The spectrum of the cool carbon star W Orionis is dominated by a multitude of molecular bands (rectified spectrum recorded with a SA-200 grism):
I have tried to assign these bands to their corresponding transitions. The vibronic structure of the Swan bands of the C2 radical can be nicely resolved and even the multiplet structures of the rovibronic levels of the CN radical are resolved in the near infrared:
The SiC2 Merrill-Sanford absorption bands were well detectable between the Swan bands:
The SiC2 has three normal modes of vibration (v1, v2, v3) and the strongest transition can be assigned to the (0,0,0) < (0,0,0) transition between the electronic ground state and the first excited electronic level (taken from : https://academic.oup...9/1/103/1074946 )
While the notation of the combined vibrational modes is clear to me, the notation of the vibronic transitions in the upper scheme remained unclear to me (e.g. 302) (see red box). In my basic spectroscopy textbooks I couldn't find any explanation. Can somebody help me here in understanding this notation? Thanks in advance.