Vibrational Feshbach Resonances Research Articles

The role of vibrational doorway states in positron annihilation with large molecules

Positron annihilation rates in many polyatomic molecular gases are anomalously high. Qualitatively, this can be explained by positron capture in vibrational Feshbach resonances, which can occur for molecules with positive positron affinities [Gribakin, Phys. Rev. A 61 (2000) 022720]. To verify this idea quantitatively, we examine the densities of vibrational excitation spectra of alkanes. To understand the energy dependence of the annihilation rates for alkanes, we propose that positron capture is mediated by vibrational doorway states, in which positron binding is accompanied by the excitation of fundamentals.

The role of vibrational doorway states in positron annihilation with large molecules

Positron annihilation rates in many polyatomic molecular gases are anomalously high. Qualitatively, this can be explained by positron capture in vibrational Feshbach resonances, which can occur for molecules with positive positron affinities [Gribakin, Phys. Rev. A 61 (2000) 022720]. To verify this idea quantitatively, we examine the densities of vibrational excitation spectra of alkanes. To understand the energy dependence of the annihilation rates for alkanes, we propose that positron capture is mediated by vibrational doorway states, in which positron binding is accompanied by the excitation of fundamentals.

High resolution study of cluster anion formation in low-energy electron collisions with OCS clusters

Using the laser photoelectron attachment (LPA) method involving a supersonic target beam with carbonyl sulfide molecules (OCS) seeded in helium, we have studied the formation of cluster anions in low-energy electron attachment (E=1–200meV) to molecular clusters of OCS at high resolution (energy width 1–2meV). Homogeneous cluster anions (OCS)q− (q=1–12) are predominantly formed. In addition, due to CS2 impurities in the OCS sample, mixed cluster anions ((OCS)q−1CS2−, q≥1) are observed at levels of a few percent. The energy dependences of cluster anion formation are characterized by a strong rise towards zero energy attributed to s-wave attachment as well as—below the onsets of the (010), (001), and (020) vibrational modes of the OCS molecule—by vibrational Feshbach resonances (VFR) whose importance decreases towards larger cluster sizes q. Formation of the (OCS)2− anion is especially enhanced at near-zero electron energies; moreover, the attachment spectrum exhibits a sharp VFR resonance just below the onset for excitation of the (020) mode in the OCS molecule. The electron attachment behaviour of OCS clusters is intermediate between that of CO2 clusters (which is dominated by VFRs) and that of CS2 (which exhibits a strong zero energy peak, but no VFRs). This finding is correlated with the properties of the respective molecular neutrals and anions which have been studied using high level ab initio methods. We also report the energetics of the neutral and the anion dimer species. The role of VFRs as doorway states into valence-type anion states is discussed.

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

A survey of resonance and threshold phenomena in low-energy electron collisions with molecules and clusters is presented. Following an introduction into the role of resonances to promote vibrational excitation and anion formation through electron attachment, we discuss recent progress in achieving high energy resolution, using optimized conventional setups and photoelectron methods (the latter achieving sub-meV energy widths in attachment studies). Discussing selected cases, we highlight threshold and resonance phenomena in electron scattering and attachment channels, as observed in recent high resolution experiments and characterized by improved theoretical models. The threshold behaviour for dissociative electron attachment proceeding through s-wave and p-wave capture is demonstrated for CCl4 and Cl2, respectively. Threshold peaks, vibrational Feshbach and outer-well resonances as well as boomerang-type oscillatory structures are discussed for the polar molecules HF, HCl, and CH3I. Narrow vibrational Feshbach resonances, observed in cluster anion formation due to electron attachment to molecular clusters of CO2 and N2O, and their size-dependent redshifts, illustrating the effects of solvation, are discussed and explained as diffuse weakly-bound electron states. We briefly address the recent observation and the relevance of vibrational resonances in positron-molecule collisions at energies below 0.5 eV, as observed in positron annihilation. We conclude with a brief summary and mention some perspectives for future work.

Energy-resolved positron annihilation for molecules

This paper presents an experimental study designed to address the long-standing question regarding the origin of very large positron annihilation rates observed for many molecules. We report a study of the annihilation, resolved as a function of positron energy ( DE;25 meV, full width at half maximum! for positron energies from 50 meV to several eV. Annihilation measurements are presented for a range of hydrocarbon molecules, including a detailed study of alkanes, CnH2n12, for n51 ‐9 and 12. Data for other molecules are also presented: C2H2 ,C 2H4 ;C D 4; isopentane; partially fluorinated and fluorinated methane (CH xF42x); 1-fluorohexane (C 6H13F) and 1-fluorononane (C 9H19F). A key feature of the results is very large enhancements in the annihilation rates at positron energies corresponding to the excitation of molecular vibrations in larger alkane molecules. These enhancements are believed to be responsible for the large annihilation rates observed for Maxwellian distributions of positrons in molecular gases. In alkane molecules larger than ethane (C2H6), the position of these peaks is shifted downward by an amount ;20 meV per carbon. The results presented here are generally consistent with a physical picture recently considered in detail by Gribakin @Phys. Rev. A 61, 022720 ~2000!#. In this model, the incoming positron excites a vibrational Feshbach resonance and is temporarily trapped on the molecule, greatly enhancing the probability of annihilation. The applicability of this model and the resulting enhancement in annihilation rate relies on the existence of positron-molecule bound states. In accord with this reasoning, the experimental results presented here provide the most direct evidence to date that positrons bind to neutral molecules. The shift in the position of the resonances is interpreted as a measure of the binding energy of the positron to the molecule. Other features of the results are also discussed, including large, qualitative changes in the annihilation spectra observed when hydrocarbon molecules are fluorinated.

A study of dissociative electron attachment to CHBr3 and CHI3

Dissociative electron attachment (DEA) has been studied in the systemsBr−/CHBr3 and I−/CHI3 in a crossed electron/molecular beams experiment in theelectron energy range from about 0 to 10 eV. In addition to prominent low-energyresonances (also observed in previous studies), we find a resonance in the systemBr−/CHBr3 at 4.65 eV and we find three additional resonant features in the systemI−/CHI3 at 2.1, 2.9 and 4.1 eV. The absolute partial cross-section for theBr−/CHBr3 DEA reaction was estimated as (3 ± 1) × 10−17 cm2 at 0.5 eV and (2 ± 1) ×10−18 cm2 at 4.65 eV. Analysis of the energy dependence of the I−/CHI3cross-section reveals s-wave attachment near threshold (≈0 eV) and an additionalpronounced feature located at 58 meV. We tentatively assign this featureto vibrational Feshbach resonance associated with excitation of the ν2vibrational quanta of CHI3.

High resolution study of cluster anion formation in low-energy electron collisions with molecular clusters of CO 2, CS 2, and O 2

Using a high resolution (Δ E=1–2 meV) laser photoelectron attachment (LPA) method, we have studied the formation of (CO 2) q − ( q=4–32), (CS 2) q − ( q=1, 2, 3, 5), and (O 2) q − ( q=5–14) cluster anions in collisions of low-energy electrons (0–200 meV) with molecular clusters of CO 2, CS 2, and O 2 molecules in a skimmed supersonic beam. For CO 2 clusters, the attachment spectrum is dominated by rather narrow vibrational Feshbach resonances (VFRs) of the type [(CO 2) N−1 CO 2( ν i )] − which involve a vibrationally excited molecular constituent ( ν i ≥1 denotes excited vibrational mode). The size-dependent values of the observed redshifts of the VFRs (about 12 meV per added CO 2 unit for small q) in conjunction with their narrow widths and with simple model calculations of the electron binding energies suggest that the size q of the observed (CO 2) q − anions is essentially the same ( q= N or N−1) as the size N of the neutral precursor (CO 2) N which captures the electron. For cluster sizes q above 24, a doubling of the VFR peak structure is observed which we tentatively attribute to the transition from icosahedral (low N) to bulk cubic (high N) cluster structure, previously predicted to occur for N around 30. The size distribution in the (CO 2) q − anion mass spectrum, due to attachment of electrons with well-defined energies, shows undulatory structure which reflects the presence or absence of VFRs at that particular energy. In contrast, the energy-integrated cluster anion mass spectrum shows a rather smooth dependence on cluster size q. The yield spectra for (CS 2) q − cluster anions exhibit an s-wave attachment peak near 0 eV and a smoothly falling cross section towards higher energies without significant structure. In the yield for (O 2) q − anions we observe a rise towards 0 eV, indicative of s-wave electron capture, and a slowly varying and essentially structureless attachment continuum between 10 and 200 meV. The attachment characteristics of the different clusters systems are discussed in the light of the basic properties of the monomers.

Enhancement of positron annihilation on molecules due to vibrational Feshbach resonances

The zero-range potential model is used to investigate positron collisions and annihilation with molecules. The Kr 2 dimer is considered as an example. It is shown that (i) although positrons do not bind to individual Kr atoms, they do form bound states with Kr 2. (ii) A sequence of vibrationally excited states of the positron–molecule complex extends into the positron continuum, where it manifests as vibrational Feshbach resonances. (iii) These resonances give a very large contribution to the positron annihilation rate. Even after averaging over the thermal positron energy distribution, the contribution of the lowest Feshbach resonance exceeds that of the non-resonant background by an order of magnitude.

Energy broadening due to photoion space charge in a high resolution laser photoelectron source

The results of experimental and theoretical studies, aiming at a quantitative characterization of photoion-induced energy broadening effects in a laser photoelectron source, are reported. The electron source is based on two-step cw laser photoionization of potassium atoms in a collimated beam. In the experimental studies, the attachment spectra for the formation of (N2O)9O− cluster ions through a narrow vibrational Feshbach resonance (full width at half maximum 2.3 meV) were measured as a function of the photocurrent. The theoretical studies involved Monte Carlo simulations of the broadening effects and were based on potential distributions caused by realistic spatial distributions of the photoions. Using the corresponding electric field distribution, trajectories were calculated for a representative ensemble of electrons, and effective electron energy distributions were obtained from averages over the electron trajectories in the volume relevant for electron attachment. Furthermore, the effects of additional weak electric fields, applied along the atomic beam direction, have been simulated. For our geometry (ionization volume about 2 mm3) the effective space charge related energy width is found to be about 16 μeV/pA.

Feshbach states of the propadienylidene anion H2CCC–

The absorption spectra of the propadienylidene anions, H2CCC− and D2CCC−, in the visible and near-UV range were recorded by photodetachment spectroscopy. The lowest excited state is the well characterized dipole bound A2A1 showing bound levels and vibrational Feshbach resonances. The higher lying electronic states are Feshbach resonances 0.5–2 eV above the electron detachment threshold and are assigned to the 2A″, 2A1 and 2B1←2B1 electronic transitions by means of ab initio calculations. The autodetachment rate increases with energy. A comparison of the laboratory data with diffuse interstellar bands shows that not only the four strong transitions of the A 2A1←2B1 system but also several of the Feshbach resonances lie close to astronomical absorptions.

Stability and production of positron–diatomic molecule complexes

The energies at geometries close to the equilibrium for the e+LiF and e+BeO ground states were computed by means of diffusion Monte Carlo simulations. These results allow us to predict the equilibrium geometries and the vibrational frequencies for these exotic systems, and to discuss their stability with respect to the various dissociation channels. Since the adiabatic positron affinities were found to be smaller than the dissociation energies for both complexes, we propose these two molecules as possible candidates in the challenge to produce and detect stable positron–molecule systems. Moreover, low-energy positron scattering on LiF and BeO targets may show vibrational Feshbach resonances as fingerprints of the existence of stable ground states of e+LiF and e+BeO.

Comparative studies of dissociative electron attachmentto methyl halides

The dissociative electron attachment cross sections for themethyl halides vary in an enormous range from the virtuallyunmeasurable 10-23 cm2 for CH3Cl at roomtemperature to 10-14 cm2 for CH3I. In this paperwe supplement our previous studies by calculations ofdissociative electron attachment to CH3Br and compare resultsfor all methyl halides studied so far. The rate as a function oftemperature for CH3Cl and CH3Br exhibits an exponentialdependence on 1/T (Arrhenius law) with the activation energylower for CH3Br. CH3I does not obey the Arrhenius lawsince the crossing point of the neutral and anion potentialcurves occurs near the lowest vibrational levels. The crosssection as a function of electron energy for all of the methylhalides studied here exhibits structure at the vibrationalexcitation thresholds that is associated with a vibrationalFeshbach resonance like that observed recently in CH3Iby Schramm et al (Schramm A, Fabrikant I I, Weber J M, Leber E,Ruf M-W and Hotop H 1999 J. Phys. B: At. Mol. Opt. Phys. 32 2153).

Vibrational Feshbach resonances in electron attachment to carbon dioxide clusters

Using a high resolution ( meV) laser photoelectron attachment method, we have studied the formation of (CO 2) q − ions (q = 4−22) in collisions of low energy electrons (1−180 meV) with (CO2) N ( ) clusters. The previously reported “zero energy resonance”, observed at much larger electron bandwidths, actually consists of several narrow vibrational Feshbach resonances of the type [(CO 2) N −1CO which involve a vibrationally-excited molecular constituent ( denotes vibrational mode) and a diffuse electron weakly bound to the cluster by long range forces. The resonances occur at energies below those of the vibrational excitation energies of the neutral clusters [(CO 2) N −1CO ]; the redshift rises with increasing cluster ion size q by about 12 meV per unit; these findings are recovered by a simple model calculation for the size dependent binding energies. The size distribution in the cluster anion mass spectrum, resulting from attachment of very slow electrons, mainly reflects the amount of overlap of solvation-shifted vibrational resonances with zero energy; the cluster anion size q is identical with or close to that of the attaching neutral cluster.

Vibrational Feshbach resonances in electron attachment to nitrous oxide clusters: decay into heterogeneous and homogeneous cluster anions

Using a high-resolution (Δ E≈1 meV) laser photoelectron attachment method, we have studied cluster anion formation in collisions of low-energy electrons (1–180 meV) with (N 2O) N clusters. We show that formation of both heterogeneous cluster anions (N 2O) q O − ( q< N) and (as a minor channel) of homogeneous anions (N 2O) p − ( p⩽ N) is mediated through narrow, weakly red-shifted vibrational Feshbach resonances of the type [(N 2O) N−1 ·N 2O( ν i )] − ( ν i denotes vibrational excitation). For the first time direct information has been obtained on the relationship between the size q of the heterogenous anions and the size p of the homogeneous anions ( p– q≈2–3 for p=8, 9, 11, 13), as formed from the same precursor cluster size involved in the vibrational Feshbach resonance. The electron-cluster binding energies are recovered from model calculations based on appropriate short- and long-range forces.

Effects of solvation on dissociative electron attachment to methyl iodide clusters

Using laser photoelectron attachment to methyl iodide clusters in a differentially-pumped seeded supersonic helium beam and mass spectrometric ion detection, we have measured the rate coefficients for formation of \(\) (q = 0-2) ions over the electron energy range 0-100 meV with an effective energy width of about 2.5 meV. Whereas a prominent vibrational Feshbach resonance just below the onset for the C-I stretch vibration ( \(\)) is observed for dissociative attachment to monomers (yielding I- ions), only weak and broad structure, shifted to lower energies, is detected for formation of \(\) ions and essentially no structure is left in the attachment spectrum for \(\). These observations are interpreted by model R-matrix calculations which successfully describe the DA cross-section for the monomer and qualitatively recover the trend observed for cluster ion formation. For the clusters, the effects of increased electron-target long-range interaction and of solvation as well as coupling to soft vibrational modes lead to strong broadening and shifting of the vibrational Feshbach resonance and, ultimately, to its disappearance.

Excitation of vibrational levels of HF up tov=4by electron impact

Relative differential cross sections for excitation of vibrational levels of HF up to $v=4$ by slow electron impact have been measured as a function of incident electron energy. Broad oscillatory structures converging to the dissociative attachment threshold, already known in the $v=\stackrel{\ensuremath{\rightarrow}}{0}3$ channel, have now also been observed in the $v=\stackrel{\ensuremath{\rightarrow}}{0}4$ channel at energies above 1.8 eV. They appear at different energies in the two final channels, revealing their boomerang character. They can be rationalized as a consequence of boomerang motion of the nuclei, initiated either by a broad repulsive ${\ensuremath{\sigma}}^{*}$ resonance, or resulting from broad overlapping vibrational Feshbach resonances. This observation complements results in other diatomic hydrides and reveals that boomerang structures are present in all members of the series $\mathrm{H}X$ $(X=\mathrm{H},$ F, Cl, Br), once thought to have no structures of vibrational origin in the cross sections at low energies, because of the extremely short lifetime (or absence) of ${\ensuremath{\sigma}}^{*}$ resonances. The shape of the $v=\stackrel{\ensuremath{\rightarrow}}{0}4$ cross section differs from those of the lower channels also near threshold---it has a weak shoulder at threshold followed by a rounded hump peaking \ensuremath{\sim}80 meV above threshold, whereas the cross sections for the $v=1--3$ channels have (within resolution) vertical onsets and narrow peaks at thresholds.

Spectroscopic observation of vibrational Feshbach resonances in near-threshold photoexcitation of X-.CH3NO2 (X- = I- and Br-)

We report the observation of resonance structure in the absorption and X-/NO2- photofragment action spectra of the X-.CH3NO2 (X- = I- and Br-) complexes in the region above the electron detachment threshold. The resonance structure corresponds to peaks which appear at the onsets for vibrational excitation of the -NO2 wag, scissors, and stretch modes of neutral CH3NO2, the modes which most strongly distort upon electron capture into its pi* lowest unoccupied molecular orbital. We attribute the peaks to excitation of vibrational Feshbach resonances of the CH3NO2- transient negative ion, where near-threshold excitation of X-.CH3NO2 spectroscopically accesses states of the free electron-CH3NO2 system.

Vibrational resonance and threshold effects in inelastic electroncollisions with methyl iodide molecules

A joint experimental and theoretical study of low-energy electron collisions with methyl iodide molecules with emphasis on dissociative attachment (DA) over the range 0-170 meV has been carried out. Below the onset for the symmetric C-I stretch (3 = 1) vibrational excitation, a prominent narrow peak is observed followed by a cusp at threshold. On approach to the 3 = 2 onset, the DA cross section shows a rapid drop terminated by a cusp at threshold. The experimental findings are interpreted by an R-matrix calculation which includes non-local effects and involves realistic model potentials for the neutral molecule and negative ion. Mediated by the combined effects of dipolar and polarization attraction in the electron CH3I interaction, the peak below the 3 = 1 onset is identified as a vibrational (`nuclear-excited') Feshbach resonance which so far may represent the clearest example of its kind. The higher vibrational onsets are free of such resonances. The effects of initial vibrational excitation are discussed and found to be significant. Predictions are made for elastic (-wave) and vibrationally inelastic cross sections and structure at the 3 = 1 onset is discussed. Absolute DA cross sections and DA rate coefficients for Maxwellian electron gases with variable electron temperature (for the fixed gas temperature TG = 300 K) are presented.

Mode selective control of unimolecular dissociations: Survey, and model simulations for HDO → H+DO, D+HO

A survey of mode selective bond fissions, decay rates and vibrational structures in unimolecular dissociations is followed by model simulations of dissociative HDO resonance decays HDO∗( m a n b)→ H+DO, D+HO. Accordingly, the system yields extreme variations of dissociative lifetimes from ps to > ns andfissions of specific bonds, either (a) OH or (b) OD, depending on selective investments of similar energies for dominant excitations of m a plus n b quanta in the stretches of bonds a and b, in the electronic ground state. Both effects result from the decay mechanism of vibrational Feshbach resonances: diabatic transfers of preferably few vibrational quanta from the “spectator” bond that survives as product hydroxyl towards the “promoting” bond to be broken. For example, preferable transfers of single quanta from spectator bonds b and a of near-degenerate local modes resonancesHDO* (17 a1 b) and HDO* (1 a22 b) induce exclusive fissions of “promoting” bonds a (OH) and b (OD), respectively, on the picosecond-time scale. In contrast, dissociation of the near-degenerate hyperspherical-type resonance HDO*(9#a#7#b#) would require rather improbable transfers of several (>/7) vibrational quanta to be accumulated in a single bond, implying much longer (>ns) decay times. Mode selective dissociation dynamics correlate with vibrational structures: local and hypersphericalotype modes have wavefunctions which extend along the potential valleys of dissociative bonds and along ellipsoidal arcs, respectively. The nodal patterns correlate well with the quantum numbers m a n b of HDO* ( m a n b). These as well as complementary, e.g., toroidal structures are also observed for excited bound states HDO( m a n b). In general, mode selectivity in HDO is more extreme than in the symmetric reference H 2O, allowing even better control of reaction rates, and novel control of exclusive product branching ratios induced by selective bond fissions. The results are derived for the simple Thiele-Wilson model of HDO, neglecting effects of bending, rotations, and potential couplings. The simulations employ fast Fourier-transform propagations of wavepackets prepared bysuperpositions of products of Morse oscillator wavefunctions adapted to bonds of asymmetric dihydrides.