In matrix isolation spectroscopy (MIS), the species to be studied are directed towards a transparent substrate cooled to cryogenic temperatures, which they reach simultaneously with rare gas atoms provided in excess, i.e., at least 1000 rare gas atoms for each molecule. Due to the low temperature of the substrate, the molecules and rare gas atoms condense together. The latter being in excess, a rare gas matrix is formed in which the molecules are kept isolated from each other. After letting the matrix grow until a quantity of molecules has been accumulated sufficient to be detected, absorption spectroscopy is carried out by measuring the attenuation of a light beam traversing the substrate and the matrix or the reflection intensity if a reflective substrate is used.

The advantages of MIS are multiple: Species whose vapor pressure is too low to carry out absorption measurements in supersonic jets (see this project) may be studied since they can be accumulated. Moreover, as the molecules are trapped in the matrix, time-consuming measurements such as those involving scans over broad wavelength ranges can be performed without consuming much sample. Due to the cryogenic temperature, only the lowest energy levels of the molecules are populated, resulting in spectra typical for cold molecules. Finally, the molecules being isolated from each other, spectra are not affected by intermolecular interactions.

On the other hand, interactions between the molecules and the atoms of the matrix give rise to broadened and shifted absorption bands relative to measurements in supersonic jets where the molecules are truly isolated, i.e., in a collision-free environment. This means that spectra obtained by MIS cannot be directly compared with the spectra of interstellar molecules for the purpose of their identification. Still, the advantages mentioned before make MIS an extremely useful technique for a first spectroscopic approach of species with low vapor pressure, for instance large polycyclic aromatic hydrocarbons (PAHs). Over the years, a number of these molecules and their cations has been investigated by MIS in several groups.

Rare gases are usually chosen as matrix material because they are chemically inert and do not have a permanent electric dipole moment. Moreover, in order to minimize the interaction between the matrix material and the molecules, rare gases with the lowest polarizabilities are used, namely Ar and Ne. The polarizability of He is even lower, but its properties are such that it can only be used in the form of a liquid matrix (see this project).

The matrix gas is deposited on a substrate kept typically at 6 K when using Ne as the matrix material, at 12 K when using Ar. For MIS in the UV-VIS wavelength range, CaF2 substrates are chosen, and, when working in the IR range, we employ KBr substrates.

 

pyrene in neon matrix at 6 K

Fig. 1. UV absorption spectrum of the PAH pyrene (C16H10) in neon. The Ne matrix was prepared at 6 K on a CaF2 substrate.

 

As PAH samples are solid under normal conditions, it is usually necessary to heat them up in order to reach useful vapor pressures. With pyrene, however, the vapor pressure is high enough even at room temperature. Adjusting the Ne flow to 5 sccm, the deposition and growth of the pyrene-doped Ne matrix shown in fig. 1 took 45 min.

We have also measured the absorption spectra of benzo[g,h,i]perylene (C22H12) [1], corannulene (C20H10) [2], hexa-peri-hexabenzocoronene (C42H18) [3], dibenzorubicene (C30H14) [4], several ethynyl derivatives [5], and tetraphenyldibenzoperiflanthene (C64H36) [6].

Further development of the current setup is considered. A higher sensitivity of the technique can be achieved by measuring the absorption through the length of the matrix instead of its thickness [7]. Moreover, narrower bandwidths can be obtained by applying the so-called site selection technique [8].

 

References:

[1] G. Rouillé, M. Arold, A. Staicu, S. Krasnokutski, F. Huisken, Th. Henning, X. Tan, and F. Salama: S1(1A1) ← S0(1A1) transition of benzo[g,h,i]perylene in supersonic jets and rare gas matrices, J. Chem. Phys. 126, 174311/1-174311/11 (2007). [DOI]
[2] G. Rouillé, C. Jäger, M. Steglich, F. Huisken, Th. Henning, G. Theumer, I. Bauer, and H.-J. Knölker: IR, Raman, and UV/Vis spectra of corannulene for use in possible interstellar identification, ChemPhysChem 9, 2085-2091 (2008). [DOI]
[3] G. Rouillé, M. Steglich, F. Huisken, Th. Henning, and K. Müllen: UV/visible spectroscopy of matrix-isolated hexa-peri-hexabenzocoronene: Interacting electronic states and astrophysical context, J. Chem. Phys. 131, 204311/1-204311/7 (2009). [DOI]
[4] G. Rouillé, M. Steglich, C. Jäger, F. Huisken, Th. Henning, G. Theumer, I. Bauer, and H.-J. Knölker: Spectroscopy of dibenzorubicene: Experimental data for a search in interstellar spectra, ChemPhysChem 12, 2131-2137 (2011). [DOI]
[5] G. Rouillé, M. Steglich, Y. Carpentier, C. Jäger, F. Huisken, Th. Henning, R. Czerwonka, G. Theumer, C. Börger, I. Bauer, and H.-J. Knölker: On the relevance of polyynyl-substituted polycyclic aromatic hydrocarbons to astrophysics, Astrophys. J. 752, 25/1-25/12 (2012). [DOI][arXiv.org]
[6] G. Rouillé, T. Kirchhuebel, M. Rink, M. Gruenewald, J. Kröger, R. Forker, and T. Fritz: Identification of vibrational excitations and optical transitions of the organic electron donor tetraphenyldibenzoperiflanthene (DBP), Phys. Chem. Chem. Phys. 17, 30404-30416 (2015). [DOI]
[7] V. E. Bondybey, T. A. Miller, and J. H. English: Electronic absorption spectra of molecular cations, J. Chem. Phys. 72, 2193-2194 (1980).
[8] B. F. MacDonald, J. L. Hammons, R. R. Gore, J. R. Maple, and E. L. Wehry: Site-selection fluorescence spectrometry of polycyclic aromatic hydrocarbons in vapor deposited argon matrices, Appl. Spectrosc. 42, 1079-1083 (1988).

 

For more information contact Dr. Cornelia Jäger.

Related funding(s): DFG HU 474/18, DFG HU 474/24.

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