Resonance Raman spectra of mass-selected Mo2 and Mo3 in Argon matrices

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Abstract

We report on mass-selected molybdenum dimers (Mo2) and trimers (Mo3) in argon matrices via resonance Raman spectroscopy. The resonance Raman spectrum of Mo2 displays lines at 394.5(12), 445.0(18), and 473.3(15)cm−1, with several overtones observed for the most intense 394.5cm−1 line.

The trimer (Mo3) displays absorption in the 575–625 nm region with a maximum at ∼600 nm. Excitation into this region gives Raman lines at 224.5(5), 236.2(8), 386.8(12), and 446.9(7)cm−1. They are assigned to ν3(a1), ν2(b1), ν1(a1) (the bend, antisymmetric stretch, and symmetric stretch) and 2ν2, respectively.

Introduction

Molybdenum dimer, Mo2, is noted for having one of the strongest metal–metal bonds with a likely bond order greater than four, [1], [2] and it is worthwhile examining whether this strong bonding extends to the trimer. Efremov et al. [3] resolved vibrational and rotational frequencies of three electronic band systems of Mo2 via flash photolysis of crystallized molybdenum hexacarbonyl. They found vibrational fundamentals in each of the states involved, from which they obtained the ground state fundamental of ωe=477cm−1 for Mo2, as well as ωe=443.4cm−1 for the first excited A(1Σu+) state at 19303.7cm−1. Pellin et al. [4] employing time resolved fluorescence techniques, investigated Mo2 in argon and krypton matrices. When exciting at various wavelengths ranging from 498 to 543 nm, they observed fluorescence from the dimer in a spectral range of ∼724–850 nm. Their fluorescence spectra gave two sets of nearly identical emissions that were slightly shifted in spectral range from one another, which they termed red and blue emission attributed to the existence of two sites in the argon matrix. They determined the ground state fundamental of the red and blue emission to be ωe=473.3 and 477 cm−1, respectively. Their findings were later confirmed by observation of a single Raman line at 475cm−1[5].

In this Letter we present the resonance Raman and absorption characteristics of Mo2 and Mo3 in argon matrices. Exciting with laser lines 488, 496.5, or 501.7 nm as well as with dye lasers, we observe Raman features for Mo2 in a spectral region of ∼451–461 nm. The Raman spectrum of Mo2 displays three distinct lines at 394.5(12), 445.0(81), and 473(15)cm−1. The 394.5cm−1 line is the most intense of the three and includes a series of three overtones up to 1547cm−1. We also detect fluorescence from Mo2 when exciting in a region of 440–685 nm, which is the same as that observed by Pellin.

For Mo3, there are no previously reported resonance Raman experiments. In our experiments, the trimer displays a broad absorption band extending from ∼575 to 625 nm, with an absorbance maximum around 600 nm. We excite near the absorbance maximum of Mo3 and observe four resonance Raman lines at 224.5(5), 236.2(8), 386.8(12), and 446.9(7)cm−1. We assign the 386.8cm−1 peak as the ground state fundamental of the symmetric stretch ν1(a1). The lines at 236.5 and 224.5cm−1 we attribute to the antisymmetric stretch ν2(b1) and the bend ν3(a1), respectively. The line at 446.9cm−1 is assigned as an overtone to the antisymmetric stretch.

Section snippets

Experimental

The CCNY metal cluster deposition source has been described in detail in previous publications [6], [7] but will be briefly detailed here. A 10 mA argon ion beam (at 25 keV) sputters a water-cooled molybdenum metal target, purchased from Alfa Aesar at 99.9% purity. A series of einzel lenses collect Mon+ from the metal sputtering region and focuses the molecules to the Wien filter, where mass selection occurs. After mass selection, molybdenum clusters are focused onto the deposition region where

Results and discussion: Mo2

The resonance Raman spectrum of Mo2 gives three unique Raman lines at 394.5, 445.0, and 473.3cm−1, with three overtones of 394.5cm−1 at 783.0(52), 1167.0(49), and 1547(9)cm−1 (Fig. 1). No Raman lines were observed below 300cm−1. Normally, only a single fundamental is observed for a diatomic molecule, unless tied to an origin of a low lying electronic state [8], [9]. Surprisingly, we observe several lines with the same spectral origin (i.e., the Rayliegh line). The 394.5cm−1 line is, by far, the

Results and discussion: Mo3

The resonance Raman spectrum of an ∼220 nA-h sample of Mo3 in an argon matrix is shown in Fig. 2. The absorption (SDS) spectrum and resonance Raman excitation profile, both shown in the inset of Fig. 2, are in agreement and confirms our assignment of the spectrum of the mass-selected sample to the trimer. The absorption of Mo3 displays a broad band in the region of ∼575–625 nm. The maximum for this absorption is in the region of ∼600 nm. Exciting at 590 nm, we observe resonance Raman lines for

Acknowledgements

We wish to acknowledge Dr. Michael Pellin and Dr. Steven Mitchell for helpful conversations pertaining to this work. We are especially grateful to Professors Michael Morse and Doreen Leopold for extensive and helpful comments concerning this manuscript. These experiments were funded by the National Science Foundation under Cooperative Agreement No. RII-9353488, Grant No. CHE-0091362, and by the City University of New York (CUNY) PSC-BHE Faculty Research Award Program.

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