New electronic transitions of the C2 molecule in absorption in the vacuum ultraviolet region

Abstract

Three new electronic transitions of the C2 molecule have been observed in absorption in the region 1300–1450 Å. The system of shortest wavelength is readily identified as a 1Πu–1Σg+ transition; the lower state is the ground state X1Σg+ of the molecule. The other two systems arise by absorption from the low-lying a3Πu state; the upper states are new 3Σg− and 3Δg states. Rotational and vibrational constants of the three new states have been determined. The new states are Rydberg states. Their correlation to the separated atoms is briefly discussed.

ULTRAVIOLET MEASUREMENTS OF INTERSTELLAR C2

“…Its potential energy curve is affected by avoided crossings at about 2.8 Bohr with the curves associated with 3 1 Π u and 4 1 Π u , yielding 3 bound vibrational levels for the F state. We note that Herzberg et al (1969) observed the lower two levels, which are also the only ones associated with interstellar spectra. The depth of the potential well is about 0.5 eV.…”

“…The only laboratory-based spectroscopic study is that of Herzberg et al (1969), whose molecular constants are used for analysis of interstellar absorption. The derivation of these constants relied on lines with J ′′ ≥ 10 for the most part.…”

“…The derivation of these constants relied on lines with J ′′ ≥ 10 for the most part. For the (0,0) band, laboratory data do not exist for lower values of J ′′ , while data only on the Q branch of the (1,0) band are presented by Herzberg et al (1969) for these rotational levels. Therefore, the only quantitative information for the perturbations involving the F − X levels at the present time is based on astronomical spectra -broadened lines and line shifts of 8 to 15 km s −1 .…”

“…Tunneling through the potential barrier created by avoided crossings among 1 Π u states and spin-orbit mixing between F 1 Π u and other close-lying triplet states of ungerade symmetry could also lead to the perturbations seen in interstellar spectra (P. Bruna, private communication), but are less likely. Tunneling is expected to affect the v = 2 level the most because it lies near the top of the barrier; such an effect may explain why lines from this level are not seen in laboratory (Herzberg et al 1969) or interstellar spectra. The spinorbit mixing considered here involves repulsive states that can predissociate the F state.…”

Line strengths and updated molecular constants for the C2 Swan system

“…The most prominent electronic system in the visible region is the Swan system, which involves the electronic transition d 3 Π g -a 3 Π u , with the (0,0) band near 19400 cm −1 . The a 3 Π u state was originally believed to be the ground state [26] as it was observed to be easily excited, which is because it lies only 1536 cm −1 above the actual X 1 Σ + g ground state. The Swan system has been investigated extensively.…”