Magnetic Bottle Electron Spectrometer Research Articles

Penning ionization electron spectroscopy of van der Waals clusters

Abstract A magnetic bottle electron spectrometer with retarding electrodes has been applied to observation of Penning ionization electron spectra (PIES) of van der Waals clusters. Considerably increased efficiency of the spectrometer by 1000 times over the conventional PIES apparatus enabled us to measure electron spectra of van der Waals clusters of rare gas atoms in collisions with metastable atoms, He*(2 3 S) and Ne*(3 3 P). Unusually fast electrons with extra kinetic energies of 1–2 eV as compared to those expected for vertical ionization have been detected for Ar and Kr van der Waals clusters in collisions with He*(2 3 S). These fast electrons are ascribed to autoionization into stable ionic structures via superexcited states embedded in the ionization continua, and the unusual electrons can be considered to be related to an adiabatically ionized state to which a direct ionization process is almost forbidden because of a very small Franck–Condon factor.

Observation of collisional ionization electron spectra of van der Waals clusters with metastable He*(2 3S) atoms: An evidence for autoionization from superexcited Ar clusters

Penning ionization electron spectra (PIES) of van der Waals clusters have been observed for the first time by means of a newly developed magnetic bottle electron spectrometer, which is 1000 times more efficient than a conventional PIES apparatus. In the PIES of Ar clusters with He*(2 3S), two bands of Ar 3p hole characters with different ionization energies (IE), band A (IE=ca.15 eV) and band B (IE=ca.13.5 eV), were observed. Band A was assigned to vertical transitions in Franck–Condon regions corresponding to the band in the ultraviolet photoelectron spectra (UPS) of clusters reported by Carnovale et al. [J. Chem. Phys. 90, 1452 (1989)]. Band B, which is an extra band missing in the UPS of clusters, can be ascribed to the autoionization into a stable structure of Arn+ via superexcited states of Arn. Band B can be considered as an adiabatic ionization which is forbidden as a direct ionization. The energy difference of ca. 1.5 eV between the vertical IE for band A and the adiabatic IE for band B was found to agree well with the reported binding energy of the trimer ion core.

Double-resonance spectroscopy of autoionizing states of ammonia

New double-resonance ionization data are presented on the autoionizing Rydberg states of NH3 excited via selected rotational levels of the C̃′ 1A1(0100) and (0200) states. Extrapolation of the Rydberg series provides a determination of the adiabatic ionization energy of NH3 that is in excellent agreement with that obtained by using zero kinetic energy photoelectron spectroscopy (ZEKE-PES). A comparison is presented between the field-free spectrum obtained by using a time-of-flight mass spectrometer and the 1-Tesla-spectrum obtained by using a magnetic-bottle electron spectrometer. Finally, the rotational and vibrational autoionization mechanisms are discussed.

Vibrational autoionization and predissociation in high Rydberg states of nitric oxide

New results on the competition between autoionization and predissociation in the high Rydberg states of nitric oxide are presented. These results provide insight into the earlier work of Park et al. [Phys. Rev. Lett. 76, 1591 (1996)] that shows evidence for substantial mixing between Rydberg series and ionization continua with even and odd orbital angular momenta (l). New data based on fluorescence-dip spectroscopy, detection of neutral predissociation products, and photoelectron spectroscopy suggest that the A′ 2Σ+ and I 2Σ+ dissociative valence states play an important role in this l mixing. Zeeman splittings observed in a magnetic bottle electron spectrometer also result in an improvement in the assignment of these high Rydberg states.

PHOTOELECTRON SPECTROSCOPY OF BINARY-METAL CLUSTER ANIONS CONTAINING SULFUR ATOM

Photoelectron spectra of [Formula: see text] (M= Al , Cu, Ag, and Au) cluster anions were measured at the photon energies of 3.49 and 4.66 eV using a magnetic-bottle electron spectrometer having ~60-meV resolution. The electron affinities of AlS−, CuS−, AgS−, and AuS− are 2.60(3), 2.10(3), 2.09(3), and 2.44(3) eV, respectively. The large EA of AlS− is attributed to large stability by six bonding electrons, and a new electronic state of A ′ 2Π can be found 0.4 eV above the AlS ground state, which is in agreement with a theoretical calculation. In the photoelectron spectrum of AuS−, two peaks due to a splitting by the L–S coupling could be observed, and the amount of the splitting is 0.18(2) eV. The results of aluminum clusters containing sulfur atom will also be presented.

PHOTOELECTRON SPECTROSCOPY OF SILICON–CARBON CLUSTER ANIONS

Photoelectron spectra of [Formula: see text] cluster anions (3≤n, m≤6) were measured at the photon energy of 3.49 eV, by using a magnetic-bottle electron spectrometer having ~60-meV resolution. The [Formula: see text] clusters were produced by laser vaporization of a carbon–silicon mixture rod in a He carrier gas. The spectra of the [Formula: see text] clusters are similar to those of pure [Formula: see text] clusters in the peak positions and their envelopes, which is attributed to the similarity in electronic structure of Si and C atoms, leading to a similar geometry. In contrast, the similarity in the photoelectron spectra is not observed between [Formula: see text] and [Formula: see text] clusters, which is attributed to a change in their geometry; from chain to ring.

Photoelectron spectroscopy of silicon–carbon cluster anions (SinC−m)

Photoelectron spectra of SinC−m cluster anions (1≤n≤7 and 1≤m≤5) were measured at the photon energies of 3.49 eV, by using a magnetic bottle electron spectrometer. The SinC−m clusters were produced either by a laser vaporization of a silicon–carbon mixed rod or by two laser vaporizations of carbon and silicon rods in a He carrier gas. The spectra of the SinC−1 (3≤n≤7) clusters are similar to those of pure Si−n+1 clusters in the peak positions and their envelopes, which is attributed to the isovalent electronic structure of Si and C atoms as well as to the similar geometrical structure. In contrast, the similarity in the photoelectron spectra is not observed between C−m+1 and Si1C−m (2≤m≤5) clusters, which is attributed to a change in their geometry; from chain to ring. These experimental conclusions are discussed with the results of our theoretical calculations.

Photoelectron spectroscopy and mass distributions of aluminum-sulfur cluster anions (AlnSm−)

Photoelectron spectra of AlnSm− cluster anions (0 ≤ n, m ≤ 10) were measured at photon energies of 3.49 and 4.66 eV, by using a magnetic bottle electron spectrometer. The AlnSm−. clusters were produced by laser vaporization of an aluminum-sulfur mixed rod in He carrier gas. The abundant AInSM clusters are three types of AlS−(Al2S3)k, AlS2−(Al2S3)k, and (Al2S3)k− , where an unit of Al2S3 aggregates with ion cores of AlS−, AlS2−, and Al2S3 , respectively. Al3S3 is exceptionally stable with its geometrical structure.

Photoelectron spectroscopy of AlnS1− clusters (n=1−9)

Photoelectron spectra of AlnS−1 (1≤n≤9) cluster anions were measured at the photon energies of 3.49 and 4.66 eV, using a magnetic bottle electron spectrometer having ∼60 meV resolution. AlS− has an electron affinity of as large as 2.6 eV, attributed to large stability by six bonding electrons. A new electronic state, A′ 2Π, was found 0.4 eV above the AlS ground state, which is in agreement with theoretical calculations. AlnS−1 cluster anions are produced efficiently up to n=5, but the production of larger cluster sizes is found difficult. The mass distribution of AlnS−1 cluster anions and electron binding energy can be explained by a geometric structure obtained by theoretical calculation.

Photoelectron Spectroscopy of AlS− Diatomic Anion

Abstract Photoelectron spectra of AlS− radical were measured using a magnetic bottle electron spectrometer. Electron affinity of AlS was determined to be 2.60(3) eV, and a new electronic excited state, A′2Π, was found 0.4 eV above the AlS ground state, which is consistent with theoretical calculations.

Photoelectron spectroscopy of iron carbonyl cluster anions (Fe n(CO) −m, n = 1−4)

Photoelectron spectra of iron-carbonyl cluster anions (Fe n (CO) − m ) were measured in the gas phase at the photon energy of 4.66 eV, using a magnetic bottle electron spectrometer. The iron-carbonyl cluster anions were produced by slow electron attachment of less than 1 eV to a supersonic expansion of Fe(CO) 5 seeded in a He carrier gas. The photoelectron spectra of the Fe n (CO) − 5 n−1 anions show unresolved broad features, and the threshold energy ( E T) and the vertical detachment energy (VDE) were determined: The E T of Fe n (CO) − 5 n−1 gradually increases by 0.1–0.3 eV with the cluster size. The study suggests that the iron carbonyl cluster anions produced are van der Waals clusters, in which Fe(CO) − 4 works as an ion core.

Photoelectron spectroscopy of (C 6F 6) −n and (Au-C 6F 6) − clusters

Photoelectron spectra of hexafluorobenzene (C 6F 6) cluster anions and its Au complex anion were measured in the gas phase at the photon energy of 3.49 eV, using a magnetic bottle electron spectrometer having ≈ 80 meV resolution. The spectra show unresolved broad features: The vertical detachment energies (VDEs) were determined from the maxima of the spectra. The VDE of (C 6F 6) − n gradually increases by 0.1–0.2 eV with the cluster size ( n=1–8) from 1.56(3) to 2.54(6) eV, and the VDE of (Au-C 6F 6) − anion is 1.99(5) eV.

Double-resonance spectroscopy of transitions between autoionizing levels of atomic oxygen.

Double-resonance spectroscopy is used to study transitions between autoionizing levels of atomic oxygen. The first laser is used to produce oxygen atoms in the 1${\mathit{s}}^{2}$2${\mathit{s}}^{2}$2${\mathit{p}}^{4}$ $^{1}$${\mathit{D}}_{2}$ level by photodissociation of ${\mathrm{N}}_{2}$O and to excite two-photon transitions from the $^{1}$${\mathit{D}}_{2}$ level to the long-lived ${(}^{2}$D\ifmmode^\circ\else\textdegree\fi{})3p $^{1}$${\mathit{F}}_{3}$ autoionizing level. The second laser is used to probe transitions from the $^{1}$${\mathit{F}}_{3}$ level to higher-energy autoionizing levels as well as to the ${\mathrm{O}}^{+}$ $^{2}$D\ifmmode^\circ\else\textdegree\fi{} direct-ionization continuum. The transitions are detected by using a magnetic bottle electron spectrometer to monitor photoelectrons selectively with the kinetic energy specific to the process of interest. New transitions to the ${(}^{2}$D${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{5/2,3/2}$)ns' and nd' Rydberg-series members with n=16--31 are observed. The intensity dependence of these results has also been studied, and the implications of this relatively low-power measurement for above-threshold ionization are discussed.

Rotational state distributions from vibrational autoionization of H2 revisited

Photoelectron spectra of vibrationally autoionized (X 2Σ+g)np, v=1,2 Rydberg states of H2 were obtained by using an optical–optical double-resonance technique. The spectra were obtained by using a hemispherical electrostatic electron energy analyzer that had sufficient resolution to determine the rotational state distributions of the H+2 ions. The ionization process occurred in a magnetic and electric field-free region. All of the results are consistent with the assumptions that the photoelectrons are ejected as p waves and that spin effects are negligible. This is in sharp contrast to our earlier results obtained by using a magnetic bottle electron spectrometer, in which the ionization process occurred in a 0.5–1.0 T magnetic field [J. L. Dehmer et al., J. Chem. Phys. 90, 6243 (1989)]. It is concluded that the magnetic field can significantly modify the rotational distributions observed for vibrational autoionization. The implications of this conclusion are discussed, with particular attention to other experiments performed with the magnetic bottle spectrometer.

Photoelectron spectroscopy of vibrationally excited H2 (E,F 1 Sigma g+).

Photoelectron energy spectra following (2+1)-multiphoton ionization of ${\mathrm{H}}_{2}$ via the double-well E,${\mathit{F}}^{1}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{g}}^{+}$ state have been measured using a magnetic-bottle electron spectrometer. The range of vibrational levels studied, ${v}_{E}$,F=3--9, includes states localized at the bottom of the E well, and also states that span both wells. The branching among the vibrational ionization channels is governed by the strong R dependence of the electronic wave function and the degree of localization of the vibrational wave function in the E or the F well. Theoretical analysis confirms that at least two mechanisms contribute to the observations: a direct process involving only the E,F state and the ${\mathrm{H}}_{2}^{+}$ ionization continuum, and an indirect process involving the $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ autoionizing state. Mass spectroscopic measurements carried out for ${v}_{E}$,F=9, J=1 show that about 25% of the ions produced were ${\mathrm{H}}^{+}$. Qualitative arguments suggest that most of these protons arise from dissociative ionization to the ${\mathrm{H}}_{2}^{+}$ continuum.

Rotational state distributions from vibrational autoionization of H2

Optical–optical double-resonance excitation together with electron spectroscopy was used to measure the H+2 rotational state distributions produced by vibrational autoionization of singlet np Rydberg states of H2 . In the two-color excitation scheme, one laser was used to excite the two-photon transition to the H2 E, F 1∑+g, v′=1, J′=1 state, and a second laser was used to probe single-photon transitions to the vibrationally autoionized np Rydberg series converging to the X 2∑+g, v+=1, N̄+=1 and N̄+=3 levels of the ion. The expected P(1)npσ, Q(1)npπ, R(1)np1, and R(1)np3 Rydberg series converging to v+ =1 were observed and assigned, as were several interlopers converging to higher vibrational levels of the ion. Rotationally resolved photoelectron spectra were determined for all of the autoionizing transitions by using a magnetic bottle electron spectrometer. Under the normal assumptions that p waves are ejected and that spin effects are negligible, vibrational autoionization of the upper levels of the P(1)npσ and Q(1)npπ transitions should produce only v+ =0, N̄+ =1, while vibrational autoionization of the upper levels of the R(1)np1 and R(1)np3 transitions should produce a mixture of v+ =0, N̄+ =1 and v+ =0, N̄+ =3. Significant deviations from these expectations were observed. For example, vibrational autoionization of the upper levels of the Q(1)npπ transitions produced substantial amounts of v+ =0, N̄+ =3, while vibrational autoionization of the upper levels of certain Q(1)npπ, R(1)np1, and interloper transitions produced nonnegligible amounts of v+ =0, N̄+ =5. This indicates that vibrational autoionization of npπ Rydberg states is accompanied by rotational state changes in the H+2 core to an unexpected degree, and that additional mechanisms for exchange of angular momentum within the excited complex must be considered. Possible contributing mechanisms are critically assessed.

Magnetic bottle electron spectrometer using permanent magnets

Design principles and initial results are presented for a magnetic bottle electron spectrometer of high collection efficiency (2π sr) and good energy resolution. The instrument duplicates the carefully shaped axially decreasing magnetic field configuration of Kruit and Read [J. Phys. E 16, 313 (1983)], but replaces the high-field electromagnet central to the Kruit–Read design with two simple pole pieces energized by permanent magnets. The use of permanent magnets requires modifications of the transition region (between the high-field electron production zone and the low-field drift tube analyzer) to avoid axial field reversals associated with fringing fields.

Vibrational branching ratios following two-color excitation of autoionizing n p Rydberg states of H2

Two-color resonantly enhanced multiphoton ionization-photoelectron spectroscopy (REMPI-PES) was used to determine vibrational branching ratios following autoionization of the ungerade npσ 1Σ+u and npπ 1Πu Rydberg states of H2. In this two-step experiment, one laser used to excite the two photon transition to the E,F 1Σ+g, v′=E2, J′=1 state, and a second laser was used to access the autoionizing Rydberg states near the H+2X 2Σ+g, v+=2 ionization limit. Electrons corresponding to the formation of H+2X 2Σ+g, v+=0 and 1 were collected and energy analyzed using a magnetic bottle electron spectrometer. In agreement with the well-known propensity rule for vibrational autoionization, the vibrational branching ratios strongly favor the final ionic state that corresponds to the minimum change in vibrational quantum number. In general, the branching ratio into the v+=1 channel is 94%–96%, while that into the v+=0 channel is 4%–6%; however, two major deviations from this trend were observed for Rydberg states that are perturbed by the 3pπ 1Πu, v=9 and 4pσ 1Σ+u, v=7 states. Although these low n/high v interlopers were not observed in the present work (since their ionization efficiency is near zero), interchannel coupling apparently causes their influence to be felt by nearby Rydberg states, resulting in v+=0 branching ratios as high as 18%. A number of additional studies suggested by these initial results are discussed.

Four-photon dissociation and ionization ofH2

We report a quantitative investigation of competition between dissociation and ionization in four-photon excited ${\mathrm{H}}_{2}$. The C $^{1}\ensuremath{\Pi}_{u}$ v=0--4, N=1--3 states are prepared by three-photon excitation from the ground state. Absorption of a fourth photon from the same laser pulse leads either to ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$+e or H+H(n=3,4). This branching is monitored in photoelectron spectra, taken with a novel magnetic bottle electron spectrometer based on a permanent magnet design. The competition between dissociation and ionization shows a pronounced dependence on the symmetry, vibrational, and rotational quantum numbers of the C state. We attempt to explain this dependence in terms of excitation of dissociative ${\mathrm{H}}_{2}$ states in the ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$ ionization continuum from which both autoionization and dissociation may occur.

Photoionization dynamics of H2 C 1Πu: Vibrational and rotational branching ratios

Angle-integrated, rotationally resolved photoelectron spectra were determined for three-photon resonant, four-photon (3+1) ionization of H2 via the C 1Πu, v′=0–4 intermediate levels using a magnetic bottle electron spectrometer. The results confirm and extend our previous angle-resolved results obtained using a hemispherical electron energy analyzer. As predicted by both Franck–Condon factors and recent ab initio Hartree–Fock level calculations, ionizing transitions in which the vibrational level of the resonant intermediate state is preserved in the ion are most probable; however, large deviations from theoretical expectations are observed for photoionization of all intermediate levels. These deviations take the form of a progressive broadening of the ionic vibrational distribution as the vibrational level of the resonant intermediate state is increased. The rotational branching ratios also exhibit systematic variations with both the vibrational level of the C 1Πu state and the vibrational level of the ion. Photoelectron signal from ionization of electronically excited atomic hydrogen produced in the photodissociation of H2 C 1Πu is also observed.