Threshold Electrons Research Articles

The Heat of Formation of 2-C3H7+and Proton Affinity of C3H6Determined by Pulsed Field Ionization−Photoelectron Photoion Coincidence Spectroscopy

Pulsed field ionization combined with ion coincidence has been used with synchrotron radiation at the Advance Light Source (ALS) operated in the multibunch mode to energy select ions and to measure their breakdown diagram. The resolution for ion state selection achieved with Ar on this instrument is 0.6 meV. The production of the 2-C3H7+ ion was investigated by measuring the dissociative ionization onset from the 2-propyl chloride, bromide, and iodide. Because the dissociation limits for halogen atom loss lie in the Franck−Condon gap, the yield of threshold electrons generated by pulsed field ionization was found to be extremely low. Nevertheless, the heat of formation of 2-C3H7+ determined from the three molecules agreed to within 2 kJ/mol. The average and now recommended values for this heat of formation are 825.0 ± 1.5 kJ/mol (0 K) and 807.5 ± 1.5 kJ/mol (298 K). This can be combined with the known heats of formation of C3H6 and H+ to yield a proton affinity of C3H6 of 742.3 ± 1.5 kJ/mol (298 K). This new value, which is 9.3 kJ/mol lower than the recommended value of Hunter and Lias, is in much better agreement with two theoretical values (744 kJ/mol) as well as the determination of Szulejko and McMahon (746.4 kJ/mol).

Fragmentation of the valence electronic states of NF 3+ studied by threshold photoelectron–photoion coincidence spectroscopy

The valence threshold photoelectron spectrum of NF 3 is reported for the first time in the literature, and threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has measured, state-selectively, the decay dynamics of the valence states of NF 3 + in the range 13–23 eV. Vacuum–UV radiation from the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monochromator photoionises the parent molecules. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing coincidence ion yields of the fragment ions and the breakdown diagram to be obtained. A comparison of the integrated threshold photoelectron and the total ion signals as a function of energy suggests that, in the range 16–19 eV, autoionisation via Rydberg states of NF 3 makes a significant contribution to the production of threshold electrons. The 50% crossover energy for production of NF 2 + from NF 3 + is determined to be 14.10±0.05 eV. The first onsets for NF 2 + and NF + production are 13.95±0.05 and 17.6±0.1 eV, respectively. The majority of the Franck–Condon region of the X ̃ 2 A 1 ground state of NF 3 + is stable with respect to dissociation to NF 2 +, whereas the unresolved A ̃ 2 E / B ̃ 2 A 2 states and most of the C ̃ 2 E state dissociate exclusively to NF 2 +. The D ̃ 2 A 1 and E ̃ 2 E states dissociate to NF +. Translational kinetic energy releases have been measured in NF 2 + and NF + at the energies of the Franck–Condon maxima of the valence states of NF 3 +. The results are compared with models assuming statistical and impulsive dissociation. The Ã/B̃ states of NF 3 + dissociate directly from the excited-state potential energy surface to NF 2 +, whereas the higher-lying C̃ state probably dissociates off the ground-state surface following rapid internal conversion. It is not possible to correlate unambiguously the formation of NF + with either F 2 or 2F, although on energetic grounds the latter products are more likely. Assuming that the neutral products are 2F, no information is obtained whether the two N–F bonds break simultaneously or sequentially.

Angular distributions in photoelectron spectroscopy of small tungsten clusters: competition between direct and thermionic emission

Single-photon photodetachment of mass-selected W n - clusters has been studied by velocity map imaging. Photoelectron imaging allows us to measure simultaneously the kinetic energy spectrum and the angular distribution of photoelectrons, providing a clear distinction between the isotropic thermionic emission and the anisotropic direct photoemission. A careful study of threshold electrons shows that the thermal distribution p(e) cannot be described, even qualitatively, by a simple exponentially decreasing Boltzmann function, as is usually assumed. On the contrary, our results are in excellent agreement with more refined theoretical models. Our results indicate that the transition towards a bulk-like statistical behavior of the internal energy redistribution occurs in very small systems, because of the high density of states in metal clusters. The asymmetry parameter β of the most intense band observed in direct photoemission for each cluster decreases monotonically with size; the direct photoemission of small systems is strongly anisotropic, and becomes isotropic as the size of the system increases. This probably indicates the loss of coherence induced by electron–electron collisions occurring in large systems prior to electron–phonon coupling.

Ethene loss kinetics of methyl butanoate ions studied by threshold photoelectron–photoion coincidence:: The enol ion of methyl acetate heat of formation

Threshold photoelectron–photoion coincidence spectroscopy has been used to investigate the unimolecular chemistry of gas-phase methyl butanoate ions (CH 3CH 2CH 2COOCH 3 ·+). This ester ion isomerizes to a lower energy distonic ion (CH 2CH 2CH 2COHOCH 3 ·+) prior to dissociating by the loss of C 2H 4. The asymmetric time of flight distributions, which arise from the slow rate of dissociation at low ion energies, provide information about the ion dissociation rates. By modeling these rates with assumed k( E) functions, the thermal energy distribution for room temperature and molecular beam samples, and the analyzer function for threshold electrons, it was possible to extract the dissociative photoionization limit for methyl butanoate which at 0 K is 10.275 ± 0.010 eV as well as the dissociation barrier of the distonic ion of 0.735 ± 0.010 eV. By combining these with an estimated heat of formation of methyl butanoate, we derive a 0 K heat of formation of the distonic ion CH 2CH 2CH 2COHOCH 3 ·+ of 119.0 ± 2.0 kcal/mol. The product ion is the enol of methyl acetate, CH 2COHOCH 3 ·+, which has a derived heat of formation at 0 K of 122.0 ± 2.0 kcal/mol. This energy agrees well with a derived energy obtained recently from the rate analysis of energy selected methyl acetate ions. Also measured was the adiabatic ionization energy of the methyl butanoate, 9.95 ± 0.05 eV.

Ethene loss kinetics of methyl 2-methyl butanoate ions studied by threshold photoelectron–photoion coincidence: the enol ion of methyl propionate heat of formation

Threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has been used to investigate the unimolecular chemistry of gas-phase methyl 2-methyl butanoate ions [CH 3CH 2CH(CH 3)COOCH 3 ·+]. This ester ion isomerizes to a lower energy distonic ion [CH 2CH 2CH(CH 3)COHOCH 3 ·+] prior to dissociating by the loss of C 2H 4. The asymmetric time of flight distributions, which arise from the slow rate of dissociation at low ion energies, provide information about the ion dissociation rates. By modeling these rates with assumed k( E) functions, the thermal energy distribution for room temperature sample, and the analyzer function for threshold electrons, it was possible to extract the dissociative photoionization threshold for methyl 2-methyl butanoate which at 0 K is 9.80 ± 0.01 eV as well as the dissociation barrier of the distonic ion of 0.86 ± 0.01 eV. By combining these with an estimated heat of formation of methyl 2-methyl butanoate, we derive a 0 K heat of formation of the distonic ion CH 2CH 2CH(CH 3)COHOCH 3 ·+ of 101.0 ± 2.0 kcal/mol. The product ion is the enol of methyl propionate, CH 3CHCOHOCH 3 ·+, which has a derived heat of formation at 0 K of 106.0 ± 2.0 kcal/mol.

New Method for the Study of Dissociation Dynamics of State-Selected Doubly Charged Ions: Application toCO2+

A new technique is described which is capable of observing the stability of specific states of doubly charged molecular ions with vibrational level resolution. The method is based on the process of double photoionization using synchrotron radiation. Pairs of threshold electrons identify the initial state, and the resulting ions are characterized by their times of flight. Lifetimes have been measured for specific levels of ${\mathrm{CO}}^{2+}$ and confirm, except for the very lowest levels, the intrinsic instability of this species on the $\ensuremath{\mu}\mathrm{s}$ time scale.

Substrate Mediated Suppression of Postcollision Interaction Effects

The line shapes and kinetic energies of argon ${2p}_{3/2}$ and ${2p}_{1/2}$ photoelectrons from monolayers adsorbed on a Ru(001) substrate were measured with an electron time-of-flight detector in the near-threshold region. Compared with gas phase results, the two main effects due to postcollision interaction, i.e., line broadening and redshift of the kinetic energy of threshold electrons, are strongly suppressed on the surface. We show that these changes are brought about by an effective, screening induced reduction of the energy loss encountered by the threshold electron upon transition from a singly to a doubly charged central argon atom at core-hole decay.

Postcollision interaction effects induced by Auger cascades following Xe L3-shell ionization.

Yield spectra of multiply charged ions have been measured in coincidence with threshold electrons near the Xe ${\mathit{L}}_{3}$-shell ionization region. The ion-yield spectra exhibit characteristic profiles of postcollision interaction effects induced by Auger cascades following Xe ${\mathit{L}}_{3}$-shell ionization. The peak shifts of the spectral profiles increase gradually along with the increase of the charge states, but the peak widths are almost independent of the charge states. The spectral profiles are reproduced by a simple model based on the assumption that Coster-Kronig and Auger shake-off decays play a major role in the Auger cascades following the Xe ${\mathit{L}}_{3}$-shell vacancy creation. \textcopyright{} 1996 The American Physical Society.

Decay channels following 1s photoexcitation of argon

Charge state resolved ion yield spectra in the 1s excitation region of Ar have been measured in coincidence with threshold electrons. Decay channels of the Ar 1s-14p resonance were studied by combining these ion yields and those obtained by a conventional time-of-fight mass spectrometer. The branching ratio of the resonant Auger shake-off to two-step decays for the 1s-14p resonance was estimated to be 71:29%. The resonant Auger shake-off decay plays an important role in the 1s-14p resonance decay.

A threshold-photoelectron coincidence spectrometer using a single detector and time-of-flight analysis

A simple instrument is described for the coincidence measurement of two near-zero energy (threshold) electrons, which can be used for the study of double-photo-ionization processes. The threshold electrons are collected by means of the penetrating field technique and the coincidence measurements are performed using a single detector (channel plates). The application of time-of-flight techniques leads to much improved spectral resolution and yields information on the double-photo-ionization process.

Recent results of crossfertilization between electron spectroscopy and magnetism

The probing depth in transition metals for low energy electrons of 5–10 eV above the Fermi-energy is much lower than previously assumed. It increases with the number of unoccupied d-states, that is the early transition metals have the shortest probing depth of, e.g., only one monolayer for threshold electrons in Gd. Spin polarized electron spectroscopy combined with the art of fabricating ultrathin magnetic structures permits accurate measurement of these probing depth and allows detailed verification of the underlying assumptions through measurement of the transport polarization acquired by electrons traversing ferromagnetic material. While the phenomenological meaning of the probing depth is clearly defined by the experimental procedure, meaningful theoretical analysis is not available.

Ar 2p shake-up structures studied by threshold electron spectroscopy

Threshold electrons and photoions have been measured in the vicinity of the Ar 2p-13p-1 nl shake-up region using a coincidence technique. The threshold electron spectrum exhibits conjugate shake-up structures due to dipole transitions on the lower energy side of normal shake-up structures due to monopole transitions. The shake-up structures are identified with the aid of multiconfigurational Dirac-Fock energy calculations. The Ar3+ photoions in coincidence with threshold electrons are observed with significant intensities at the shake-up states. It is proposed that the Ar3+ formation is promoted by two-step decays besides Auger shake-off decays from the shake-up states.

Vibrational structure of the ground state of Ar2+ by the threshold photoelectron-ion coincidence technique

The coincidence between threshold electrons and ions from Ar2+ was taken to separate the monomer ions and the dimer ions between 790 and 855 AA using synchrotron radiation as a light source. The vibrational structure of the ground state of the Ar2+ ion was clearly observed. The energy of the Ar2+ ionic state is determined to be 116 432+317.2( nu +1/2)-2.5( nu +1/2)2(cm-1) from the zero vibrational state of Ar2.

Photoionization Dynamics of Small Molecules With Synchrotron Radiation: State Preparation by Pepico and Tpepico

The use of synchroton radiation combined with TPEPICO experiments to study the photoionization dynamics of molecules at Orsay's synchroton radiation facility is discussed. The initial state preparation by Threshold Photoelectron Spectroscopy and final state-mass and internal energy spectroscopy by time of flight analysis of photoelectrons as well as ions in coincidence with threshold electrons is illustrated by the TPES of HCl, the TOF TPES of O2 and the TPEPICO spectra of NO+ fragments from the decay of state selected N2O+ ions.

Photoion spectra in coincidence with threshold electrons near the Xe 4d ionization limits

The yield spectra of Xe2+ and Xe3+ ions near the 4d ionization limits have been measured by a threshold electron-photoion coincidence method. These spectra consist of some prominent peaks corresponding to the 4d-1np Rydberg states and two dominant peaks corresponding to 4d threshold ionization. It is proposed that threshold-electron emission from the inner-shell resonance states is due to not only shake-off (e.g. Xe 4d-16p to Xe2+ 5p-2) but also two-step autoionization (e.g. Xe 4d-16p to Xe+ 5s-15p-17p to Xe2+ 5s-15p-1). The two-step autoionization process plays an important role in decays from the lower n Rydberg states. Two dominant peaks somewhat above the 4d ionization limits show the effect of post-collision interaction, namely, shifts of peak positions and asymmetric profiles. The shifts are almost the same in the Xe2+ and Xe3+ spectra.

Photoion spectra in coincidence with threshold electrons near the Kr 3d ionisation limits

Photoion spectra of Kr near the 3d ionisation limits have been measured using a threshold electron-photoion coincidence technique. The spectra of Kr2+ and Kr3+ ions show structures corresponding to the 3d-1np Rydberg states below the ionisation limits and to 3d5/2 and 3d3/2 ionisation above the ionisation limits. It is proposed that the lower Rydberg states can decay by emitting threshold electrons through two-step autoionisation, such as Kr 3d-15p to Kr+ 4p-2(1S)6p to Kr2+ 4p-2(3P) and Kr 3d-15p to Kr2+ 4s-14p-2np (n>or=5) to Kr3+ 4p-3. The structures due to 3d threshold ionisation show the effect of post-collision interaction, namely peak shifts and asymmetric profiles. The shifts are almost the same in the Kr2+ and Kr3+ spectra.

Coincidence spectra of threshold electrons and ions around the Kr 3d and Xe 4d delayed onset regions

Photoion spectra as a function of photon energy have been measured around the Kr 3d and Xe 4d delayed onset regions using a threshold-photoelectron photoion coincidence technique. The spectra of doubly charged ions show structures corresponding to the shake-up states of Kr 3d−14p−1nl and Xe 4d−15p−1nl. It is clarified that the spectra of triply charged ions are composed of the shake-off processes and the double Auger process after the Kr 3d and Xe 4d ionization.

Construction of a new apparatus for angle- and energy-resolved measurements of photoelectrons and photoions

A new apparatus for the study of photoionization processes has been designed and constructed. The main capabilities of the apparatus include measurements of angle-resolved photoelectrons, as well as time-of-flight measurements of photoions in coincidence with total, energy selected, or threshold electrons.

Photoion spectra in coincidence with threshold electrons near the Ar 2p ionisation limits

The yield spectra of Ar2+ and Ar3+ ions in coincidence with threshold electrons near the 2p ionisation limits have been measured using a time-of-flight mass spectrometer and synchrotron radiation. Below the 2p ionisation limits, the Ar2+ yield spectrum exhibits a prominent peak corresponding to the the 2p-14d, 5d and 6d resonance states. It is proposed that these resonance states can decay by emitting threshold electrons through two-step autoionisation, such as Ar 2p-15d to Ar+ 3p-2(1D)6d to Ar2+ 3p-2(3P). Above the 2p ionisation limits, the Ar2+ and Ar3+ yield spectra exhibit two dominant peaks. These peaks show the effect of post-collision interaction, namely shifts of peak positions and asymmetric profiles. The shifts are almost the same in the Ar2+ and Ar3+ yield spectra.

Study of the C and D states of CO+by the threshold photoelectron-ion coincidence technique

The coincidence between threshold electrons and ions from CO was used to separate the parent and fragment ions between 500 and 580 AA using synchrotron radiation as the light source. Most of the C and D state above the dissociation limit were confirmed to predissociate to C++O.