic products was evaluated by measuring the position of ions arriving to the detector. Theoretical calculations of the energy and structure of dissociation product ions were performed to provide further information on the dynamics of the charge separation reactions following the photoionization event.     

Double photoionization and dication fragmentation of CF3I: Experiment and theory

The double photoionization of CF(3)I and the electronic structure and the dissociation dynamics of the CF(3)I(++) dication have been investigated using large ab initio calculations and coincidence techniques. The double photoionization spectrum of CF(3)I consists of a continuous background with a number of narrow bands superimposed. The spectrum is attributed here to the population of groups of close lying electronic states interacting mutually by spin-orbit, spin-spin, and rovibronic couplings. At energies near the vertical double ionization threshold, CF(3) (+)+I(+) ionic fragments are produced. At higher energies, a very specific dissociation with double charge retained on one fragment, CF(3)I(++)-->CF(2)I(++)+F becomes dominant and is attributed to a specific group of dication electronic states.     

Metastable decomposition and hydrogen migration of ethane dication produced in an intense femtosecond near-infrared laser field

We investigated a formation channel of triatomic molecular hydrogen ions from ethane dication induced by irradiation of intense laser fields (800 nm, 100 fs, ～1 × 10(14) W∕cm(2)) by using time of flight mass spectrometry. Hydrogen ion and molecular hydrogen ion (H,D)(n)(+) (n = 1-3) ejected from ethane dications, produced by double ionization of three types of samples, CH(3)CH(3), CD(3)CD(3), and CH(3)CD(3), were measured. All fragments were found to comprise components with a kinetic energy of ～3.5 eV originating from a two-body Coulomb explosion of ethane dications. Based on the signal intensities and the anisotropy of the ejection direction with respect to the laser polarization direction, the branching ratios, H(+):D(+) = 66:34, H(2)(+):HD(+):D(2)(+) = 63:6:31, and H(3)(+):H(2)D(+):HD(2)(+):D(3)(+) = 26:31:34:9 for the decomposition of C(2)H(3)D(3)(2+), were determined. The ratio of hydrogen molecules, H(2):HD:D(2) = 31:48:21, was also estimated from the signal intensities of the counter ion C(2)(H,D)(4)(2+). The similarity in the extent of H∕D mixture in (H,D)(3)(+) with that of (H,D)(2) suggests that these two dissociation channels have a common precursor with the C(2)H(4)(2+)...H(2) complex structure, as proposed theoretically in the case of H(3)(+) ejection from allene dication [A. M. Mebel and A. D. Bandrauk, J. Chem. Phys. 129, 224311 (2008)]. In contrast, the (H,D)(2)(+) ejection path with a lower extent of H∕D mixture and a large anisotropy is expected to proceed essentially via a different path with a much rapid decomposition rate. For the Coulomb explosion path of C-C bond breaking, the yield ratios of two channels, CH(3)CD(3)(2+)→ CH(3)(+) + CD(3)(+) and CH(2)D(+) + CHD(2)(+), were 81:19 and 92:8 for the perpendicular and parallel directions, respectively. This indicates that the process occurs at a rapid rate, which is comparable to hydrogen migration through the C-C bond, resulting in smaller anisotropy for the latter channel that needs H∕D exchange.     

Revision of the second ionization energy of toluene

Charge stripping (CS) of the molecular ion of toluene, C(7)H(8) (+)-->C(7)H(8) (2+)+e, is often used as a reference for the determination of second ionization energies in energy-resolved CS experiments. For calibration of the kinetic energy scale, a value of IE(C(7)H(8) (+))=(15.7+/-0.2) eV derived from the appearance energy of the toluene dication upon electron ionization has been accepted generally. Triggered by some recent discrepancies between CS measurements on the one hand and different experimental methods as well as theoretical predictions on the other, we have reinvestigated the photon-induced double ionization of toluene using synchrotron radiation. These photoionization measurements yield phenomenological appearance energies of AE(C(7)H(8) (+))=(8.81+/-0.03) eV for the monocation and AE(C(7)H(8) (2+))=(23.81+/-0.06) eV for the dication. The former is in good agreement with a much more precise spectroscopic value, IE(C(7)H(8))=(8.8276+/-0.0006) eV. Explicit consideration of the Franck-Condon envelopes associated with photoionization to the dication in conjunction with the application of the Wannier law leads to an adiabatic ionization energy IE(a)(C(7)H(8) (+))=(14.8+/-0.1) eV, which is as much as 0.9 eV lower than the previous value derived from electron ionization. Because in many previous CS measurements the transition C(7)H(8) (+)-->C(7)H(8) (2+)+e was used as a reference, the energetics of several gaseous dications might need some readjustment.     

Two-body Coulomb explosion and hydrogen migration in methanol induced by intense 7 and 21 fs laser pulses

Two-body Coulomb explosion with the C-O bond breaking of methanol induced by intense laser pulses with the duration of Delta t=7 and 21 fs is investigated by the coincidence momentum imaging method. When Delta t=7 fs, the angular distribution of recoil vectors of the fragment ions for the direct C-O bond breaking pathway, CH(3)OH(2+)-->CH(3) (+)+OH(+), exhibits a peak deflected from the laser polarization direction by 30 degrees -45 degrees , and the corresponding angular distribution for the migration pathway, CH(2)OH(2) (+)-->CH(2) (+)+H(2)O(+), in which one hydrogen migrates from the carbon site to the oxygen site prior to the C-O bond breaking, exhibits almost the same profile. When the laser pulse duration is stretched to Delta t=21 fs, the angular distributions for the direct and migration pathways exhibit a broad peak along the laser polarization direction probably due to the dynamical alignment and/or the change in the double ionization mechanism; that is, from the nonsequential double ionization to the sequential double ionization. However, the extent of the anisotropy in the migration pathway is smaller than that in the direct pathway, exhibiting a substantial effect of hydrogen atom migration in the dissociative ionization of methanol interacting with the linearly polarized intense laser field.     

Evidence of CH2O (a3A2) and C2H4 (a3B1u) produced from photodissociation of 1,3-trimethylene oxide at 193 nm

We investigated the dissociative ionization of formaldehyde (CH(2)O) and ethene (C(2)H(4)) produced from photolysis of 1,3-trimethylene oxide at 193 nm using a molecular-beam apparatus and vacuum-ultraviolet radiation from an undulator for direct ionization. The CH(2)O (C(2)H(4)) product suffers from severe dissociative ionization to HCO(+) (C(2)H(3) (+) and C(2)H(2) (+)) even though photoionization energy is as small as 9.8 eV. Branching ratios of fragmentation of CH(2)O and C(2)H(4) following ionization are revealed as a function of kinetic energy of products using ionizing photons from 9.8 to 14.8 eV. Except several exceptions, branching ratios of daughter ions increase with increasing photon energy but decrease with increasing kinetic energy. The title reaction produces CH(2)O and C(2)H(4) mostly on electronic ground states but a few likely on triplet states; C(2)H(4) (a(3)B(1u)) seems to have a yield greater than CH(2)O (a(3)A(2)). The distinct features observed at small kinetic energies of daughter ions are attributed to dissociative ionization of photoproducts CH(2)O (a(3)A(2)) and C(2)H(4) (a(3)B(1u)). The observation of triplet products indicates that intersystem crossing occurs prior to fragmentation of 1,3-trimethylene oxide.     

Kinetic energy release of protonated methanol clusters using the low-temperature fast-atom bombardment: experiment and theory combined

Low-temperature fast-atom bombardment was found to be an excellent method for generating large protonated methanol clusters, (CH(3)OH)(n)H(+) (n = 2 to 15). Metastable dissociations of these clusters, involving elimination of one methanol molecule, were studied using mass-analyzed ion kinetic energy spectra (MIKES). From metastable peak profiles kinetic energy release (KER) distributions were obtained, even for clusters as large as (CH(3)OH)(15)H(+). The results were analyzed by a simple thermal model, by the finite heat bath theory (FHBT) and by the RRKM-based MassKinetics algorithm. The KER distribution was shown to correspond to a three-dimensional translational energy distribution, implying statistical energy partitioning in the transition state. The mean KER values and transition state temperatures were found to increase with cluster size, reaching 25 meV and approximately 210 K for large clusters (n = 10).     

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.     

Dissociative double photoionization of N2 using synchrotron radiation: appearance energy of the N2+ dication

Photoionization cross sections for the production of the doubly charged ion N2+ from N2 have been measured by means of synchrotron radiation in the photon energy range from 50 to 110 eV. The appearance energy for N2+ has been determined as 55.2+/-0.2 eV, i.e., about 1.3 eV higher than the spectroscopic dissociation limit leading to the charge asymmetric dissociation channel N2+(2P)+N(4S) at 53.9 eV. The onset of a second threshold at 59.9+/-0.2 eV is detected and the energy dependence of photoion intensities near the threshold regions is interpreted in terms of the Wannier theory. The production of the N2+ dication is discussed in terms of direct and indirect mechanisms for dissociative charge asymmetric photoionization and by comparison with the potential energy curves of the intermediate N(2)2+ dication. Experimental evidences for the opening of the Coulomb explosion channel N2++N+ at high photon energies are provided by measuring the kinetic energy release spectra of N2+ fragments at selected photon energies.     

A study of selected fragmentation paths of the ethyne dication: theory and experiment

The fragmentation of the C(2)H(2)(2+) dication, formed upon inner shell ionization and the subsequent Auger decay, has been studied by means of Auger electron-ion and Auger electron-ion-ion coincidence spectroscopy at four different kinetic energies of the Auger electron. The experimental investigation of three fragmentation paths leading to the C(2)H(+)/H(+), C(2)(+)/H(+) and C(+)/H(+) pairs has been complemented by theoretical calculations of the Potential Energy Surfaces (PES). It is found that when the amount of internal energy of the dication increases this is mainly transferred into the kinetic energy of the fragments of the second step of the dissociation.     

Anisotropy of the angular distribution of fragment ions in dissociative double photoionization of N2O molecules in the 30-50 eV energy range

The double photoionization of N2O molecules by linearly polarized light in the 30-50 eV energy range has been studied by coupling ion imaging technique and electron-ion-ion coincidence. For the two possible dissociative processes, leading to N++NO+ and O++N2+, angular distributions of ionic fragments have been measured, finding an evident anisotropy. This indicates that the molecules ionize when their axis is parallel to the light polarization vector and the fragments are separating in a time shorter than the dication rotational period. The analysis of results provides, in addition to the total kinetic energy of ionic fragments, crucial information about the double photoionization dynamics.     

Dissociative double photoionization of singly deuterated benzene molecules in the 26-33 eV energy range

This work provides new experimental and theoretical results about the formation and dissociation of benzene dication. The experiment has been carried out by using a vacuum ultraviolet radiation from a synchrotron source together with a time-of-flight spectrometer and a position sensitive ion detector. Isotopically labeled benzene molecules with a single deuterium atom have been used in order to study the symmetric dissociation of the benzene dication, not well evident in previous experiments. A threshold of 30.1 ± 0.1 eV has been observed for this dissociation reaction. Moreover, the lifetime of the dissociation of the benzene metastable dication producing CH(3)(+) and C(5)H(3)(+) has been obtained as a function of the photon energy, by the use of a Monte Carlo trajectory analysis of the coincidence distributions. The determined lifetime is independent of the photon energy and has an average value of 0.75 ± 0.22 μs. Theoretical calculations of the energy and structure of dissociation product ions have been also performed to provide crucial information about the dynamics of the charge separation reactions following the photoionization event.     

Valence and inner-valence shell dissociative photoionization of CO in the 26-33 eV range. II. Molecular-frame and recoil-frame photoelectron angular distributions

Experimental and theoretical results for molecular-frame photoemission are presented for inner-valence shell photoionization of the CO molecule induced by linearly and circularly polarized light. The experimental recoil frame photoelectron angular distributions (RFPADs) obtained from dissociative photoionization measurements where the velocities of the ionic fragment and photoelectron were detected in coincidence, are compared to RFPADs computed using the multichannel Schwinger configuration interaction method. The formalism for including a finite lifetime of the predissociative ion state is presented for the case of general elliptically polarized light, to obtain the RFPAD rather than the molecular frame photoelectron angular distribution (MFPAD), which would be obtained with the assumption of instantaneous dissociation. We have considered photoionization of CO for the photon energies of 26.0 eV, 29.5 eV, and 32.5 eV. A comparison of experimental and theoretical RFPADs allows us to identify the ionic states detected in the experimental studies. In addition to previously identified states, we found evidence for the 2 (2)Δ state with an ionization potential of 25.3 eV and (2)Σ(+) states with ionization potentials near 32.5 eV. A comparison of the experimental and theoretical RFPADs permits us to estimate predissociative lifetimes of 0.25-1 ps for some of the ion states. Consideration of the MFPADs of a series of (2)Π ion states indicates the importance of inter-channel coupling at low photoelectron kinetic energy and the limitations of a single-channel analysis based on the corresponding Dyson orbitals.     

Theoretical study of HCN(+) + C2H2 reaction

A detailed theoretical investigation for the ion-molecule reaction of HCN (+) with C 2H 2 is performed at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels. Possible energetically allowed reaction pathways leading to various low-lying dissociation products are probed. It is shown that eight dissociation products P 1 (H 2C 3N (+)+H), P 2 (CN+C 2H 3 (+)), P 3 (HC 3N (+)+H 2), P 4 (HCCCNH (+)+H), P 5 (H 2NCCC (+)+H), P 6 (HCNCCH (+)+H), P 7 (C 2H 2 (+)+HCN), and P 8 (C 2H 2 (+)+HNC) are both thermodynamically and kinetically accessible. Among the eight dissociation products, P 1 is the most abundant product. P 7 and P 3 are the second and third feasible products but much less competitive than P 1 , followed by the almost negligible product P 2 . Other products, P 4 (HCCCNH (+)+H), P 5 (HCNCCH (+)+H), P 6 (H 2NCCC (+)+H), and P 8 (C 2H 2 (+)+HNC) may become feasible at high temperatures. Because the intermediates and transition states involved in the reaction HCN (+) + C 2H 2 are all lower than the reactant in energy, the title reaction is expected to be rapid, as is consistent with the measured large rate constant at room temperature. The present calculation results may provide a useful guide for understanding the mechanism of HCN (+) toward other pi-bonded molecules.     

Distributions of angular anisotropy and kinetic energy of products from the photodissociation of methanol at 157 nm

We investigated distributions of angular-anisotropy parameter beta and kinetic energy of fragments after photodissociation of methanol using time-of-flight (TOF) mass spectrometry. Fragments, in particular CH(3)O and CO, were successfully detected using tunable radiation from a synchrotron for photoionization. Following O-H bond fission, a CH(3)O fragment with internal energy greater than 104 kJ mol(-1) dissociates to CH(2)O+H. Elimination of two H(2) accompanies formation of CO. The beta value of hydroxyl hydrogen is -0.26 whereas that of methyl hydrogen is zero. H(2) has two distinct components in TOF spectra; these rapid and slow components have beta values -0.30 and -0.18, respectively. The CH(3)+OH dissociation exhibits a highly anisotropic angular distribution with beta= -0.75. The beta values of fragments from CD(3)OH photolysis are addressed. From measurements of angular-anisotropy parameters of various fragments, we surmise that the transition dipole moment mu is almost perpendicular to the C-O-H plane and that n-3p(x) (2 (1)A") is the major photoexcited state at 157 nm.     

溴乙烷光电离解离的理论计算和实验

分子的电离电势、键能和离子的标准生成焓等都是非常重要的物理化学数据,它们对化学反应机理等研究有很大帮助,精确测定离子的出现势,就可以获得这些热力学常数.迄今为止,有关溴乙烷(Ｃ2Ｈ5Ｂｒ)电离解离过程的研究已有若干报导[1-3],但这些结果均是在常温条件下,用电子轰击电离、彭宁电离或真空紫外灯辐照等方法获得的,由于常伴有热带效应、离子分子反应、离子对形成等过程[4],其结果的准确性往往较差.本文首次报导使用同步辐射光源对Ｃ2Ｈ5Ｂｒ进行光电离解离研究.通过准确测量母体离子以及几种主要碎片离子的出现势,结合已有的公认的热力…     

Spectroscopy and metastability of CO2 2+ molecular ions

The spectroscopy and metastability of the carbon dioxide doubly charged ion, the CO(2) (2+) dication, have been studied with photoionization experiments: time-of-flight photoelectron photoelectron coincidence (TOF-PEPECO), threshold photoelectrons coincidence (TPEsCO), and threshold photoelectrons and ion coincidence (TPEsCO ion coincidence) spectroscopies. Vibrational structure is observed in TOF-PEPECO and TPEsCO spectra of the ground and first two excited states. The vibrational structure is dominated by the symmetric stretch except in the TPEsCO spectrum of the ground state where an antisymmetric stretch progression is observed. All three vibrational frequencies are deduced for the ground state and symmetric stretch and bending frequencies are deduced for the first two excited states. Some vibrational structure of higher electronic states is also observed. The threshold for double ionization of carbon dioxide is reported as 37.340+/-0.010 eV. The fragmentation of energy selected CO(2) (2+) ions has been investigated with TPEsCO ion coincidence spectroscopy. A band of metastable states from approximately 38.7 to approximately 41 eV above the ground state of neutral CO(2) has been observed in the experimental time window of approximately 0.1-2.3 mus with a tendency towards shorter lifetimes at higher energies. It is proposed that the metastability is due to slow spin forbidden conversion from bound excited singlet states to unbound continuum states of the triplet ground state. Another result of this investigation is the observation of CO(+)+O(+) formation in indirect dissociative double photoionization below the threshold for formation of CO(2) (2+). The threshold for CO(+)+O(+) formation is found to be 35.56+/-0.10 eV or lower, which is more than 2 eV lower than previous measurements.