Breakdown Diagram Research Articles

On the dissociation dynamics of energy-selected dimethylether ions

Dissociation of the dimethylether ion has been studied using threshold photoelectron-photoion coincidence (TPEPICO) mass spectrometry. Our TPEPICO breakdown curves showed a significant occurrence of electronic autoionization at around 12.2 eV and over 16 eV. For the formation of [CH 3O] +, rectangular flight-time distributions owing to a large, sharp kinetic energy release were observed at high energies, and about 80% of internal excess energy of the molecular ion was found to be released as translational energy in this fragmentation. This remarkably high conversion was consistent with the spectator model of direct dissociation rather than with statistical theories (RRKM—QET). However, the relative abundance of [CH 3O +] remained less than 20% in the breakdown diagram although direct dissociation predicts its predoiminace. These results may be intepreted as a direct dissociation to CH 3O + on a repulsive state reached by transition from an initial state or as a statistical fragmentation to CH 2O + H strongly controlled by a tight transition exit on a high reverse activation barrier.

A photoelectron–photoion coincidence study of H2O, D2O, and (H2O)2

Photoelectron–photoion coincidence (PEPICO) data for OH+(OD+), H+(D+), and H2O+ (D2O+) from H2O (D2O) have been obtained in the region of 625–700 Å. The PEPICO measurements allow the construction of breakdown diagrams for the unimolecular dissociation of energy-selected H2O+ and D2O+ in the B̃ 2B2 state. The breakdown diagrams for H2O+(B̃ 2B2) and D2O+(B̃ 2B2) in the internal energy range of 129–166 kcal/mol are essentially identical. The branching ratios observed for H+ (D+) are higher than those reported previously. About 3%–5% of stable H2O+ (D2O+) is observed in the time scale of ≊10 μs. These stable H2O+ (D2O+) ions are attributed to ultrafast B̃ 2B2→Ã2A1 nonradiative relaxation followed by the radiative stabilization from H2O+(Ã2A1) [D2O+(Ã2A1)] to H2O+ (X̃2B1) [D2O+(X̃2B1)]. This observation also supports that the formation of H+ (D+) via the H2O+ (Ã 2A1)[D2O+(Ã2A1)] state is a viable process. The relative state-selected cross sections for the reaction H2O+ (X̃2B1,Ã2A1; ν1, ν2)+H2O→H3O+ +H at center-of-mass collision energies ≤1.9 eV have been examined using an effusive beam arrangement. Experimental evidence supports that vibrational and electronic excitations suppress the cross section of the latter reaction. We have also performed PEPICO measurements of the processes, (H2O)2+hν→(H2O)+2+e− and H3O+ +OH +e−, in the region of 1020–1110 Å. The sums of the PEPICO intensities for H3O+ and (H2O)+2 at corresponding photoionization wavelengths yield the photoelectron spectrum for (H2O)2.

On the distinction between tight and loose transition states in unimolecular dissociations

Measuring the cross-over point in breakdown diagrams for different sample temperatures provides a simple method to determine the threshold energy, E 0( J), for the unimolecular dissociation of many ions. The variation of E 0 J) with the angular momentum of the molecule depends strongly on the characteristics of the potential energy surface. It is shown by comparison with available data for the dissociation of ethyl chloride and some alkane ions that this method allows one to distinguish tight from loose transition states.

Vacuum ultraviolet photodissociation and photoionization studies of CH3SCH3 and CH3S

We have measured the translational energy releases of the laser photodissociation processes CH3SCH3+hν (193 nm)→CH3+CH3S [process (1)] and CH3SCH2+H [process (2)]; and CH3S+hν (193 nm)→S+CH3 [process (3)]. The onsets of the translational energy distributions for photofragments of processes (1) and (2) allow the direct determination of 74.9±1.5 and 91±2.5 kcal/mol for the dissociation energies of the CH3–SCH3 and H–CH2SCH3 bonds at 0 K, respectively. The threshold observed for S formed by process (3) is consistent with the conclusion that the production of S(3P) is small compared to S(1D). The photoelectron–photoion coincidence (PEPICO) spectra for CH3SCH+3, CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ resulting from photoionization of CH3SCH3 have been measured in the wavelength region of 900–1475 Å. The PEPICO study allows the construction of a detailed breakdown diagram for the formation of CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ from energy-selected CH3SCH+3 ions.

Vacuum ultraviolet photodissociation and photoionization studies of CH3SH and SH

The kinetic energy releases of the photodissociation processes, CH3SH+hν (193 nm)→CH3+SH, CH3S+H, and CH2S+H2, have been measured using the time-of-flight mass spectrometric method. These measurements allow the direct determination of the dissociation energies for the CH3–SH and CH3S–H bonds at 0 K as 72.4±1.5 and 90±2 kcal/mol, respectively. The further dissociation of SH according to the process SH+hν (193 nm)→S+H has also been observed. The appearance energy (AE) of S produced in the latter process is consistent with the formation of S(3P)+H. The photoelectron–photoion coincidence (PEPICO) spectra for CH3SH+, CH3S+ (or CH2SH+), and CH2S+ from CH3SH have been measured in the wavelength range of 925–1460 Å. The PEPICO measurements make possible the construction of the breakdown diagram for the unimolecular decomposition of internal-energy-selected CH3SH+ in the range of 0–83 kcal/mol. The AE measured for CH2S+ is consistent with the conclusion that the activation energy is negligible for 1,2-H2 elimination from CH3SH+.

Threshold photoelectron photoion coincidence study of the ethane loss from energy selected pentane ions cooled in a supersonic expansion

The loss of C2H6 from energy selected pentane ions has been investigated by the threshold protoelectron photoion coincidence method. Breakdown diagrams are reported at room temperature and for two different conditions in a molecular beam. From the observed shift in cross-over energies the 0 K onset for the C2H6 loss is determined to be 690 ± 20 meV. This result together with information about kinetic energy release and other thermochemical data supports the earlier conclusion that the corresponding ion in the C2H6 loss is the propene ion rather than the cyclopropane ion. It is shown that rotational and vibrational energies are both active in this dissociation. The rate constants of dissociation have been measured as a function of the ion internal energy and are shown to agree with the results of Rice-Rampsperger-Kassel-Marcus calculations. Evidence is given that isomerisation of pentane to methylbutane prior to dissociation does not play an important role.

Breakdown diagrams for low‐mass cations from angle‐resolved mass spectrometric measurements

AbstractAngular scatter distributions, P(θ), of 4 keV collisionally activated ions of propan‐1‐ol, ethanol, dimethyl ether, propyne and nitromethane undergoing specific dissociation reactions were obtained by measuring the ‘scatter profiles’ of fragment ions. The most probable (and root mean square) angles associated with these distributions were found to be proportional to the root mean internal energy depositions, but there was not a one‐to‐one relationship between individual angles and energies. However, by using as a basis set the angular distributions found for some non‐dissociative reactions involving ‘single‐valued’ distributions of internal energy gain, it was possible to modify the P(θ) functions to give approximate energy deposition functions which could be presented in breakdown‐diagram form. Such data were generally found to be in agreement with those observed previously.

Shifts in photoionization fragmentation onsets. A direct measure of cooling in a supersonic molecular beam

The dissociation of isobutane and ethyl chloride cooled in a supersonic expansion was investigated by photoelectron-photoion coincidence (PEPICO). The breakdown diagrams for various expansion conditions were compared to the breakdown diagram for a thermal source. The shift in the cross-over energy provides a direct measurement of the internal energy of molecules in the beam. For isobutane the lowest internal energy was observed for a seededbeam with 〈 E int〉 = 5 meV (corresponding to T< 100 K) compared to 〈 E int〉 = 123 meV at room temperature. Theexperiments revealed that rotational energy is available for dissociation in isobutane ions. For ethyl chloride the lowest internalenergy observed was 5 meV compared to 73 meV at room temperature. The results indicated that part of the rotational energy is available forovercoming the dissociation barrier. This is in spite of the tight transition state associated with the isomerization barrier.

A study of the unimolecular dissociation of SO+2(C̃,D̃,Ẽ) using the photoelectron–photoion coincidence method

Photoelectron–photoion coincidence spectra for SO+2, SO+, and S+ resulting from the photoionization of SO2 over the photon energy range of 15.5–17.2 eV (720–800 Å) have been obtained at an electron energy resolution of ≊40 meV (full width at half maximum). The breakdown diagram is presented for SO+2, SO+, and S+. For excited SO+2(C̃,D̃,Ẽ) ions formed initially with internal energies in the range of 4.18–4.87 eV, the breakdown curves for the three ions are relatively smooth. The relative yields for the formation of S+, SO+, and SO+2 via radiative stabilization are approximately 1:80:20. The lack of structure observed in the breakdown curves in this internal energy region is consistent with the conclusion that rapid internal conversion occurs prior to ionic decay. For the SO+2 internal energy range of 3.88–4.18 eV, the S+ breakdown curve is structured, suggesting that the formation of S+ follows a state-specific dissociation pathway. The results of the present experiment do not support the previous findings that the SO+2 and S+ product channels are only formed for photon energies ≤16.7 eV.

Scatter profiling: a potentially useful technique for the study of energy deposition in collision-activated decomposition reactions

Plots of fragment-ion current as a function of 'off-plane' displacement (scatter profiles) can be partitioned into fragmentational and collisional contributions. For reactions CH4+ to CHn+ (n=0-3) occurring in 8 keV collisions with Ar atoms, the latter are dominant with mean displacement values which correlate with the known mean energy depositions. The sensitivity of such curves to the reaction energy requirement should allow the technique to distinguish between ions of different internal energy or structure, or to determine breakdown diagrams.

Mechanisms of photodissociative ionization of HCOOH: The heat of formation of COOH+

The dissociative ionization of HCOOH to form COOH+ +H+e has been reinvestigated. Experiments with DCOOH and HCOOD demonstrate that rearrangement competes with direct bond cleavage in this dissociation, even near threshold. Extrapolation of the photoion yield curve of COOH+ to the background level is ambiguous. A photoelectron–photoion coincidence experiment was thereupon performed, to obtain a breakdown diagram. From the 0 K cross over energy of 12.30±0.02 eV, we infer ΔHf00 (COOH+)=143.2±0.5 kcal/mol, and proton affinity (PA) (CO2)=129.2±0.5 kcal/mol. Some possible mechanisms for the rearrangement process are explored, but the calculated barriers are too high to explain the observation.

Bimodal kinetic energy release in the unimolecular dissociation of energy-selected formic acid ion

Dissociation of HCOOH+ and DCOOH+ have been studied using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown diagrams were obtained over internal energies of the molecular ion less than 8 eV. Kinetic energy release distributions (KERDs) for HCO+ and DCO+ were extracted from their flight time distributions by using a multiple linear regression method. The breakdown curve of HCO+ not only exhibited several maxima but also the surprisal analysis of its KERD demonstrated a considerable deviation from a single statistical distribution. Bimodal KERDs were observed for the formation of HCO+ and DCO+. The large KER component results from dissociation to the products in the ground state, HCO+ (1Σ+)+⋅OH (2Πi). Average kinetic energy releases observed for this process are, however, about twice higher than those calculated by QET/RRKM. The small KER component shows average KERs remarkably smaller than the statistical expectations and this value remains constant over the energy range of 4.5 eV. The origin of this small KER component was ascribed to dissociation after fluorescent transition of initial vibronic states to the first excited state (1 2A″). In contrast with these results average KERs measured for H2O+ and HDO+ are in good agreement with the RRKM/QET calculations.

The dissociation dynamics of energy selected o-nitrotoluene ions

The photoelectron photoion coincidence technique has been used to measure the breakdown diagram and the OH loss rates of energy-selected o-nitrotoluene ions. In addition, the subsequent dissociation of C 7H 6NO + ions via CO loss which occurs at higher ion internal energies has also been measured. The measured rates are consistent with a mechanism in which the parent o-nitrotoluene ion rearranges to a more stable structure prior to OH loss. This rearranged ion is calculated to be 1.62 eV more stable than the o-nitrotoluene ion and dissociates with an activation energy of 1.5 eV. The heat of formation for the OH loss product, C 7H 6NO +, is found to be 900 ± 15 kJ mol −1. The results are discussed in light of the earlier photodissociation data of Beynon and co-workers.

Unimolecular dissociation of energy-selected methyl formate ion

Fragmentation mechanisms of the methyl formate and methyl formate-d1 ions have been investigated using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The breakdown diagrams and ion kinetic energies of fragment ions were measured at internal energies of the molecular ion from 0 to 7 eV, and fragmentation pathways were elucidated. The formation of HCO+ showed a large kinetic isotope effect. The formation of CH3O+ from methyl formate-d1 is accompanied by a large kinetic energy release, about twice that calculated using quasiequilibrium theory (QET). Its relative abundance in the breakdown curve also appears to be inconsistent with statistical theory. Direct dissociation from an isolated state of the molecular ion was one possible explanation. Slow dissociations were observed for the formation of methanol ions resulting from a migration of the formyl hydrogen while QET predicts no metastable dissociation for this process. Two-component reaction rates for methanol ion formation suggest participation of more than one state or isomer. The heat of formation of HCOO was found to be −40±6 kcal/mol, indicating that it is less stable than COOH (ΔHf =−58+4 kcal/mol).

The effect of temperature on photoionisation mass spectra

This paper is devoted to a study of the effect of ion source temperature on fragmentation reactions. The literature concerning the subject is briefly reviewed. Experimental results on the temperature dependence of photoionisation mass spectra of n-hexane, 3-methylpentane, and ethanol are presented. A mathematical relationship between ion source temperature and internal energy of molecular ions is dervied, which in the case of n-hexane and ethanol enables the construction of thermal breakdown diagrams. The lowest appearance energies obtained from these breakdown diagrams are in very good agreement with literature photoionisation data, indicating that thermal energy is fully effective in the fragmentation reactions studied.

Photoionization mass spectrometry of 1,1-difluoroethane

Photoionization efficiencies for the various fragments of 1.1-difluuoroethane (Freon 152 A) as well as the threshold photoelectron spectrum at photon energies between 10 and 20 eV have been measured. The appearance energy of the parent ion AP = 11.865 eV and Δ 1 H 2?? o = 644.0±6 kJ/mol. A number of excess-energy corrections for fragmentation reactions have been obtained and the decomposition of the CH 3CHF 2 + ion is documented with energy derivatives of the photoionization efficiencies and a breakdown diagram. At photon energies above 16 eV isolated-state behaviour was detected.

The rate/energy functions for the competitive fragmentation processes of ethylene and ethane cations

A generally applicable strategy is presented in order to derive the rate/energy functions of competing unimolecular fragmentations of excited organic cations directly from their breakdown diagrams obtained from photoelectron/photoion coincidence spectroscopy. As an illustration, the energy dependence of the unimolecular rate constants for the loss of an H 2 molecule or an H atom from ethylene and ethane cations is determined. The main difficulties connected with the analysis of larger systems are discussed.

Unimolecular dissociations of excited C3H6O+: A photoelectron—photoion coincidence study of cyclopropanol and allyl alcohol

Abstract The photoelectron spectrum of cyclopropanol has been determined and the first few bands have been assigned to the respective ionization processes. The dissociative photoionization of cyclopropanol and allyl alcohol has been studied by means of HeIα photoelectron—photoion coincidence spectroscopy. The breakdown diagrams of the two corresponding radical cations are practically indistinguishable within the entire energy range investigated (= 5 eV). This implies that extensive isomerizations to a single or to a mixture of common precursor structure(s) precede the dissociative processes. The currently available information on the structure of this reactant and, in particular, the role of other C 3 H 6 O + isomers is discussed. The kinetic and thermodynamic aspects of the present results are outlined. Although three generations of daughter ions are involved, the relative importance of the different fragmentation pathways could be determined. The RRKM analysis of the coincidence data suggests that the formation of the ethylene cation by loss of a CO molecule from the parent ion hardly competes with the other four primary fragmentation reactions in the sense of the statistical theory of unimolecular reactions.

Fundamental Aspects of Ionic Dissociations: The Fragmentation Pathways of Excited Bicyclobutane Cations

AbstractA most recently developed method to quantify the fragmentation pathways of excited radical cations is presented. Using bicyclobutane cation as an illustrative example, the RRKM analysis of the breakdown diagram determined by He‐Iα photoelectron‐photoion coincidence spectroscopy is outlined. The results imply complete isomerization to 1,3‐butadiene cation preceding the dissociative processes. The rate‐energy functions of four competitive primary fragmentation reactions, leading to C3H, C4H, C4H and C2H are established. There is compelling evidence that the production of C2H fragment ions does not compete effectively with these four reactions. The extent of kinetic and competitive shift effects is determined. The derived enthalpies of formation are in excellent accord with the available high quality reference data. The relative importance of different fragmentation pathways which ultimately lead to fragment ions of identical mass to charge ratio is assessed.

Photoionization mass spectrometry of NH2OH: Heats of formation of HNO+ and NOH+

Energy-dependent mass spectra are presented for the most prominent ions produced upon photoionization of hydroxylamine. Two distinct thresholds are oberved for mass 31, which we attribute to onsets for formation of HNO+ and NOH+. The corresponding heats of formation are ΔH°f,0 (HNO+) = 256.8±1.4 kcal/mol and ΔH°f,0 (NOH+) = 274.8±0.7 kcal/mol. The thresholds for mass 32, which we tentatively ascribe to H2NO+, corresponds to 224.6±0.2 kcal/mol for its heat of formation. Each of these entities can therefore be produced in an exothermic reaction of H+3 with NO, and represent mechanisms for depleting NO in the interstellar clouds. Experimental breakdown diagrams are generated and compared with predictions of QET. It is found necessary to invoke a three-body decomposition to explain some observations at higher energy. A theory is presented to accommodate this process within the framework of QET.