Stuart Pollack

Optical absorption spectroscopy of sodium clusters as measured by collinear molecular beam photodepletion

Collinear molecular beam photodepletion was used to obtain particle specific electronic absorption information for Na3, Na4, and Na8 in a wavelength range from 370–835 nm. We critically discuss the experimental method used and the deconvolution procedure applied to the resulting data to yield absolute absorption cross sections. The spectra contain much information on the cluster‐size–dependent transition from molecular to bulk‐like optical response and are interpreted in terms of various computational approaches ranging from classical electrostatic to a b i n i t i o large scale configuration interaction.

Molecular excited states versus collective electronic oscillations: Optical absorption proves of Na4 and Na8

Abstract Photodepletion spectra were obtained for Na 4 and Na 8 over the wavelength range from 435 to 825 nm. For Na 4 , six band systems were found with cross sections from 0.25 to 3.39 A 2 . They are assigned to various electronic transitions from rhombic ground state Na 4 predicted in ab initio large-scale configuration interaction calculations. For Na 8 , we confirm the observation of a strong transition centered at 490 nm which has been interpreted in terms of a classical surface-plasma oscillation. The enhanced signal-to-noise available to us allows the first resolution of fine structure incompatible with this classical picture.

ON THE OPTICAL RESPONSE OF NA20 AND ITS RELATION TO COMPUTATIONAL PREDICTION

We compare cluster size specific measurements of Na2 0absorption response to the predictions of semiclassical electrostatic and quantum mean field treatments and discuss the implications for size trends in the optical properties of alkali microclusters.

Photodepletion probes of Na5, Na6, and Na7. Molecular dimensionality transition (2D→3D)?

Photodepletion spectroscopy was used to obtain electronic absorption data for Na5, Na6, and Na7 in a wavelength range from 420–770 nm. Measurements were rationalized using several models, ranging from classical electrostatic to a b i n i t i o large‐scale configuration‐interaction calculations. As in previous studies of Na4, Na8, and Na2 0, classical Mie–Drude theory applied to appropriately dimensioned metal droplets describes the gross but not a number of fine‐structure features of the spectra observed. In contrast, a b i n i t i o large‐scale configuration‐interaction predictions of vertical electronic excitations from specific ground‐state geometries can more fully account for the measurements. Theory–experiment comparison suggests that Na6 is predominantly formed under our molecular‐beam conditions in a planar or near‐planar structure. In contrast, the spectrum obtained for Na7 is consistent with a three‐dimensional topology.

On the optical absorption spectrum of Na2Cl

Abstract One-color resonant two-photon ionization (R2PI) and photodepletion measurements of Na 2 Cl generated in continuous molecular beams by coexpansion of sodium and sodium chloride equilibrium vapors are presented. The spectra extend from 1.9 to 3.0 eV as well as encompassing several wavelengths in the 1.4–1.7 eV range. We observe two electronic band systems centered respectively at 2.24 and 2.64 eV, which are tentatively assigned as X( 1 A 1 )→B( 1 A 1 ) and →C( 1 B 1 ) transitions from the Na 2 Cl C 2v ground state via comparison with theoretical predictions.

Optical absorption probes of LixNa4−x (x=1,2): Geometry, substitution isomers, and composition‐dependent electron delocalization

Photodepletion spectra (415–763 nm, 10 A effective resolution) were obtained for LiNa3 and Li2Na2 and compared to previous Na4 and Li4 measurements as well as to model calculations ranging from classical electrostatic to a b i n i t i o molecular orbital. Findings include (i) the assignment of planar rhombic singlet ground states for both molecules; (ii) in Li2Na2, the possibleobservation of predominantly one of three energetically close‐lying planar rhombic or near rhombic substitution isomers; and (iii) in the sodium/lithium tetramer series, evidence for increasing localization of itinerant valence electrons with increasing cluster lithium content.

Metal-insulator transitions in microclusters? Mixed sodium/sodium chloride aggregates

Photoionization measurements on cluster beams obtained by coexpanding sodium and sodium chloride vapors allow inferences regarding the thermodynamic stability of a number of mixed metal/salt aggregates. We discuss the possibility of observing composition dependent metal/insulator transitions among clusters with a fixed number of monomer units.