Laser spectroscopy of the Fourth Positive System of carbon monoxide isotopomers
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Carbon monoxide (CO) is a diatomic molecule of particular interest in astrophysics, due to its high abundance in interstellar space. The Fourth Positive System A1Π−X1Σ+ of CO is an important feature in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum in astronomical observations, especially in high-resolution spectra recorded by satellite-based spectrographs. The interpretation of these astronomically detected spectra requires accurate laboratory wavelengths to serve as rest wavelengths and to resolve possible Doppler-shifts. Such rest wavelengths are known for the 12C16O, 13C16O and 12C18O isotopomers for all astronomically observed spectral lines of the Fourth Positive System. The only laboratory wavelengths currently available for the Fourth Positive System of the 12C17O isotopomer have been determined in a previous work carried out in our laboratory for the vibronic band A1Π(v0 = 3)−X1Σ+(v00 = 0). The present study continues this work for the other vibronic bands which have been detected astronomically, namely A1Π(v0 = 2 − 5)−X1Σ+(v00 = 0). The A1Π(v0 = 0− 1)−X1Σ+(v00 = 0) vibronic bands have also been investigated due to their probability for future astronomical detection. Rotationally-resolved spectra of these six vibronic bands were obtained by selective rovibronic laser excitation, and subsequent detection of the undispersed fluorescence, observed as a function of the excitation wavelength in the VUV. A tunable narrow-bandwidth VUV laser source is used for excitation, and the CO gas sample is introduced by supersonic expansion. Flow-cooling in the supersonic expansion to rotational temperatures roughly corresponding to temperatures in the interstellar medium simplifies and aids the spectral analysis of the spectral lines of interest. The cold conditions in the supersonic expansion facilitates a high sensitivity for detection of the low-J lines, and allows the detection of rare isotopomers of CO in natural abundance. The experimental setup has been improved in the present study by the addition of a vacuum monochromator, facilitating an improved characterisation of the VUV source. Furthermore, a number of experimental conditions have been optimised for the detection of rare CO isotopomers, significantly increasing the signals of these lines in the spectra. The combination of this increase in sensitivity and the addition of the vacuum monochromator to the experimental setup, allowed the simultaneous detection of absorption spectra with the fluorescence spectra as an additional source of information in spectral analysis. The increased sensitivity also contributed to the detection of a large number of spectral lines of interest, with some additional lines identified in the previously studied vibronic band. Spectral lines of 12C16O, 13C16O, 12C18O and 12C17O were detected in each vibronic band, allowing accurate calibration of the spectra. A total of 29 new lines of 12C17O were recorded in the six vibronic bands investigated. Additionally, 10 new singlet-triplet lines of 12C16O were recorded in the wavelength regions investigated. The new wavelengths of 12C17O have been applied to calculate consistent heliocentric velocities of a gas cloud toward the star X Persei, obtained from spectra of the different CO isotopomers taken by the Hubble space telescope.