Observation of the electronic band system 23Σg−–a3Πu of C2 in the vacuum ultraviolet region

Abstract

A systematic spectroscopic study of the dicarbon molecule C2 has important applications in various research fields, such as astrochemistry and combustion. In the short vacuum ultraviolet (VUV) wavelength region, recent theoretical calculations have predicted many absorption band systems of C2, but only few of them have been verified experimentally yet. In this work, we employed a tunable VUV laser radiation source based on the two-photon resonance-enhanced four-wave mixing method and a time-of-flight mass spectrometer to investigate the absorption bands of C2 in the VUV range of 64 000–66 000 cm−1. The electronic transition 23Σg−(v′)–a3Πu(v″) of C2 has been observed and identified experimentally for the first time. The term value Te for the 23Σg− state is determined to be 66 389.9 ± 0.5 cm−1 above the ground state X1Σg+, and the vibrational and rotational constants are also determined. The experimentally measured spectroscopic parameters in this study are in excellent agreement with the theoretical results based on high-level ab initio calculations.

Spectroscopic Study of a New Electronic Band System 33Δg-a3Πu of C2

“…In the experimental conditions of DC-discharge, most produced C 2 molecules are populated at the ground X 1 Σ g + and first excited a 3 Π u states, and the c 3 Π u state can also be populated. 21,29,35 From the following hole-burning experiments, it is confirmed that a 3 Π u is the lower level of all of the transition bands observed here. Furthermore, the obtained spectra all show strong Q and R branches with relatively weaker P branches according to the tentative spectral simulations by using the PGOPHER program, 36 indicating an electronic transition of ΔΛ = +1.…”

“…T e (a 3 Π u ) and G 0 (a 3 Π u ) are 716.2 and 817.8 cm −1 , respectively, adopted from previous reports. 21,38,39 The zero-point energy G 0 (3 3 Δ g ), which is equal to 1 2 ω e − 1 4 ω e x e , relies on the determination of the vibrational constants ω e and ω e x e of the 3 3 Δ g state. To estimate the vibrational constants of ω e and ω e x e for the 3 3 Δ g state, two different fitting processes were performed, both with the v′ = 2 and v′ = 5 levels excluded due to their obvious irregularities.…”

“…To determine the lower levels of the transition bands observed here, hole-burning experiments have been carried out, which have been proved to be powerful in analyzing the experimental spectra of C 2 . ,, Three strong rovibronic peaks with good signal-to-noise ratios in the (1VUV+1′UV) photoionization spectra in Figures c and b,d are chosen for the hole-burning experiments, and the corresponding lower vibrational levels of the selected rovibronic transitions have been undoubtedly identified from the burning spectra as shown in Figure . In Figure a, the VUV laser is fixed at 74354.2 cm –1 , which corresponds to the rotational transition branch of R1 as shown in Figure c.…”

“…v 00 (3 3 Δ g ) can be independently deduced from the measured transition bands from the v ″ = 0, 1, 2, 3 levels of the a 3 Π u state to the 3 3 Δ g ( v ′ = 0) level, as listed in Table , and the average value 77549.7 cm –1 is used for v 00 (3 3 Δ g ). T e ( a 3 Π u ) and G 0 ( a 3 Π u ) are 716.2 and 817.8 cm –1 , respectively, adopted from previous reports. ,, The zero-point energy G 0 (3 3 Δ g ), which is equal to 1 2 ω e – 1 4 ω e x e , relies on the determination of the vibrational constants ω e and ω e x e of the 3 3 Δ g state. To estimate the vibrational constants of ω e and ω e x e for the 3 3 Δ g state, two different fitting processes were performed, both with the v ′ = 2 and v ′ = 5 levels excluded due to their obvious irregularities.…”

“…Up to 21 single photon electronic transition band systems have been identified so far for C 2 , covering the wavelength ranges from infrared, visible, ultraviolet to VUV, as shown in Figure . , In the past 15 years or so, Schmidt and co-workers have put great effort on studying the electronic transitions of C 2 in the visible and UV region by using the resonance-enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF) methods and have identified several new transition band systems. ,− They recently successfully measured the photodissociation of C 2 via velocity-map imaging and unraveled the mystery why the green color can only be seen in the cometary comae but not in the tails . However, in the higher energy region above the dissociation threshold of C 2 , experimental observations of electronic transition bands of C 2 were rare.…”