Laser Induced Spectra of SiN Molecule in Near IR Region

Abstract

Laser-induced spectra of SiN molecule are recorded in the region of 670–1060 nm using laser-induced spectroscopy technique and about 80 bands are observed. Out of total 80 bands, 49 bands are attributed to F-B system and the rest 27 bands are analyzed into J-D system. The rest 18 bands are unidentified. The molecular constant of the B²Σ⁻, D²Σ⁻, F²Π and J²Π states is determined and reported.

1. Introduction

Thirty-five red degraded emission bands of SiN molecule, lying in the region of 380–495 nm, were first observed by Jevons [1]. Mulliken [2] reinvestigated the spectra in 380–530 nm region and observed 53 red degraded bands including the bands reported by Jevons [1] and attributed these bands to B²Σ⁺-X²Σ⁺ system of SiN molecule. In addition to B²Σ⁺-X²Σ⁺ transition, a new transition C-A²Π was also observed by Mulliken [2] in near Uv-green region. Jenkins and De Laszlo [3], on the basis of rotational analysis of B²Σ⁺-X²Σ⁺ transition, concluded that the 𝑣=4 level of B²Σ⁺ is perturbed. Schoffield and Broida [4] investigated the chemiluminescence spectra of SiN molecule and observed two new electronic transitions in 518–602 and 554–725 nm regions along with B²Σ⁺-X²Σ⁺, and C-A²Π systems. Bredohl et al. [5] reinvestigated the spectra in 310–560 nm region and observed three new electronic transitions D²Π-A²Π, L²Π-A²Π, and K²Π-A²Π along with B²Σ⁺-X²Σ⁺ system.

Foster [6], on the basis of rotational study of four bands, renamed K²Π-A²Π system reported by Bredohl et al. [5] as B²Σ⁺-A²Π_𝑖 system of SiN molecule and calculated 𝑇_𝑒=994.4 cm⁻¹ for A²Π_𝑖 state. Foster et al. [7] rotationally analyzed (2,0) band of the A²Π_𝑖-X²Σ⁺ system along with B²Σ⁺-X²Σ⁺, D²Π-A²Π, and L²Π-A²Π systems and improved the molecular constants of A²Π_𝑖 and X²Σ⁺ states. Cai et al. [8] calculated rotational and vibrational constants for X²Σ⁺and A²Π states of the SiN molecule using complete active space SCF (CASSCF), multireference CI (MRCI), and coupled cluster (CCSD(T)) levels. The potential energy curves of the SiN molecule for the low-lying electronic states were also computed by Cai et al. [9] using internally contracted multireference configuration interaction (CMRCI) level.

Laser-induced spectroscopy technique is being widely used in the field of molecular spectroscopy to provide laboratory data of molecules. Fu et al. [10], Bondybey [11], Hopkins et al. [12], and Ojha and Gopal [13, 14] have observed a number of new band systems of different molecules using this technique. Therefore, it was decided to investigate the emission spectra of SiN molecule in near IR region. In the present work, the emission spectra of SiN molecule were investigated in 670–1060 nm region to provide the laboratory data.

2. Experimental Technique

The experimental setup comprised of a Pulsed Nd: YAG laser (Spectra Physics, USA, repetition rate 10 Hz), a laser ablation chamber (10^′′ diameter) mounted on the throat of rotary and diffusion pumps, and a computer-controlled Spex TRIAX 320 M monochromator (Jobin Yovn, USA) fitted with TE cooled ICCD detector (Jobin Yovn, USA) system having spectral resolution of 0.06 nm for 1200 grs/mm grating. The silicon nitride rod (purity 99.999%, Goodfellow, UK) was clamped inside the chamber. The ablation chamber was evacuated up to 10⁻³ torr using rotary pump. The chamber was flashed two-three times with high purity Argon gas (purity 99.99%) to remove the contaminants. The chamber was then filled with the same argon gas up to a pressure of 0.1 mbar. The silicon nitride plasma was produced by focusing 355 nm of Pulsed Nd: YAG laser (70 mJ) on continuously rotating and translating silicon nitride rod with the help of a convex lens of focal length 50 cm. The produced plasma was allowed to cool adiabatically from 80 to 500 ns delay time and the cooled plasma was focused on the entrance slit of computer-controlled monochromator using a cylindrical lens of focal length 25 cm. The TE cooled ICCD detector with gating speed of 5 ns was synchronized with Nd: YAG laser in Q-switch mode. For spectroscopic investigation, signals were sent to computer and data acquisition was made using Spectra Max software. The Grams 32 software was used for peak picking and peak marking. The spectra of SiN molecule were observed at 120 ns delay time using 600 groves/mm grating blazed at 750 nm in this region.

3. Analysis and Results

The spectra of SiN molecule are investigated in the region of 670–1060 nm using laser-induced spectroscopy technique. Total of 80 bands are observed in this region. Out of total 80 bands, 62 bands are analyzed into two new doublet-doublet electronic transition F²Π-B²Σ⁺ and J²Π-D²Σ⁻ reported by Cai et al. [9] of SiN molecule. The rest 18 bands are unanalyzed. The details of the analyses of these systems are given below.

3.1. New F²Π-B²Σ⁺ System

The F²Π-B²Σ⁺ system lies in the region of 680–1060 nm. Total 49 red degraded and single headed bands of this system are identified with (0,0) at 974.60 nm. The observed 49 bands are assigned to Δ𝑣=0, 1, 2, 3, 4, 5, 6, 7, and 8 sequences. The bands of Δ𝑣=−1 sequence are not observed due to the ICCD response. The bands of Δ𝑣=0 sequence are observed with very low to signal noise ratio, while the bands of the rest of sequences are well developed and observed with good intensity. The value of 𝑇_𝑒 calculated in this experiment for F²Π state is found to be 34908.76 cm⁻¹ which is close to that theoretically calculated by Cai et al. [9]. The experimentally calculated molecular constant of F²Π and B²Σ⁻ states is very close to that reported by Cai et al. [9] and presented in Table 3. Figures 1, 2, 3, and 4 represents the spectra of F²Π-B²Σ⁺ system and the bandhead data are given in Table 1.

Table 1 

Bandhead data of F²Π-B²Σ⁺ system.

Figure 1 

Figure 2 

Figure 3 

Figure 4 

3.2. New J²Π-D²Σ⁻ System

The system lies in the region of 715–1045 nm with (0, 0) at 897.42 nm. The total 27 violet degraded and single headed bands of the J²Π-D²Σ⁻ system are identified and attributed to Δ𝑣=0, ±1, ±2 and ±3 sequences. All the bands of the system are well developed and observed with good intensity. The value of  𝑇_𝑒 calculated experimentally is found to 44871.20 cm⁻¹ which is very close to 44861.00 reported by Cai et al. [9]. The experimentally calculated values of molecular constants for J²Π and D²Σ⁻ states is very close to those reported by Cai et al. [9], and presented in Table 3. The spectra of the system are shown in Figures 1, 2, 3, and 4. The bandhead data of this system are given in Table 2.

Table 2 

Bandhead data of J²Π-D²Σ⁻ system.

Table 3 

Molecular constants of B²Σ⁺, D²Σ⁻, F²Π, and J²Π states of SiN molecule.

4. Discussion

The molecular constants calculated experimentally for B²Σ⁻, D²Σ⁻, F²Π, and J²Π states are very close to those reported by Cai et al. [9]. The close matching of molecular constants of this state along with the 𝑇_𝑒 values of F²Π and J²Π sates confirmed that the spectra are due to SiN molecule.