Vacuum ultraviolet laser spectroscopy of CO molecules in a supersonic jet

[摘要] ENGLISH ABSTRACT:A tunable narrow-bandwidth laser source combined with a supersonic gas jet as sample is wellsuitedfor obtaining high-resolution spectra of cold isolated molecules and complexes. In thepresent study such a laser source in the vacuum ultraviolet was applied to the spectroscopicinvestigation of rare carbon monoxide (CO) isotopomers and CO-noble gas van der Waalscomplexes in supersonic gas pulses seeded with natural CO gas.Tunable coherent vacuum ultraviolet radiation was generated by two-photon resonant fourwavesum-frequency mixing of two pulsed dye laser beams in a magnesium vapour medium.Laser induced fluorescence excitation spectra of the A(v'=3)-X(v=0) vibronic band of COmolecules in a noble gas (neon or argon) jet were obtained by measuring the total undispersedfluorescence from the irradiated sample volume in dependence of the excitation wavelength. Thedynamics of the flow-cooling process in the supersonic jet were investigated and the experimentalparameters optimised using the rotational temperature of 12C160as determined from rotationalline intensities. Rotational temperatures as low as 2 ± I K were observed.Spectroscopic detection of the rare 12C170and 12C180isotopomers was facilitated by the lowrotational temperature and high spectral resolution. Six rotational lines of 12C170and four of12C180were detected in the A(v'=3)-X(v=0) vibronic band. This demonstrates the low detectionlimit (circa 3 parts per million) obtained in the experiment. The line wavelengths weredetermined to an accuracy of 0.2 pm using the well-documented 12C160and 13C160lines forcalibration. The spectral results on 12C170are, to our knowledge, the first rotationally resolvedlaboratory measurements published on the A-X band of this isotopomer. Accurate wavelengthdata of the stable isotopomers of CO is of importance in the interpretation of astrophysicalobservations of CO in the interstellar medium. The newly determined 12C170wavelengths weresuccessfully applied to a recent problem in astrophysics (Astrophys. J. Lett. 2003).The conditions in a supersonic jet facilitate the study of weakly bound van der Waalscomplexes, of which CO-noble gas complexes are prototypes. However, there is no experimentaldata available on the electronic excitation spectra of the CO-noble gas complexes, lying in thevacuum ultraviolet region. In the present experiment evidence of extensive complexation of the CO in the noble gas jet has been found, but in the spectral region around the A(v'=3)-X(v=O)band of CO no distinct spectral features that could be associated with these complexes wereobserved. Having considered the existing knowledge on CO and CO-noble gas complexes andexperimental studies on the excitation and dissociation dynamics of Iz-noble gas complexes, weregard complex induced inter-system crossing or electronic predissociation as the most likelycauses for these observations.The results on the rare CO isotopomers demonstrate the potential of our experimental setupfor high-resolution, isotope and state selective spectroscopy in the vacuum ultraviolet with a highsensitivity for fluorescent species. The availability of the now well-characterised experimentalsetup in our laboratory opens the way for further investigation of molecular or complex specieswith spectroscopic features in the vacuum ultraviolet region.

[发布机构] Stellenbosch University