Vibrational spectra and structures of neutral Si₆X clusters (X = Be, B, C, N, O)

Abstract

aNeutral silicon clusters doped with first row elements (Si 6 X) have been generated (X = B, C, N, O) and characterized by infrared-ultraviolet (IR-UV) two-photon resonance-enhanced ionization spectroscopy (X = C, O) and quantum chemical calculations (X = Be, B, C, N, O, Si). In the near threshold UV photoionization, the ion signal of specific cluster sizes can be significantly enhanced by resonant excitation with tunable IR light prior to UV irradiation, allowing for the measurement of the IR spectra of Si 7 , Si 6 C, and Si 6 O clusters.Structural assignments are achieved with the help of a global optimization procedure using density functional theory (DFT). The most stable calculated structures show the best agreement between predicted and measured spectra. The dopant atoms in the Si 6 X clusters have a negative net charge and the Si atoms act as electron donors within the clusters. Moreover, the overall structures of the Si 6 X clusters depend strongly on the nature of the dopant atom, i.e., its size and valency. While in some of the Si 6 X clusters one Si atom in Si 7 is simply substituted by the dopant atom (X = Be, B, C), other cases exhibit a completely different geometry (X = N, O). As a general trend, doping of the Si 7 cluster with first-row dopants is predicted to shift the optically allowed electronic transitions into the visible or even near-IR spectral range due to symmetry reduction or the radical character of the doped cluster.

Characterization of neutral boron-silicon clusters using infrared spectroscopy: The case of Si6B

“…Most of the computational results have already been provided in our recent work [25] and support the previous study of Tam et al [19]. Generally, the three used methods agree in predicting the most stable structure (6a), while for the substantially higher-energy structures (≥30 kJ/mol) slight differences in their relative energies are observed.…”

“…An increase in the internal energy of the cluster upon IR absorption therefore results in an enhancement of the ionization yield. An IR-UV2CI spectrum is obtained from the relative ionization enhancement normalized by the IR photon flux as a function of the IR laser frequency [25,50,51], which closely reflects the linear vibrational absorption spectrum of the neutral cluster.…”

“…Anionic SinB -clusters (n=1-6) formed by direct laser ablation from a mixed B/Si sample were studied by mass spectrometry and DFT calculations [24]. In that work, Si6B -was predicted to have C5v symmetry ( 1 A1), whereas its neutral counterpart was suggested to have Cs symmetric geometry ( 2 A') [25]. Recently, Tam et al studied thermochemical parameters and the growth mechanism of B-doped silicon clusters (SinB q , n=1-10, q=0, ±1) using B3LYP, G4, and CCSD(T) approaches [19].…”

“…Surprisingly, no spectroscopic data are available for B-doped silicon clusters in any charge state. Here, we combine infraredultraviolet two-color ionization (IR-UV2CI) spectroscopy [27] with quantum chemical simulations to determine the structure of Si6B, a strategy recently applied to a variety of (doped) silicon clusters [25,[27][28][29][30].…”

“…We 3 demonstrate that with this cluster source an almost arbitrary mixing ratio between the different components can be achieved. As an example for the spectroscopic characterization of the clusters produced in this source, we apply the IR-UV2CI technique to neutral Si6B to determine its geometric structure, as suggested previously [25].…”