The – electronic transition in

The first assigned electronic gas phase spectrum of a magnesium chain species dates to 1996, when laser induced fluorescence studies reported the A2P–X2R origin band of MgCCH [1]. In subsequent investigations, the corresponding transitions due to MgC4H and MgC6H were observed [2,3]. However, the MgC4D isotope has not been studied spectroscopically so far. In this note, the line positions and a rotational analysis for the origin band of the A2P–X2R transition of this molecule are presented. Magnesium containing species such as MgCN have been identified in circumstellar envelopes of carbon rich stars through pure rotational spectroscopy [4]. Because of this astrophysical interest, magnesium capped carbon chains have been the topic of many recent spectroscopic studies, including both MgCH3 and MgCCH in the mm-wave [5,6]. In more terrestrial environments, molecular-electronics may one day replace silicon-based components in semiconductor devices [7]. The simplest electronic device, the one-dimensional wire, has the basic motif of a conjugated pbonding system. Thus promotion of an electron across the band-gap of the free molecule plays a crucial role in the behavior of the molecular device and can be studied in a straightforward manner using optical spectroscopy [8–10]. The MgC4D radicals were generated in a plasma expansion through laser ablation of a magnesium rod in the presence of 0.1% DCCD in argon. Laser induced fluorescence was used to observe the species with a 0.07 cm 1 bandwidth dye laser used for excitation and a wavemeter for frequency calibration. The fluorescence signal was collected by an f/1 lens and detected using a photomultiplier tube and a digital oscilloscope. Fig. 1 shows the X 1⁄4 2 spin orbit component for the origin band of the A2P–X2R transition in MgC4D, with the assignments