M. Y. Hahn

Magic numbers in CN+ and CN− abundance distributions

Abstract Charged carbon clusters (C + N and C − N ; N = 9–87) have been produced by growth from laser-generated carbon vapor in a helium flow channel, without the aid of subsequent laser ionization/attachment. The resulting abundance distributions therefore reflect relative cluster stabilities, rather than the consequences of specific photophysical processes. Under all conditions, the N = 60 magic number is reproduced in the C + N distribution, while these and other observed numbers are not seen in the C − N distribution.

Spectroscopic manifestations of structural shell filling in (benzene)n clusters, N = 1–20

Abstract Well resolved ultraviolet spectra of the smaller benzene clusters (B N , N ⩽20) have been obtained by resonant two-photon ionization spectroscopy of a cluster beam. Under the coldest conditions, optical lineshapes show a remarkable narrowing at N = 13, and probable secondary linewidth minima at N = 7 and 19. This pattern is interpreted according to the principles of shell filling in clusters of nonpolar molecules.

Multiple ionization of benzene clusters by ultraviolet radiation

Efficient multiple ionization of large benzene clusters has been observed following irradiation of a supersonic cluster beam by ArF‐laser pulses at low fluences. The mechanism of multiple ionization is shown to be distinct from that believed to predominate in electron impact or synchrotron ionization experiments. The fluence dependence is instead interpreted by considering independent ionization of molecules within the cluster. The efficiency of multiple ionization over a range of laser fluence can be predicted quantitatively using a recently proposed exciton annihilation model, and explains the 106 enhancement of the double ionization rate over the molecular value. The critical sizes N*(z) for stability of a z‐charged drop with respect to Coulomb fission are N*(2)=23, N*(3)=52, and N*(4)=92 and can be accounted for by liquid drop models. However, the fission of doubly charged clusters could not be detected in dynamical mass spectrometry experiments on the 10−6 to 10−4 s time scale. The surprising critica...

Photoionization and excitation energies of an Al atom in ArN clusters

Photoionization measurements on Al · ArN clusters (N < 150) in a cold cluster beam have been used to determine ionization threshold energies El, th(N) of the metal atom in a variable-size dielectric medium. The El, th(N) values descend from the atomic value [5.99 eV (1 eV ≈ 1.602 × 10–19 J)] to below 5.5 eV near N≈ 55. The El, th(N) functional form is compared to dielectric continuum predictions and to N-specific Monte Carlo calculations incorporating polarization and model-potential interactions. Satisfactory agreement is obtained when a strong neutral Al–Ar interaction is assumed. Measurements of the slow fragmentation probability indicate that near-threshold ionization of smaller clusters (N < 25) produces negligible fragmentation, resulting in N-specific threshold measurements. Comparison of neutral cluster distributions, measured by threshold photoionization to the corresponding cold Al+·ArN distribution (produced in the source) supports a predominant charge-invariance of the icosahedral-based structures, also found in Monte Carlo simulations. By contrast, photo-absorption spectra in bound states, as exemplified by R2PI spectra of the Al 3p → 4s transition, exhibit only a small fraction (ca. 0.12 eV) of the bulk shift (ca. 0.6 eV). This result may be partly explicable in terms of the distinction between sudden and adiabatic polarization energies in the two experiments.

The ultraviolet spectra of size‐resolved (benzene)N, N=2–60

Photoabsorption spectra of benzene clusters BN containing up to N=60 molecules have been obtained on an N‐resolved basis over the B2u region of the spectrum (λ=265–245 nm). The resonant enhanced two‐photon ionization of a supercooled cluster beam yields distinctive spectra for each value of N. The energy of the 610 resonance is found to decrease monotonically with increasing N, but at N=50 is still far from condensed‐phase values.