Elsevier

Chemical Physics

Volume 315, Issues 1–2, 8 August 2005, Pages 41-52
Chemical Physics

Infrared spectroscopic detection of the methylsilyl (CH3SiH2, X2A′) and the silylmethyl (CH2SiH3, X2A′) radicals and their partially deuterated counterparts in low temperature matrices

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Abstract

Both the methylsilyl, CH3SiH2(X2A′), and the silylmethyl, SiH3CH2(X2A′), together with their partially deuterated isotopomers were identified for the first time via infrared spectroscopy in low temperature silane–methane matrices at 10 K upon exposure of the matrices with mono energetic electrons. Three fundamentals at 654 cm−18), 1414 cm−112), and 1251 cm−15) (methylsilyl) and one absorption at 645 cm−18) (silylmethyl) were detected; these assignments were confirmed by identifying also partially deuterated radicals through their absorptions at 781 cm−17), 1415 cm−112), 1419 cm−14), and 1546 cm−111) (d2-methylsilyl, CH3SiD2) and 582 cm−18), 749 cm−15), and 771 cm−113) (d3-silylmethyl, SiD3CH2) in d4-silane–methane matrices. Since both silane and methane have been observed in the circumstellar envelope of the carbon star IRC+10216, our investigations assist in future infrared spectroscopic, astronomical searches for these species in interstellar space. Likewise, the knowledge of the infrared absorption features might help to follow the chemical evolution of organo-silicon CVD process via time resolved infrared spectroscopy.

Introduction

Small, binuclear organosilane molecules and their radicals of the generic formula SiCHx (x = 1–6) are important precursors to prepare amorphous silicon carbide (a-Si–C) films via chemical vapor deposition (CVD) [1], [2]. The fully dehydrogenated silicon monocarbide (SiC) is considered as a promising class of materials for high temperature and high power electronic devices since silicon carbide is chemically and mechanically inert and presents a wide-band gap (2.2–2.3 eV) semiconductor [3], [4]. In the early years, amorphous silicon carbide films were produced via chemical vapor deposition of mixtures of silicon and carbon bearing gases such as silane (SiH4) and methane (CH4) [5], [6]. Due to the strong carbon–hydrogen bond in the methane molecule, this approach required excessive growth temperatures, often inducing high tensile stress in the silicon carbide films. Recently, the use of single precursor molecules such as methylsilane, CH3SiH3, has been exploited successfully [7], [8]. However, to optimize and even to manipulate the nucleation processes, it is crucial to understand the fundamental, underlying chemical mechanisms in depth. Reaction networks which actual model the chemical vapor deposition processes of organo silanes [9], [10] demand crucial input parameters. These are rate constants of the critical reactions involved, the reaction intermediates together with the final reaction products, and their thermochemical data [11], [12], [13], [14], [15], [16], [17].

An understanding of the energetics and spectroscopic properties of simple silicon–carbon bearing species is also of crucial importance to untangle the elementary chemical reactions involved in circumstellar envelopes of dying carbon stars. About 15% of the observed interstellar molecules contain silicon ranging from simple diatomics (SiC, SiN, SiO, SiS), triatomics (SiCN) and a silicon-terminated cummulene (CCCCSi) to two cyclic species (SiC2, SiC3) [18], [19]. Here, the silane and methane molecules – precursors to more complex organo silicon molecules – have been detected in the outflow of carbon stars [20].

However, despite the importance of the organosilyl molecules as rate limiting growth species of a-Si–C films in chemical vapor deposition processes and their astrophysical potential to form silicon carbide molecules, the vibration levels of the corresponding radicals have not been investigated experimentally. Here, a detailed knowledge of the infrared absorption features might help to follow the chemical evolution of CVD processes in real time not only via mass spectrometry [4], but also through time resolved infrared spectroscopy. Here, the methylsilane (CH3SiH3; ΔHf0=-31kJmol-1) molecule as well as the corresponding silylmethyl CH2SiH3 (ΔHf0=174kJmol-1) and methylsilyl CH3SiH2 (ΔHf0=139kJmol-1) doublet radicals have received particular attention [21], [22]. The spectroscopic properties of the methylsilane molecule (CH3SiH3) have been studied extensively [23], and both microwave [24], [25], [26], [27] as well as infrared spectra [28], [29] are well known. The bond distances have been determined to be 109.6 nm (CH bond), 148.3 nm (Si–H bond), and 186.9 nm (Si–C bond); the H–C–Si and H–Si–C bond angles were calculated to be 110.9° and 110.5°, respectively. The SiCH5 radicals are thought to be the key intermediates in the growth of amorphous silicon carbide films. A theoretical study of CVD processes utilizing an ethylsilane (C2H5SiH3) precursor suggests that the CH2SiH3 species can be formed via a barrier-less methyl radical loss pathway; this process was calculated to be endothermic by 330 kJ mol−1[30] and 371 kJ mol−1 [31]. Recently, the CH2SiH3 and CH3SiH2 radicals were proposed to be formed via methylidene (CH) insertion into a silicon–hydrogen bond of silane and silylidene (SiH) insertion into a carbon–hydrogen bond of methane [32]; both reactions are barrier-less. Alternatively, kinetic studies in the temperature range of 291–1360 K suggest that hydrogen atoms can abstract a hydrogen atom from the methylsilane molecule (CH3SiH3) via a barrier of about 11 kJ mol−1; however, the nature of the SiCH5 isomer could not be identified since the experiments only follow the decay kinetics of the atomic hydrogen reagent [33], [34], [35]. Guided ion beam studies suggest that the enthalpy of formation of the CH3SiH2 radical ranges between 130 and 160 kJ mol−1 [36]. Note that solely the CH3SiH2 radical was observed in a matrix via ESR at 77 K as a reaction product of vibrationally excited methyl radicals via hydrogen abstraction from neighboring CH3SiH3; the CH2SiH3 isomer is thermodynamically less stable by 41 kJ mol−1 and was not observed [37]. Note that a 60Co γ irradiation of methylsilane yielded solely the energetically preferred CH3SiH2 radical [38], [39]. Similar to the methylsilane molecule, the infrared [40], [41] and millimeter spectra [42] of the closed shell H2CSiH2 species are also known; various theoretical investigations have been carried out on the SiCH4 potential energy surfaces as well [43], [44]. Smaller, hydrogenated silicon–carbon clusters have been investigated, too. For instance, the vibrational levels of the H2CSiH(2A′) and CH3Si(2A″) isomers have been determined via negative ion photoelectron spectroscopy [45]. Smith et al. assigned the fundamentals of H2CSi(1A1) via wavelength resolved fluorescence and stimulated emission pumping (SEP) [46]. The rearrangement to the HCSiH(1A′) and CSiH2(1A1) isomers has also been probed theoretically [47], [48]. Likewise, the vibrational levels and rovibronic spectra of HCSi(2Σg) have been recorded in the gas phase [49], [50]. A recent matrix isolation study determined the ν1 mode of the HCSi radical to be at 1010 cm−1 [51]; this measurement is in close agreement with the gas phase value of 1013 cm−1 [52] and a theoretical investigation [53].

Despite the importance of the SiCHx species in chemical vapor deposition processes and in the chemistry of circumstellar envelopes, the infrared spectroscopic properties of the SiCH5 radicals are still elusive. This is of particular importance to identify structural isomers and open shell radicals which are thought to be essential growth species in CVD and circumstellar environments. In this paper, we present a combined experimental and theoretical study on the SiCH5 radical and elucidate for the first time the position of the most intense, hitherto elusive infrared absorption frequencies of the methylsilyl and silylmethyl isomers together with their partially deuterated counterpart in low temperature silane–methane matrices. We demonstrated earlier that an interaction of energetic electrons can cleave a carbon–hydrogen bond in a methane molecule to form atomic hydrogen and a methyl radical at 10 K, reaction (1). Two neighboring methyl radicals were found to recombine forming an ethane molecule (2) [54]. Upon further irradiation with electrons, the ethane molecule decomposes to a hydrogen atom plus an ethyl radical, Eq. (3). In a similar manner, the dilsilyl (Si2H5) radical was synthesized in low temperature silane matrices (reactions (4), (5), (6)) [55]. Here, we export this concept and attempt to synthesize two SiCH5 radical isomers via a homologous reaction sequence involving a cross recombination of a methyl with a silyl radical, followed by an atomic hydrogen loss (reactions (7), (8))CH4(X1A1)CH3(X2A2)+H(2S1/2)CH3(X2A2)+CH3(X2A2)C2H6(X1A1g)C2H6(X1A1g)C2H5(X2A)+H(2S1/2)SiH4(X1A1)SiH3(X2A1)+H(2S1/2)SiH3(X2A1)+SiH3(X2A1)Si2H6(X1A1g)Si2H6(X1A1g)Si2H5(X2A)+H(2S1/2)SiH3(X2A1)+CH3(X2A2)CH3SiH3(X1A1)CH3SiH3(X1A1)CH3SiH2/CH2SiH3(X2A)+H(2S1/2).

Section snippets

Experimental

The experiments were carried out in a contamination-free ultrahigh vacuum (UHV) setup consisting of a 15 l cylindrical stainless steel chamber of 250 mm diameter and 300 mm height; this system can be evacuated down to 8 × 10−11 Torr by a magnetically suspended turbopump backed by an oil-free scroll pump [56]. A rotatable, two stage closed cycle helium refrigerator is attached to the lid of the machine and holds a polished silver mono crystal. This crystal is cooled to 10.2 ± 0.3 K and serves as a

Theoretical approach

We have employed the hybrid density functional B3LYP method [60] with the 6-311G(d,p) basis functions in order to obtain the optimized structures and vibrational frequencies for SiCHx (x = 5, 6) systems. The relative energies are calculated by using the coupled cluster CCSD(T) method [61], [62] with the aug-cc-pVTZ basis functions [63] at the structures obtained by the B3LYP method with the correction of B3LYP zero-point vibrational energies without scaling. All calculations were carried out with

Computational results

Fig. 2 shows the optimized structures of SiCH6 and SiCH5 systems. The staggered conformation of CH3SiH3 is located at the energy minimum; the eclipsed conformation is the transition state of internal rotation around the carbon–hydrogen bond. This energy barrier of 6 kJ mol−1 is much smaller than the rotation barrier around the carbon–carbon bond in ethane. The structure CH3SiH2(X2A′) is very similar to the structure of methylsilane. Since the bond strength of Si–H is weaker than that of C–H bond,

Experimental results

Upon irradiation of the silane–methane sample, prominent absorptions of the silyl radical, SiH3(X2A1), and of the methyl radical, CH3(X2A2″), developed instantaneously at 722 and 609 cm−1, respectively (Table 4). The positions of both ν2 umbrella modes correlate nicely previous studies utilizing hydrogen, neon, argon, and krypton (721–738 cm−1; silyl) [68], [69], [70] as well as neon, argon, and nitrogen matrices (603–611 cm−1; methyl) [71]. Note that the umbrella mode holds the highest absorption

Discussion and summary

Our results suggest that the response of the silane–methane target upon electron irradiation is shaped initially by silicon–hydrogen and carbon–hydrogen bond rupture processes in the silane and methane molecules, respectively. This leads to the synthesis of the silyl radical, SiH3(X2A1), and the methyl radical, CH3(X2A2″), plus atomic hydrogen via Eqs. (1), (4). However, each hydrogen atom demands an excess energy – the lattice binding energy – of a few tens of kJ mol−1 to escape from the

Acknowledgments

The experiments were supported by the University of Hawai’i at Manoa (DS, CJB, RIK). The computations were carried out at the computer center of the Institute for Molecular Science, Japan, and supported by the Grants-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science, and Culture, Japan (Y.O.).

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