In this Letter, we report the near infrared spectrum of C2 generated in AC hollow-cathode discharges of ethylene/helium mixtures using high resolution laser absorption spectroscopy with concentration modulation. The high sensitivity achieved by this zero-background technique allows the first detection of the very weak Δv = −4 sequence of the d3Πg←a3Πud3Πg←a3Πu system (Swan system). The detailed analysis of the spectrum between 13 750 and 14 130 cm−1 revealed that the (4, 8) band was heavily perturbed. On the other hand, the (5, 9) band was unperturbed to yield accurate molecular constants from least-squares fitting of the observed transitions.Graphical abstractHighlights► High sensitivity zero-background absorption spectroscopy. ► First observation of Δv = −4 sequence of Swan system of C2. ► Near infrared spectrum of C2 at Doppler-limited resolution.

High-resolution laser absorption spectroscopy using concentration modulation has been applied to record the near-infrared spectrum of C2 in an AC hollow-cathode discharge of acetylene/helium mixtures. The (2,1) and (1,0) vibronic bands of the d3Πg– c3Σu+ system have been observed in the spectral region between 12 010 and 12 540 cm–1. While the analysis of the (1,0) band was straightforward, the (2,1) band was found to be perturbed. Using the effective Hamiltonians for 3Π and 3Σ+ states, molecular constants for the vibrational levels of the c3Σu+ state were retrieved in least-squares fits of the observed spectral lines. The experimental conditions, detailed analysis of the perturbations, and the determined molecular constants are reported.

The electronic transition 1Σ+–X1Σ+ of scandium monophosphide (ScP) has been observed around 11 900 cm−1 using laser induced fluorescence spectroscopy. This is the first observation of the ScP spectrum. The spectrum covering ten vibronic bands with v up to 4 was assigned and analyzed. Molecular constants for the vibronic levels involved were obtained by least-squares fitting of the measured line positions. The equilibrium bond length in the upper 1Σ+ state was found to be 2.1590 Å, which was slightly shorter than that of 2.1995 Å in the X1Σ+ state, in agreement with results from recent calculations.Graphical abstractHighlights► First observation of spectrum of ScP. ► First determination of equilibrium bond lengths of ScP. ► Experimental determination of accurate structural parameters for ScP.

The fine structure of the rovibrational W1(0) transition (v = 1 ← 0, J = 6 ← 0) of solid parahydrogen has been observed using high-resolution near infrared diode laser spectroscopy. Similar to the case of the pure rotational W0(0) transition, the W1(0) transition also exhibits triplet structure which is interpreted as the crystal-field split M components of the J = 6 manifold. The observed splitting is consistent with the theoretical prediction based on the localized J = 6 rotons. The crystal-field splitting parameters obtained by least-squares fitting are in good agreement with the previous work on W0(0) transition.The crystal-field split M components of the W1(0) transition of parahydrogen observed using high-resolution spectroscopy.