Article pubs.acs.org/JPCA
Vibrationally Hot Bands of the SiCN Ã 2Δ − X̃ 2Π System Masaru Fukushima* and Takashi Ishiwata Faculty of Information Sciences, Hiroshima City University, Hiroshima 731-3194, Japan S Supporting Information *
ABSTRACT: We have generated SiCN in supersonic free jet expansions and observed the laser induced fluorescence (LIF) spectrum. In addition to the vibronic bands from the vibrationless level of the X̃ 2Π state, the hot bands from the bending vibrational level, à (0110) 2Φ − X̃ (0110) 2Δ and à (0110) 2Π − X̃ (0110) 2Σ(−), have been measured. The rotational energy levels were reasonably analyzed as those of the 2K′ − 2 K″ transitions, but their line intensities calculated from the Hönl−London factors derived in the intermediate case between Hund’s case (a) and (b) could not reproduce the observed spectra. The Hönl−London factors derived in the 2Λ′ − 2Λ″ (2Δ − 2 Π) transition reasonably reproduced the spectra. It indicates that coupling between the electronic orbital and vibrational angular momenta is weak in the SiCN 2Δ − 2Π system, and a basis set of |Λ v2 l Σ;J P MJ⟩, the so-called “l-basis”, better describes the system than that of |Λ v2 K Σ;J P MJ⟩.
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INTRODUCTION Because of the considerable cosmic abundance of silicon, much attention has been devoted to silicon containing species in the astrochemical and astrophysical fields. The target molecule in this study, SiCN, was actually identified in the circumstellar envelope of IRC+10216,1 based on the laboratory observation.2,3 The gas phase observation of SiCN is restricted to the pure rotational transitions, but in the condensed phase, the species was detected in an infrared spectrum as a photochemical reaction intermediate.4 The isomerization reaction, SiCN ⇌ SiNC, in the X̃ 2Π state was computationally studied.5,6 The electronically excited states of these isomers were also examined by ab initio calculations.7 We have studied the à 2 Δ − X̃ 2Π transition of SiCN under jet cooled conditions and analyzed three vibrationally cold bands;8 the vibronic bands from the vibrationless level of the X̃ 2Π state are called cold bands.9 We report here the rotational analysis of the vibrationally hot bands, and discuss the transition strengths of the molecules affected by the Renner−Teller interaction.
of the pulsed laser system, and the resolution was then improved to be ∼0.03 cm−1. The absorption spectrum of I2 recorded by the red fundamental light source, which was simultaneously measured with the SiCN spectrum, was used to establish the absolute frequency. For the measurement of the dispersed fluorescence (DF) spectrum, a monochromator with 50 cm focal length was used. The resolution of the DF spectrum was about 10 cm−1, and the accuracy in determining the peak positions would be better than 3 cm−1. The spectral settings of the monochromator were established using the Hg atomic lines. To measure the scattered light of the excitation laser in the monochromator, spectra were recorded after the DF spectrum measurements without the ablation laser. The LIF signals were calibrated against the power of the UV source by monitoring it on each shot.
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RESULTS Figure 1 shows a survey spectrum of the à 2Δ − X̃ 2Π transition of SiCN. Three vibronic band systems observed in the spectrum have been analyzed and will be reported in a separate paper.8 One of them consists of the 29 195 and 29 327 cm−1 sub-bands. They have been assigned as the 000 band, à (0000) 2Δ − X̃ (0000) 2Π, with the spin−orbit splitting of about 132 cm−1.8 The rest designated by α and β in Figure 1 lie at 29 802 and 29 934 cm−1 and 29 864 and 29 995 cm−1, respectively. They have been assigned to the transitions from the vibrationless level of the X̃ 2Π state to the vibronic levels affected by the Fermi resonance between the (0200) 2Δ and (0001) 2Δ levels in the upper à 2Δ state.8 There are weak
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EXPERIMENTAL SECTION Applying the laser ablation technique, the SiCN radicals were generated in supersonic free jet expansions of Ar. The metallic surface of a silicon rod was ablated using the second harmonics of a Nd3+ YAG laser, and the Si in the radical would be supplied from the Si ablated. The decomposition products from acetonitrile in the plasma generated by the ablation would be the source for CN. The second harmonics of a pulsed dye laser were used to detect SiCN. The energy resolution of the UV light source was ∼0.3 cm−1. The absolute frequency was calibrated from opto-galvanic signals of Ne for the red fundamental outputs, whose signals were simultaneously recorded with the SiCN spectrum. To measure the rotationally resolved spectrum, an étalon was installed in the resonant cavity © 2013 American Chemical Society
Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: November 27, 2012 Revised: March 4, 2013 Published: March 5, 2013 9435
dx.doi.org/10.1021/jp311684e | J. Phys. Chem. A 2013, 117, 9435−9443
The Journal of Physical Chemistry A
Article
Figure 1. The LIF excitation spectrum of SiCN observed in the supersonic free jet expansions of Ar. The vibrationally hot bands focused on in this study are marked by arrows. They lie at 29 302, 29 421, and 29 451 cm−1. Atomic lines of Si are identified in the same spectral region, which are pointed out by dots (•). There are three vibrationally cold bands, 000, α, and β, of the à 2Δ − X̃ 2Π transition of SiCN.
vibronic bands around the 000 band, which are marked by arrows in Figure 1. From the band origin of the α band, the transition, Ã (0110) 2Δ − X̃ (0000) 2Π, is estimated to lie ∼270 cm−1 above the 000 band, if it occurs. The ab initio values of ω2(π−) ≈ 210 cm−1 5 and 206 cm−1 6 also hold this estimation. Since the weak bands lie at
