Formation Of Parent Ions Research Articles

Multiphoton breakdown of acetylene; formation of organic building block fragments.

Mass resolved REMPI spectra and electron and ion velocity map images were recorded for REMPI of acetylene in the case of two-photon resonant excitations to low lying 3p and 4p Rydberg states. Combined data analysis of ion signal intensities and electron and ion kinetic energy release distribution revealed multiphoton-fragmentation processes in terms of photodissociation and photoionization channels to form the molecular ion, C2H2+ and the fragment ions H+, C+, CH+, CH2+, C2+ and C2H+. The ratio of fragment ion formation over the parent ion formation increases with excitation energy. To a large extent, multiphoton-fragmentation involves the initial breakdown of the molecule into ground and excited state neutral fragments by two-, three- and four-photon dissociation processes prior to multiphoton ionization. The three-photon dissociation processes via superexcited molecular state(s) are found to be the most important and electronically excited fragment species playing a significant role in the overall multiphoton-fragmentation. Furthermore, the data are indicative of the involvement of secondary photodissociation processes and provide information on fragment energetics as well as state interactions. The question, whether acetylene could be an important source of building block fragments for the formation of organic molecules in interstellar space, is addressed.

Dominance of parent ion formation in the ultrafast, intense-field ionization of the valence isoelectronic molecules and

We report on the photodynamics of the valence isoelectronic molecules carbon dioxide () and carbon disulfide () under intense-field, single-molecule conditions in focused 50 fs, 800 nm laser pulses. Ion mass spectra are recorded as a function of intensity in the range from to . Ion yields are measured in the absence of the focal volume effect without need for data deconvolution. In both targets, formation of the molecular parent ion () is by far the dominant process; the yield of other species is only a few percent. We observe that the yield of this parent ion is strictly proportional to an integer power of the intensity, until saturation sets in. We conclude the parent ion is formed through multiphoton single-electron ionization, with the photoelectron originating from the doubly degenerate, non-bonding highest-occupied molecular orbital (HOMO). The non-bonding nature of this HOMO explains the remarkable stability of the parent ion.

Photoemission and photofragmentation of butanoic, hexanoic and octanoic acids in the gas phase

The valence and C 1 s core photoelectron spectra of three carboxylic acids (butanoic, hexanoic and octanoic) as well as the near-edge absorption fine structure spectra and photo-electron photo-ion coincidence spectra have been measured. Changes in the valence spectra are related to the lengthening of the aliphatic chains, and the core spectra have been assigned. Small shifts of the carboxylic carbon 1 s binding energy are related to screening of the core hole by the aliphatic group. The C K edge near-edge absorption fine structure spectra probe the unoccupied density of states and the main changes in the spectra are due to increasing length of the aliphatic chain and related Rydberg states. The coincidence spectra show that soft (near threshold) ionization of all compounds leads to parent ion formation for butanoic and octanoic acids. For hexanoic acid, both parent ions and fragments are produced for near threshold ionization. Fragmentation occurs at the C2C3 bond for all carboxylic acids at the lowest energies, and other channels open for higher internal energy.

Photoelectron–photoion coincidence momentum imaging for dissociative ionization of ethanol in intense laser fields

Ionization dynamics of ethanol in intense laser fields is investigated by photoelectron–photoion coincidence momentum imaging. Comparisons are made among the photoelectron images for the four different single ionization channels; the parent ion formation, H elimination, CH 3 elimination, and OH elimination. At the intensity of 9.3 TW/cm 2 with the duration of 35 fs, the electronically excited states are prepared directly in the ionization for the CH 3 and OH elimination. As the pulse becomes longer and stronger, the ionization to the electronic ground state becomes dominant even for the CH 3 and OH elimination, suggesting the efficient electronic excitation subsequent to the ionization.

High-temperature mass spectrometry: Instrumental techniques, ionization cross-sections, pressure measurements, and thermodynamic data (IUPAC Technical Report)

Abstract An assessment of high-temperature mass spectrometry and of sources of inaccuracy is made. Experimental, calculated, and estimated cross-sections for ionization of atoms and inorganic molecules typically present in high-temperature vapors are summarized. Experimental cross-sections determined for some 56 atoms are generally close to theoretically calculated values, especially when excitation–autoionization is taken into account. Absolute or relative cross-sections for formation of parent ions were measured for ca. 100 molecules. These include homonuclear diatomic and polyatomic molecules, oxides, chalcogenides, halides, and hydroxides. Additivity of atomic cross-sections supplemented by empirical corrections provides fair estimates of molecular cross-sections. Causes of uncertainty are differences in interatomic distances and in shapes of potential energy curves (surfaces) of neutral molecules and of molecular ions and tendency toward dissociative ionization in certain types of molecules. Various mass spectrometric procedures are described that render the accuracy of measured thermodynamic properties of materials largely independent of ionization cross-sections. This accuracy is comparable with that of other techniques applicable under the conditions of interest, but often only the mass spectrometric procedure is appropriate at high temperatures.

Reactivity of anomeric 1alpha- and 1beta-pentofuranosyl azide derivatives in ammonia chemical ionization.

Electron impact (EI), fast atom bombardment (FAB) and ammonia chemical ionization [CI(NH3)] mass spectrometry were applied with the aim of differentiating between the anomeric 1alpha- and 1beta-azidopentofuranosyl derivatives. Calculated ammonium affinities [AA(M)] and proton affinities [PA(M)] show that beta-anomers have higher affinities for H+ and NH4+ ions than alpha-azides. Protonated molecules, obtained by CI(NH3) of azidofuranosyl derivatives, lose HN3 giving abundant furanosyl (S+) ions. Ammonia solvation of MH+ ions competes with the previous reaction producing the [SNHN2NH3]+ ion, a competitive product to the ammonium-attached [SN3NH4]+ ion. The fragmentation pathways of the stable and metastable [MNH4]+, MH+ ions, and several other important fragment ions, were determined using mass analyzed ion kinetic energy spectrometry (MIKES). The abundance of the [SN3NH4]+ and/or [SNHN2NH3]+ ions was found to correlate inversely with the exothermicity of ammonia solvation of the MH+ ion. The abundance of the fragment ions [SNHNH3]+, [SNH3]+ and SNH+ in some examples correlates with the exothermicity of the corresponding [MNH4]+ and MH+ parent ion formation. The fragment ions SNH3+ and SNHNH3+ can be formed, at least in part, in the ammonia solvation reaction of the S+ and SNH+ ions taking place within the high-pressure region of the CI ion source.

Multiphoton dissociation/ionisation of dimethyl sulphide (CH3SCH3) at 355 and 532 nm

Multiphoton dissociation/ionization has been studied for CH3SCH3 at 355 and 532 nm using a time-of-flight mass spectrometer. The major ion signals observed at 355 nm are C+, CH3 +, HCS+, CH2S+, CH3S+ and CH3SCH3 +. Power dependence studies at 355 nm show a (2+1) REMPI process for the formation of parent ion. Peaks atm/e = 46, 47 and 61 show two-photon laser power dependence whereasm/e = 15 and 45 peaks show four-photon dependence. However, in 532 nm photo-ionisation, no parent ion signal is observed. A peak atm/e = 35 corresponding to SH3 + has been observed. SH3 + has been suggested to originate from CH3SCH2 + via a cyclic transition state. Photoionisation results of CH3SCH3 have been compared with those of CH3SSCH3, at these two wavelengths.

Imaging studies of the multiphoton excitation, dissociation and ionization of Br2

Both ions and photoelectrons arising in the multiphoton excitation of molecular bromine at various wavelengths in the range 430–475 nm have been investigated using imaging methods. The ion time-of-flight (TOF) spectra show only Br+ fragment ions (and no parent ions) under the prevailing experimental conditions. However, analysis of the photoelectron images shows clear evidence of both parent ion formation and predissociation at the energy of four absorbed photons. Highly excited Br atoms in the 4P1/2 and/or 2P3/2 states derived from the electronic configuration ···4s24p4[3P1]5s (with respective term values of 66883.9 and 67183.6 cm−1) are amongst the predissociation products. Analysis of the ion images reveals efficient one (and two) photon excitation and dissociation of the parent Br2+ ions (depending on excitation wavelength and the extent of internal excitation within the parent ion)—thereby accounting for the observed absence of parent ions in the ion TOF spectrum. The study also serves to reinforce previous conclusions regarding spin–orbit branching in the Br atomic fragments resulting from one photon dissociation of Br2 in this wavelength range, and provides tantalizing glimpses of the rich variety of multiphoton ionization dynamics exhibited by the atomic Br fragments.

Electron impact ionization of the hydroxyl radical

We employed the fast-neutral-beam technique in a measurement of absolute partial cross sections for the electron-impact ionization and dissociative ionization of the hydroxyl free radical from threshold to 200 eV. The deuterated OD radical rather than the protonated OH radical was used as a target in our studies in order to allow a better separation of the various product ions in our apparatus. The total (single) OD ionization cross section was found to have a value of slightly less than 2.0×10−16 cm2 at 70 eV. The ionization of OD is dominated by the formation of parent ions with a parent ionization cross section of 1.85×10−16 cm2 at this energy. A comparison of the experimentally determined total single OD ionization cross section with a calculated OH cross section using a modified additivity rule showed good agreement in terms of the absolute value and the cross section shape (at least above 50 eV). In the course of this work, we also measured the partial ionization cross sections for the D2O molecule and found good agreement between our cross sections and the most recent measurements of Straub et al. [J. Chem. Phys. 108, 109 (1998)] as well as with recent calculations.

Time-of-flight mass spectrometry of aromatic molecules subjected to high intensity laser beams

The recent introduction of femtosecond technology to pulsed lasers has led to the development of femtosecond laser mass spectrometry (FLMS). The present paper describes an FLMS investigation of the aromatic molecules, benzene, toluene and naphthalene. Wavelengths of 750 and 375 nm were used with beam intensities up to 4 × 1014 W cm−2. Pulse widths were of the order of 50–90 fs. The laser system was coupled to a linear time-of-flight mass spectrometer. This experimental method of chemical analysis is gaining momentum, often replacing its nanosecond forerunner, resonant enhanced multiphoton ionization. For the said molecules, predominant parent ion production is found, making identification unambiguous. In fact this characteristic is being consistently attained in small to medium mass molecules irradiated under similar conditions, leading to the conclusion that a universal chemical detection system is a possibility. Such soft ionization is particularly evident at longer wavelengths (∼750 nm) with less relative fragmentation, daughter ion formation, compared to results at shorter wavelengths (∼375 nm). In terms of parent ion formation, similar numbers are produced with laser intensities around 1014 W cm−2 for both wavelengths. It has also been shown that at a threshold of about 5 × 1013 W cm−2, double ionized molecules appear for the 750 nm wavelength. These interesting new mass spectra display intense single, double and even triple ionized peaks without significantly increased dissociation. Such effects are less pronounced at 375 nm. © 1998 John Wiley & Sons, Ltd.

Ionization and Dissociation of Benzaldehyde Using Short Intense Laser Pulses

In a recent series of experiments, femtosecond laser mass spectrometry (FLMS) was applied to benzaldehyde utilizing laser pulse widths in the range 90 fs to 2.7 ps. Beam intensities up to 2 × 1014 W cm-2 were used with wavelengths of 750 and 375 nm. Different ionization−dissociation channels were found compared with previous studies by other authors based in the nanosecond regime. The general theme emerging is one of predominant above-threshold ionization−dissociation (ID) in which predissociative states are largely bypassed via rapid optical up-pumping to the molecular ionization continuum. Above the parent ionization threshold, ladder-switching is seen to be a function of laser pulse width, intensity, and wavelength with exclusive parent ion formation being achievable in the lower intensity regions at all pulse widths. Increasing fragmentation occurs as the laser intensity increases, although parent supremacy remains. At 750 nm however, the increase of fragmentation with intensity is greatly reduced compared to 375 nm, leading to the conclusion that FLMS at longer wavelengths is preferred for chemical analysis. Moreover at 750 nm and at laser intensities close to 1014 W cm-2 C7H6O2+ becomes evident. It is also shown that the C7H6O+/C7H5O+ ratio is strongly dependent on the pulse width, suggesting that a hydrogen loss pathway has a dissociation time of about a picosecond.

Thermal energy reactions of CO+2 with chloromethanes

Rate constants and product ions have been determined for thermal energy reactions of CO2+ with CHnCl4−n(n=0–3) by using an ion-beam apparatus. Total rate constants are (8.7±3.7), (6.7±3.1), (9.1±4.1), and (4.9±1.6)×10−10 cm3 s−1 for CH3Cl, CH2Cl2, CHCl3, and CCl4, respectively. These values amount to 38%–61% of the collision rate constants estimated from either the Langevin or averaged dipole oriented theory. Although charge transfer followed by the successive loss of a Cl atom is the major product channel, parent ions are formed from CH3Cl and CH2Cl2 with branching ratios of 33%±5% and 25%±3%, respectively. The reaction mechanisms are interpreted in terms of the electronic states of the parent molecular ion accessible in the charge-transfer processes. The lack of formation of parent ions from CHCl3 and CCl4 is explained as due to complete (pre)dissociation of ionic states below 13.78 eV.

Low-energy collision-activated dissociation of n-butylbenzene. Effect of the electron energy used during parent ion formation

Low-energy collision-activated dissociation of n-butylbenzene. Effect of the electron energy used during parent ion formation

Attachment of Electrons to Substituted Benzenes

Attachment of slow (0–5 eV) electrons to a number of aromatic species has been studied in a time-of-flight (TOF) mass spectrometer using an automatic retarding potential difference (RPD) electron beam with resolution of ∼ 0.1 eV. Autodetachment lifetimes were measured for parent ions formed by two-body attachment of thermal (∼ 0.03 eV) electrons to the following molecules (lifetimes in microseconds are given in parentheses): C6F5Br (21), C6F5Cl (17.6), C6F5CN (17), C6F5CHO (36), C6H5CN (≈ 5), C6H5NO2 (17.5), C6D5NO2 (22), m-C6H4ClNO2 (47), and perfluoronaphthalene (123). The C10F8− ion and ions of the form C6F5X− had lifetimes exhibiting a systematic increase with the number of vibrational degrees of freedom consistent with the trend observed in a previous study of cyclic fluorocarbons. Cl−, Br− and I− were formed from C6F5X (X==Cl, Br, I) along with C6F5− at incident electron energies below 0.1 eV. Current ratios for X− to C6F5− were found to be approxmaltely 100/1, 3/1, and 1/30, respectively. No nondissociative attachment was observed for C6F5I. Monochlorination of nitrobenzene essentially doubled the autodetachment lifetime and added a new mode of dissociation. Cl− and NO2− ion currents from m-chloronitrobenzene showed almost identical energy dependences with peaks at ∼ 1 eV and ∼ 3.5 eV, corresponding closely to the energy dependence of NO2− dissociation from nitrobenzene. This was taken as evidence that the Cl− and NO2− ion currents result from unimolecular dissociation of the same intermediate negative ion state with the incident electron in a π-electronic orbital. Iodine substitution in nitrobenzene had the effect of elimination of long-lived parent ion formation with dissociative attachment producing I− dominating at thermal electron energy. An SF6 scavenger spectrum of benzaldehyde indicated the presence of a temporary negative ion resonance at ∼ 0.65 eV. Negative ion breakdown curves were also determined for C6H5OH, C6F5OCH3, C6F5NH2, C6H5CN, o-C6H4ClF, m-C6H4ClF, m-C6F4ClBr, and m-C6H4ICl.