A New Band System of the Dicarbon Molecule in the Vacuum Ultraviolet Region

Abstract

As one of the most abundant molecules in the universe, the long history of spectroscopic studies of the dicarbon molecule, C 2 , reaches back two centuries. While many electronic band systems with upper states below the lowest dissociation threshold have been well characterized, much less is known about transitions to higher-lying states. Here, we report the observation of a new band system of C 2 from the lowest triplet state a 3 Π u through a resonance-enhanced multiphoton ionization scheme. The upper state is identified as 1 3 Σ g + , which is determined to be 61539.0 cm −1 (7.630 eV) above ground state X 1 Σ g + . The spectroscopic parameters determined for the 1 3 Σ g + state are in excellent agreement with those predicted by the high-level ab initio calculations. This study paves the way for systematic investigations of the photoabsorption and photodissociation of C 2 in the vacuum ultraviolet region, which has important applications in the field of astrochemistry.

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.…”

“…The experimental setup, which includes a time-of-flight (TOF) mass spectrometer and a tunable VUV laser source, has been described in detail elsewhere. ,− The C 2 molecules are produced in a supersonic expansion of C 2 H 2 (0.5%) seeded in argon gas (4 bar) followed by a DC discharge (800–1000 V, duration of 200 μs) device. A filament is put in front of the discharge device to stabilize the discharge process.…”

“…In this work, we report a new electronic transition band system of C 2 in the photon energy region from 72000 to 81000 cm −1 , which is assigned as 3 3 Δ g -a 3 Π u , using the same method as before. 21,28,29 Irregular vibrational intervals are observed, indicating perturbations from nearby electronic states.…”

“…31 For the 1 3 Σ g + and 2 3 Σ g − states, their calculated transition energies, bond lengths, vibrational and rotational constants were in excellent agreement with the experimental measurements. 21,29,31 However, the F 1 Π u state at the v′ = 0 level was calculated to have much longer predissociation lifetime than that deduced from astronomical observations; 5,28,30 thus, further experimental and theoretical studies are still needed. As shown in Figure 1, only five electronic states above ∼60000 cm −1 have been experimentally identified so far, compared with the complicated electronic structures predicted by theory.…”

“…28 Furthermore, two new strong absorption electronic band systems of C 2 were identified for the first time, which were assigned as transitions from the a 3 Π u state to the 1 3 Σ g + and 2 3 Σ g − states, respectively. 21,29 On the theoretical side, several high-level ab initio calculations have been performed for unraveling the electronic structures of C 2 in the high-lying energy region. In 2001, Bruna and Grein reported the first systematic investigation on the potential energy curves (PECs) of C 2 in the energy region of 7−10 eV.…”