Stuart R. Mackenzie

Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

The geometric structures of small cationic rhodium clusters Rh(n)(+) (n = 6-12) are investigated by comparison of experimental far-infrared multiple photon dissociation spectra with spectra calculated using density functional theory. The clusters are found to favor structures based on octahedral and tetrahedral motifs for most of the sizes considered, in contrast to previous theoretical predictions that rhodium clusters should favor cubic motifs. Our findings highlight the need for further development of theoretical and computational methods to treat these high-spin transition metal clusters.

New experimental method for studying rotationally state‐selected ion‐molecule reactions

A new apparatus is described in which a beam of molecular ions in a selected vibration–rotation state is prepared by field ionization of high Rydberg states, in an adaptation of the zero‐kinetic‐energy photoelectron technique. The state‐selected ions undergo low energy reactive collisions within a molecular beam and the ionic products are detected in a quadrupole mass filter. The Rydberg states are populated by two‐color stepwise multiphoton excitation, and by tuning to the pseudocontinuum of high‐Rydberg states associated with different vibration–rotation states of the ion core, different states of the ion are selected and the effect on reactivity determined. Some preliminary results for the H2+H+2→H+3+H reaction are reported.

Infrared induced reactivity on the surface of isolated size-selected clusters: dissociation of N2O on rhodium clusters.

Multiple photon infrared excitation of size-selected Rh(6)N(2)O(+) clusters drives surface chemistry resulting in partially oxidized clusters.

Infrared-Induced Reactivity of N2O on Small Gas-Phase Rhodium Clusters

Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, Rh(n)N(2)O(+) (n = 4-8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N(2)O has been observed to result in the decomposition of the N(2)O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N(2)O decomposition mechanisms are given based on DFT calculations using exchange-correlation functionals. Similar experiments pumping the Rh-O stretch in Rh(n)ON(2)O(+) complexes, on which the same chemistry is observed, confirm the thermal nature of this reaction.

Reactions of nitric oxide on Rh6+ clusters: abundant chemistry and evidence of structural isomers

We report the first results of a new instrument for the study of the reactions of naked metal cluster ions using techniques developed by Professor Bondybey to whom this issue is dedicated. Rh6+ ions have been produced using a laser vaporization source and injected into a 3 T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer where they are exposed to a low pressure (< 10(-8) mbar) of nitric oxide, NO. This system exhibits abundant chemistry, the first stages of which we interpret as involving the dissociative chemisorption of multiple NO molecules, followed by the liberation of molecular nitrogen. This yields key intermediates such as [Rh6O2]+ and [Rh6O4]+. The formation of the latter, after adsorption of four NO molecules, marks a change in the chemistry observed with further NO molecules adsorbed (presumably molecularly) without further N2 evolution until saturation is apparently reached with the [Rh6O4(NO)7]+ species. We analyse the data in terms of a simple 12-stage reaction mechanism, and we report the relative rate constants for each step. The trends in reactivity are assessed in terms of conceivable structures and the results are discussed where appropriate by comparison with extended surface studies of the same system. Particular attention is paid to the first step in the reaction (Rh6(+) + NO --> [Rh6NO]+) which exhibits distinctly bi-exponential kinetics, an observation we interpret as evidence for two different structural isomers of the Rh6+ cluster with one reacting more than an order of magnitude faster than the other.

Following interfacial kinetics in real time using broadband evanescent wave cavity-enhanced absorption spectroscopy: a comparison of light-emitting diodes and supercontinuum sources

A white light-emitting diode (LED) with emission between 420 and 700 nm and a supercontinuum (SC) source with emission between 450 and 2500 nm have been compared for use in evanescent wave broadband cavity-enhanced absorption spectroscopy (EW-BB-CEAS). The method is calibrated using a dye with known absorbance. While the LED is more economic as an excitation source, the SC source is superior both in terms of baseline noise (noise equivalent absorbances lower than 10(-5) compared to 10(-4) absorbance units (a.u.)) and accuracy of the measurement; these baseline noise levels are comparable to evanescent wave cavity ringdown spectroscopy (EW-CRDS) studies while the accessible spectral region of EW-BB-CEAS is much larger (420-750 nm in this study, compared to several tens of nanometres for EW-CRDS). The improvements afforded by the use of an SC source in combination with a high sensitivity detector are demonstrated in the broadband detection of electrogenerated Ir(IV) complexes in a thin-layer electrochemical cell arrangement. Excellent signal to noise is achieved with 10 micros signal accumulation times at a repetition rate of 600 Hz, easily fast enough to follow, in real time, solution kinetics and interfacial processes.

Rotational autoionization dynamics in high Rydberg states of nitrogen

The decay dynamics of the high Rydberg states of N2 converging on the first few rotational levels (N+=0,1,2,3) of the ground vibronic X 2Σ+g (v+=0) state of the N+2 cation have been investigated by delayed pulsed field ionization (PFI) following two‐photon enhanced (2+1′) three‐photon excitation via the a″ 1Σ+g (v′=0) state of N2. The experiments were carried out in the presence of a weak homogeneous dc electric field and at typical ion densities of 200–2000 ions/mm3. All Rydberg states in the range of principal quantum number n=140–200 exhibit extreme stability against autoionization and predissociation and some have lifetimes which exceed 30 μs. The decay of the highest Rydberg states beyond n=200 is induced by external perturbations (field ionization and collisional ionization) and no Rydberg states beyond n=350 can be observed by delayed PFI. The Rydberg states which converge on the N+=0 and 1 rotational levels of the ion, and which therefore are not subject to rotational autoionization, decay into ne...

Zero‐kinetic‐energy photoelectron spectrum of carbon dioxide

The zero‐kinetic‐energy (ZEKE) photoelectron spectrum of carbon dioxide has been measured between 111 000 and 112 000 cm−1 at a resolution of 1.5 cm−1 using a coherent source of XUV radiation based on four‐wave mixing in krypton. The spectrum consists of six bands corresponding to transitions from the ground X 1Σ+g(v1,v2,v3=000) state of the neutral to the two spin–orbit components of the (000) vibrational level and the four Renner–Teller states associated with the (010) vibrational level of the ground electronic state (X 2Πg) of the ion. The analysis of the partially resolved rotational structure of the various bands leads to a detailed picture of the photoionization process. The propensity rules for angular momentum transfer during photoionization are strongly dependent on the symmetry (2Πg,3/2, 2Πg,1/2, 2Δu,5/2, 2Δu,3/2, 2Σ+u, and 2Σ−u) of the different ionic states probed and on the Hund’s coupling case they follow [case (a) for the Π and Δ states and case (b) for the Σ states]. A comparison of the ex...

Communications: The structure of Rh(8) (+) in the gas phase.

The geometric structure of the Rh(8) (+) cation is investigated using a combination of far-infrared multiple photon dissociation spectroscopy and density functional theory (DFT) calculations. The energetic ordering of the different structural motifs is found to depend sensitively on the choice of pure or hybrid exchange functionals. Comparison of experimental and calculated spectra suggests the cluster to have a close-packed, bicapped octahedral structure, in contrast to recent predictions of a cubic structure for the neutral cluster. Our findings demonstrate the importance of including some exact exchange contributions in the DFT calculations, via hybrid functionals, when applied to rhodium clusters, and cast doubt on the application of pure functionals for late transition metal clusters in general.

Preparation of ions in selected rotational states by delayed pulsed field ionization

A novel method of preparing ions in selected rovibronic states is presented. It is based on pulsed field ionization of long‐lived high‐n Rydberg states lying just below the different rovibronic energy levels of the ion. The potential of the method is illustrated with the example of hydrogen: A clean population of H2+ ions is prepared in the X 2∑g+ (v+=2, N+=0, 1, 2, 3) states. The method ought to be applicable to the preparation of a wide range of small molecular ions in selected rotational states and opens new possibilities in the study of state‐to‐state ion–molecule reactions.