S. Wei

Metallo-Carbohedrenes [M8C12+ (M = V, Zr, Hf, and Ti)]: A Class of Stable Molecular Cluster Ions

Findings of magic peaks corresponding to M8C12+ (M = V, Zr, and Hf) formed from reactions of the respective metals with various small hydrocarbons, in conjunction with recent findings for the titanium system, establish metallo-carbohedrenes as a stable general class of molecular cluster ions. A dodecahedral structure of Th point symmetry accounts for the stability of these ionic clusters.

Mixed cluster ions as a structure probe: Experimental evidence for clathrate structure of (H2O)20H+ and (H2O)21H+

First direct experimental evidence for clathrate structures of (H2O)nH+ (n=20,21) is reported based on a technique allowing the number of nonhydrogen‐bonded surface hydrogens to be counted. Neutral clusters (H2O)n⋅((CH3)3N)m, prepared in a pulsed nozzle supersonic expansion, are ionized by multiphoton ionization and investigated with a reflectron time‐of‐flight mass spectrometry technique. The magic numbers (n,m) in the ion intensity distributions of (H2O)n⋅((CH3)3N)m⋅H+ studied under various experimental conditions are well correlated to the stable hydrogen‐bonding structures. For the mixed cluster ion (H2O)20⋅((CH3)3N)m⋅H+, the intensity distribution displays an abrupt intensity drop after the magic number at (20,11), while for (H2O)21⋅((CH3)3N)m⋅H+ the magic number appears at (21,10). The findings provide experimental evidence for a stable clathrate structure of (H2O)20H+, with the proton residing on the surface, while for (H2O)21H+, the H3O+ ion is encaged inside the clathrate structure of (H2O)20; th...

Water clusters: Contributions of binding energy and entropy to stability

The binding energies of water cluster cations are obtained by measuring decay fractions of metastable dissociation and employing Klots’ model of evaporative dissociation. Their variation with degree of solvation shows the commonly observed decrease, followed by a slow rise in magnitude, which typifies the trend found for solvated cations. There is no observed abrupt change in the vicinity of the well‐known magic number (H2O)21⋅H+ corresponding to (H2O)20⋅H3O+. Other data are used to deduce free energies for water clusters up to size n=28, allowing a determination of entropy changes with size. All of the thermochemical data, including prior literature values, are assessed in terms of calculations made using the liquid drop model and standard statistical mechanical equations. It is concluded that entropic rather than energetic effects give rise to the referred to magic number.

Metallo-carbohedrenes : formation of multicage structures

An unusual structural growth pattern has been found in the system of ZrmCn, in which multicage structures are formed. The experimental evidence shows that the first cage closes at Zr8C12. Surprisingly, subsequent cluster growth does not lead to the enlargement of the cage size as it usually does in the case of pure carbon clusters and water clusters, for example. Rather, multicage structures are developed, that is, a double cage at Zr13C22 and Zr14C21/23, a triple cage at Zr18C29, and a quadruple cage at Zr22C35. This feature distinguishes the class of metallo-carbohedrenes from the regular doped fullerenes.

Ultrafast laser-induced Coulomb explosion of clusters with high charge states

Abstract Iodine and argon ions, as highly charged as I17+ and Ar8+, are found to be formed upon the irradiation of HI clusters and HIArm clusters with an intense femtosecond laser beam. Double peaks are observed for each multicharged species, and identified to result from Coulomb explosion of clusters. Kinetic energy release values are calculated from the peak splittings, and also directly measured from cutoff potentials using a reflectron as an energy analyzer. The determined values are surprisingly large, on the order of several hundred electron volts.

Kinetic energy release measurements of ammonia cluster ions during metastable decomposition and determination of cluster ion binding energies

The dissociation dynamics of protonated ammonia clusters is investigated following their production from neutrals using multiphoton ionization. A very useful method of measuring the kinetic energy release arising from evaporative unimolecular dissociation is established; it employs a time‐of‐flight/reflectron to separate daughters and parents and enables the energy release to be determined from the peak shapes. The method is found to lead to values of high precision. Using a modified statistical theory analysis suggested by Engelking, it is shown that binding energies for large cluster ions can be readily determined. The results compare very well with those derived by other methods, ones which generally have much more severe size limitations. In the case of (NH3)n H+, the observed drop in binding energy from n=5 to 6 is consistent with the particularly stable protonated pentamer structure. Heretofore, unavailable results are available from n=7 to 17, with hints of slightly more stable cluster ions at n=12...

Femtosecond laser-induced Coulomb explosion of ammonia clusters

Abstract Nitrogen atoms, as highly charged as N 5+ , are found to be formed upon the irradiation of (NH 3 ) n clusters with an intense femtosecond laser beam. Kinetic energy release values of the nitrogen atoms, from the Coulomb explosion of ammonia clusters, are found to be on the order of several hundred electron volts as calculated from the peak splittings, and also directly measured from cutoff potentials using a reflection as an energy analyzer. Also, we observe a Coulomb explosion process which results in the production of intact cluster ions, and we propose a mechanism for their formation.

Dissociation dynamics: Measurements of decay fractions of metastable ammonia cluster ions

The decay fractions of metastable ammonia cluster ions which undergo unimolecular (evaporative) dissociation in a time window of 1–40 μs were measured by using a reflection time‐of‐flight mass spectrometer. Corrections concerning instrumental artifacts and ion trajectory of parents and daughters are made to imporve the precision of the measurements. The data are used to derive the Gspann parameter and heat capacity of clusters as described in evaporative ensemble model of metastable dissociation. Using the dissociation fractions measured in the present studies, in conjunction with kinetic energy release values previously measured in our laboratory, we apply Klots’ evaporative ensemble model to obtain binding energies of ammonia cluster ions (NH3)nH+, 4≤n≤17. The deduced binding energy values are found to be in very good agreement with both thermochemical data and Engelking’s modified statistical theory.

Femtosecond multiphoton ionization of ammonia clusters

Herein, we report on femtosecond time-resolved experiments in ammonia clusters. The mechanisms of their ionization and the subsequent formation of the protonated ammonia cluster ions are studied using a femtosecond pump-probe technique at 620 nm. It is found that an intermediate corresponding to [ital C][prime] states of the monomer is responsible for the ionization of ammonia clusters. Femtosecond pump--probe studies show that the lifetime of the intermediate to the formation of the protonated cluster ions (NH[sub 3])[sub [ital n]]H[sup +] ([ital n]=1--5) is the same as that leading to the formation of the unprotonated cluster ions (NH[sub 3])[sub [ital m]][sup +] ([ital m]=2--5). The results provide the first direct experimental proof that formation of the protonated cluster ions takes place through an absorption--ionization--dissociation mechanism.

Spectroscopy of phenylacetylene bound to clusters of ammonia and the surface cluster analogy

The results of a systematic study of the spectra shifts of the S1–S0 transition in phenylacetylene (PA) [1B2(1Lb)←1A1(1A)] due to solvation by NH3 are reported based on two‐photon REMPI. A unique aspect of the present work is comparison of the red shifts for clusters produced by coexpansion techiques with those where PA is attached to performed ammonia clusters. In both experiments the 1–1 complex is red shifted by 80 cm−1. In the case of the coexpansion experiments the red shift is about 50 cm−1 for PA(NH3)n, n=2–7. By contrast, the clusters produced by attachment display a much different and gradually increasing red shift which saturates at 150 cm−1 for n=13 and beyond. Structural differences for clusters produced by the two different techniques are believed to be responsible.