Photoion Yield Curves Research Articles

Photoionization studies of benzene-argon complexes with synchrotron VUV radiation

Penning ionization of benzene-argon complexes of states corresponding to the two states of (3p)5(4s)1 of argon for which j = 1, at 11.8282 and 11.6237 eV, was studied using synchrotron radiation. Photoion yield curves for complex ions, (C6H6)nArm+, where n = 1, 2 and m = 1–3, for species less than m/e 200 were determined. For n = 1, the peaks are narrow (0.03 eV) and red-shifted (0.06 eV) by essentially the same amounts for all values of m. For n = 2, the peaks are wider (0.22 eV) and blue-shifted (0.07 eV) also for all values of m. In addition, the dissociation energies of C6H6Ar and C6H6Ar+ were measured to be 0.89 ± 0.07 and 1.37 ± 0.08 kcal mol−1 (i.e., 0.039 ± 0.003 and 0.059 ± 0.0035 eV).

Nanocluster ionization energies and work function of aluminum, and their temperature dependence.

Ionization threshold energies of Al(n) (n = 32-95) nanoclusters are determined by laser ionization of free neutral metal clusters thermalized to several temperatures in the range from 65 K to 230 K. The photoion yield curves of cold clusters follow a quadratic energy dependence above threshold, in agreement with the Fowler law of surface photoemission. Accurate data collection and analysis procedures make it possible to resolve very small (few parts in a thousand) temperature-induced shifts in the ionization energies. Extrapolation of the data to the bulk limit enables a determination of the thermal shift of the polycrystalline metal work function, found to be in excellent agreement with theoretical prediction based on the influence of thermal expansion. Small clusters display somewhat larger thermal shifts, reflecting their greater susceptibility to thermal expansion. Ionization studies of free size-resolved nanoclusters facilitate understanding of the interplay of surface, electronic, and lattice properties under contamination-free conditions.

A Novel Feature in Aluminum Cluster Photoionization Spectra and Possibility of Electron Pairing at T ≳ 100 K

A unique property of size-resolved metal nanocluster particles is their "superatom"-like electronic shell structure. The shell levels are highly degenerate, and it has been predicted that this can enable exceptionally strong superconducting-type electron pair correlations in certain clusters composed of just tens to hundreds of atoms. Here we report on the observation of a possible spectroscopic signature of such an effect. A bulge-like feature appears in the photoionization yield curve of a free cold aluminum cluster and shows a rapid rise as the temperature approaches ≈100 K. This is an unusual effect, not previously reported for clusters. Its characteristics are consistent with an increase in the effective density of states accompanying a pairing transition, which suggests a high-temperature superconducting state with Tc ≳ 100 K. Our results highlight the promise of metal nanoclusters as high-Tc building blocks for materials and networks.

Photoionization Yields, Appearance Energies, and Densities of States of Copper Clusters

We present the results and analysis of photoionization measurements of neutral Cun=24–98 clusters, thermalized to T = 60 and 215 K. The photoion yield curves were used (1) to determine the cluster ionization appearance energies via a fit to the Fowler law of surface photoemission and (2) to explore the suitability of these curves for accessing information about electron states lying below the ionization threshold. The appearance energies follow a standard shell pattern up to n ≈ 40, but a subsequent structure suggests that the electronic shells become perturbed and split by crystal field effects. Nevertheless, it is apparent that the spectra are determined essentially by the number of delocalized electrons, i.e., by the sequential filling of electronic shell sublevels. The average shift of the appearance energies with cluster temperature was found to be in good agreement with a theoretical prediction for bulk copper. An important finding is that the derivatives of the ion yield curves overlap well with th...

VUV photoionization of acetamide studied by electron/ion coincidence spectroscopy in the 8–24 eV photon energy range

A VUV photoionization study of acetamide was carried out over the 8–24 eV photon energy range using synchrotron radiation and photoelectron/photoion coincidence (PEPICO) spectroscopy. Threshold photoelectron photoion coincidence (TPEPICO) measurements were also made. Photoion yield curves and branching ratios were measured for the parent ion and six fragment ions. The adiabatic ionization energy of acetamide was determined as I.E. (1 2A′) = (9.71 ± 0.02) eV, in agreement with an earlier reported photoionization mass spectrometry (PIMS) value. The adiabatic energy of the first excited state of the ion, 1 2A″, was determined to be ≈10.1 eV. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made. The neutral species lost in the principal dissociative photoionization processes are CH 3, NH 2, NH 3, CO, HCCO and NH 2CO. Heats of formation are derived for all ions detected and are compared with literature values. Some astrophysical implications of these results are discussed.

Photoionization mass spectrometric studies of N-methyl formamide and N,N′-dimethyl formamide in the 7–18 eV photon energy range

A photoionization mass spectrometric study of N-methyl formamide and N,N′-dimethyl formamide has been made using synchrotron radiation over the photon energy range 8–20 eV. Photoion yield curves were measured for the parent ion in both species and for six fragment ions in N-methyl formamide. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made on the basis of ion appearance energies in conjunction with thermochemical data and the results of electron impact mass spectral studies. Our results illuminate aspects of the effects of methylation of formamide on ionization energies and dissociative ionization channels, as well as on the relative order of analogous molecular orbitals. The principal dissociative ionization process in both compounds involves HCO-loss. This neutral product may be formed in an electronic excited state. A comparison between the ionization properties, in particular the heats of formation of the cations, of the methyl derivatives of formamide and acetamide, shows that the ionization energies of the latter require re-investigation.

Photoionization Mass Spectrometric Study of the Prebiotic Species Formamide in the 10−20 eV Photon Energy Range

A photoion mass spectrometry study of the prebiotic species formamide was carried out using synchrotron radiation over the photon energy range 10-20 eV. Photoion yield curves were measured for the parent ion and seven fragment ions. The ionization energy of formamide was determined as IE (1(2)A') = 10.220 +/- 0.005 eV, in agreement with a value obtained by high resolution photoelectron spectroscopy. The adiabatic energy of the first excited state of the ion, 1(2)A'', was revised to 10.55 eV. A comparison of the ionization energies of related formamides, amino acids, and polypeptides provides useful information on the varied effects of methylation and shows that polymerization does not substantially alter the ionization properties of the amino acid monomer units. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made on the basis of ion appearance energies in conjunction with thermochemical data and the results of earlier electron impact mass spectral studies. Some of the dissociation pathways are considered to involve coupling between the 1(2)A' ground state and the low-lying 1(2)A'' excited state of the cation. Heats of formation are derived for all ions detected and are compared with literature values where they exist. Formation of the HNCO(+) ion occurs by two separate paths, one involving H(2) loss, the other H + H. In the conclusion a brief discussion is given of some astrophysical implications of these results.

VUV photoionisation of free azabenzenes: Pyridine, pyrazine, pyrimidine, pyridazine and s-triazine

Photoionisation mass spectrometry was used to obtain the fragmentation pathways of pyridine, pyridazine, pyrimidine, pyrazine and s-triazine molecules upon absorption of 23.0, 15.7 and 13.8 eV synchrotron photons. The ionic fragments observed vary from molecule to molecule, however C 2H 2 +, HCN +and HCNH + are common to all five molecules at the three photon energies. Furthermore, the presence of C 2H 2N 2 +, C 3H 3N + and C 4H 4 + in the spectra of some of the molecules suggests dissociation pathways via loss of HCN moieties. The respective parent cations, m / q = 79 , 80 and 81 have a greater yield at low photon energies when compared to the most intense fragment peak in each spectra. We recorded two of the fragment cation yields, as well as the parent photoion yield curves of pyridine, pyridazine, and pyrimidine in the 8–30 eV range. The formation of abundant cation fragments show a strong propensity of the molecules for dissociation after the absorption of VUV photons higher than 14 eV. The differences in relative fragment yields from molecule to molecule, and when changing the excitation energy, suggest significant bond rearrangements and nuclear motion during the dissociation time. Thus, bond cleavage is dependent on the photon energy deposited in the molecule and on intramolecular reactivity. With the aid of photoion yield curves and energy estimations we have assigned major peaks in the spectra and discussed their fragmentation pathways.

Photoionization of helium nanodroplets doped with rare gas atoms

Photoionization of He droplets doped with rare gas atoms (Rg=Ne, Ar, Kr, and Xe) was studied by time-of-flight mass spectrometry, utilizing synchrotron radiation from the Advanced Light Source from 10 to 30 eV. High resolution mass spectra were obtained at selected photon energies, and photoion yield curves were measured for several ion masses (or ranges of ion masses) over a wide range of photon energies. Only indirect ionization of the dopant rare gas atoms was observed, either by excitation or charge transfer from the surrounding He atoms. Significant dopant ionization from excitation transfer was seen at 21.6 eV, the maximum of He 2p 1P absorption band for He droplets, and from charge transfer above 23 eV, the threshold for ionization of pure He droplets. No Ne+ or Ar+ signal from droplet photoionization was observed, but peaks from HenNe+ and HenAr+ were seen that clearly originated from droplets. For droplets doped with Rg=Kr or Xe, both Rg+ and HenRg+ ions were observed. For all rare gases, Rg2+ and HenRgm+ (n,m> or =1) were produced by droplet photoionization. Mechanisms of dopant ionization and subsequent dynamics are discussed.

Photoion yield curves of Dy@C 82 in the vacuum UV region

The photoion yield curves for Dy@C 82 z+ ( z = 1 and 2) from Dy@C 82 are measured by using synchrotron radiation in the photon energy range from 24.5 to 39.5 eV. Correction has been made to compensate the effect of transient change of the density of Dy@C 82 in the interaction region, with the help of the yield curve of C 60 z+ produced from C 60 remaining as a trace impurity in the sample. The yield of Dy@C 82 + exhibits a gradually descending curve with a flat region at 30–33 eV, similarly to the yield curve of C 60 + from C 60. The total photoabsorption cross section of Dy@C 82 was evaluated to be (1.2 ± 0.4) × 10 −20 m 2 at the photon energy of 39.5 eV.

Tunable Synchrotron Vacuum Ultraviolet Ionization, Time-of-Flight Investigation of the Photodissociation of trans-Crotonaldehyde at 193 nm

Photodissociation of the unsaturated trans-crotonaldehyde (CH 3 CHCHCHO) molecule at 193 nm is studied using photofragment translational spectroscopy (PTS) and detected by ionization with tunable synchrotron ultraviolet radiation. The photofragment time-of-flight (TOF) spectra at mle = 68(C 4 H 4 O + ), 55(C 3 H 3 O + ), 41(C 3 H 5 + ), 40(C 3 H 4 + ), 39(C 3 H 3 + ), 29(?), 28(CO + ), 27(C 2 H 3 + ), 26(C 2 H 2 + ), and 15(CH 3 + ) are measured. Mass 29 could not be unambiguously assigned. Analysis of these experimental results reveals three dissociation channels: H + C 4 H 5 O, CH 3 + C 3 H 3 O, and CO + C 3 H 6 . A fourth channel associated with mass 29 could reflect either formation of HCO or C 2 H 5 . The measurements also indicate that C 3 H 6 undergoes strong secondary dissociation. The center of mass (CM) translational energy distributions obtained by fitting the TOF mass spectra of the fragments yield values 42, 23, and 43 kJ/mol average CM kinetic energies for the H + C 4 H 5 O, CH 3 + C 3 H 3 O and CO + C 3 H 6 channels, respectively. The photoionization yield curve of the CHCHCHO radical is measured for the first time with an onset ≤7.6 eV. The photodissociation mechanism of crotonaldehyde is compared with the photodissociation dynamics of acrolein (CH 2 CHCHO).

Double photoionization of C(60) and C(70) in the valence region.

Photoion yields from gaseous fullerenes, C(60) and C(70), for production of singly and doubly charged ions are measured by mass spectrometry combined with tunable synchrotron radiation at hnu=25-150 eV. Since the signal of triply or highly charged ions is very weak, the total photoionization yield curve can be estimated from the sum of the yields of the singly and doubly charged ions. A distinct feature appears in the resultant curve of C(60) which is absent in the calculated total photoabsorption cross section previously reported. This difference is attributed to C(60) (2+) ions chiefly produced by spectator Auger ionization of the shape resonance states followed by tunneling of the trapped electron or by cascade Auger ionization. Ratios between the yields of doubly and singly charged ions for C(60) and C(70) are larger than unity at hnu>50 eV. These ratios are quite different from those reported in the experiments using electron impact ionization.

Fragment emission anisotropy in the dissociative photoionization of O2investigated by two-dimensional photoion spectroscopy

The photoion yield from the photoionization of O2has been measured in the 20.4–24.8 eV photon energy range as a function ofboth the incident photon energy and ion kinetic energy, at 0° and90°with respect to the electric vector of the light. From the two-dimensional photoion spectra,photoion yield curves via the formation of O2+(B 2 Σg−)are extracted. Resonances due to the autoionization of the Rydbergstates converging to O2+(c 4 Σu−)appear in both photoion yield curves measured at 0° and90°,which indicates rotation in the O2+(B 2Σg−)state prior to dissociation that smears the initial molecularorientation generated by photoabsorption. On the assumptionthat the lifetime of O2+(B 2Σg−)is longer than the mean rotational period, photoion yield curves for parallel andperpendicular transitions are obtained. The two Rydberg series appearstrongly in the parallel transition curve which implies that they have Σsymmetry. In the two-dimensional photoion spectra, a structure due to O*autoionization following the dissociation of superexcited O2states is identified. The photoion yield curves extracted for this process indicatethe superexcited states responsible for the neutral dissociation and the symmetriesof the states are determined from the observed fragment emission anisotropies.

A photoionization mass spectrometric study of acetylene and ethylene in the VUV spectral region

The molecular and dissociative photoionization of C2H2 and C2H4 has been studied using synchrotron radiation within the energy range 11–30eV. Positive and negative photoion mass spectra have been obtained. Appearance thresholds for the formation of fragment ions have been determined and are compared with the thermochemical values. Above threshold it is shown that superexcited states, involving intravalence transitions and shape resonances which can decay through ion pair formation, play an important role in the photodissociative ionization process. The positive photoion yield curves display significant hitherto unobserved structures, which are unrelated to those present in the photoelectron spectra.

The molecularand dissociative photoionization of ethane in the inner and outer valence energy regions

The molecular and dissociative photoionization of C2H6has been studied using synchrotron radiation within the energyrange 11-40 eV. Positive and negative photoion mass spectraand threshold photoelectron spectra have been obtained. Theseshow that superexcited states, which can decay through ion pairformation, play an important role in the photodissociativeionization process. The positive photoion yield curves displaysignificant and hitherto unobserved structures, which areunrelated to those present in the photoelectron spectra.

Dissociation processes of core-excited CBr 4 involving the Br(3d, 3p, 3s) and C(1s) inner-shells in the range 50–460 eV

The dissociative multiple photoionization of CBr 4 in the valence, and in the Br(3d, 3p, 3s) and C(1s) inner shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the range 50–460 eV. Total and individual photoion yield curves have been recorded as a function of the photon energy. Several discrete resonances are observed below the Br(3d) threshold region. Also a giant shape resonance in the total ion yield curve is observed beyond the Br(3d) region, which can be identified as a shape resonance owing to the diffusion of the outgoing photoelectron by the angular momentum barrier in the 3d 9ϵf outgoing channel. Various monocations of Br + and CBr n + ( n=0–3) are detected along with dications of Br 2+ and CBr 3 2+ in the whole energy range examined. Dissociation processes of multiply charged CBr 4 parent ions have also been investigated by photoelectron–photoion coincidence (PEPICO) and photoion–photoion coincidence (PIPICO) methods. The dominant dissociation pattern is found to involve C +–Br + and Br +–Br + in the whole energy range examined. Distinct energy dependence of dissociation processes is observed in the Br(3p) and Br(3s) regions. With the help of ab initio HF/6-311++G(d) calculation, we estimated the photoabsorption positions and symmetries for the discrete core-excited states using the Z+1 equivalent ionic core model. The specific excitation and dissociation of molecules provides information on energy dissipation processes in conjunction with the relevant core hole states.

Photo-ionization efficiency curves of alkali nanoclusters in a beam and determination of metal work functions

We produced free beams of cold nanometer-sized particles of lithium, sodium and potassium and measured the three corresponding photo-ionization yield curves. Quadratic (Fowler) plots, originally developed for bulk surfaces, were found to provide a good fit to the threshold shape and were used to obtain the particle ionization potentials. The latter match precisely the bulk work functions cited in the literature, suggesting that photo-ionization of free nanoclusters may form a useful complement to traditional photoelectron studies of surfaces. Within 0.25–1 eV above the threshold, the ionization efficiency begins to drop. This effect, which has parallels in bulk-surface and small-cluster photoemission, is presently not well understood but may be related to an interplay between electron emission and collective surface plasma excitations.

Photoionization mass spectrometry of six isomers of C 7H 8 in the 7–22 eV photon energy range

Photoion mass spectrometry in the 7–22 eV range was used to obtain the parent and fragment photoion yield curves and appearance energies for six isomers of C 7H 8: toluene, cycloheptatriene, norbornadiene, quadricyclane, spiro [2,4]hepta-4,6-diene and 1,6-heptadiyne. The apparent heats of formation of fragment ions m 1 + were determined for various fragmentation pathways and compared with standard thermochemical values of ΔH f(m 1 +) in order to assign the fragmentation channels. Comparisons between the mass spectra of the six isomers obtained by photoelectron–photoion coincidence measurements at a photon excitation energy of 20 eV, and between the respective apparent ΔH f(m 1 +) values for these isomers, were used to discuss the possible formation of common isomers during the various dissociative ionization processes. Interconversion of norbornadiene and cycloheptatriene ions is suggested to occur prior to the formation of C 7H 7 + and other fragment ions in these two isomers. For toluene, isomerization does not appear to be necessary for the formation of low-energy fragment ions except for the tropylium form of C 7H 7 + and its sequential product C 5H 5 +. The results also suggest that interconversion to an acyclic isomer common to all six species occurs in the formation of the three fragment ions C 4H 3 +, C 3H 3 + and C 2H 3 + at high internal energies of the parent ion.

Photoionization mass spectrometry of chlorine oxides

Photoionization mass spectrometry of four chlorine oxides (Cl 2O, Cl 2O 4, Cl 2O 6, and Cl 2O 7) is reported. Photoionization efficiency curves of parent and fragment ions are measured in the energy regime between 10 and 21 eV using monochromatic synchrotron radiation. Ionization energies (IE) of the chlorine oxides and fragmentation appearance energies (AE) are obtained from photoion yield curves of mass-selected cations. Specifically, we obtain IE(Cl 2O) = 10.88 ± 0.02 eV, IE(Cl 2O 4) = 11.23 ± 0.05 eV, IE(Cl 2O 6) = 12.66 ± 0.05 eV, and IE(Cl 2O 7) = 12.15 ± 0.05 eV. The experimental results are used to derive numerous thermochemical stability data of the neutral and ionic chlorine oxides and their fragments, as well as plausible cation fragmentation mechanisms. Resonant excitation with tunable vacuum ultraviolet radiation is applied in the case of Cl 2O to obtain further insight into autoionization processes. Three series of Rydberg excitations are assigned converging to the C̃( 2 A 2) ionization energy at 12.74 eV.

VUV peaks in absorption spectra and photoion yield curves of polycyclic aromatic hydrocarbons and related compounds

Photoion mass spectrometry (PIMS) was used to obtain the parent and fragment photoion yield curves in the 8–35 eV range of 25 aromatic hydrocarbons, mainly polycyclic aromatic hydrocarbons (PAHs) and related compounds. An intense peak, also observed in the absorption spectrum, occurs in the 16–18 eV region in the parent monocation yield curves and at similar or higher energies for fragment ions. Its origin, shape and width are discussed in terms of the electronic structure and photophysics of these carbonaceous species. The general behaviour of the ion yield curves as a function of photon energy is modelled for the prototype PAH benzene using data on the partial cross-sections for photoionization of this species. The appearance energies of fragment ions and the corresponding VUV peak profiles provide information about fragmentation mechanisms. In particular, the double peaks observed in the (M - 3) + ion yield curves for several species are interpreted as resulting from sequential mechanisms in the production of these ions. This constitutes a new diagnostic use of ion yield curves.