The infrared photodissociation spectroscopy of OCS-alkane van der Waals clusters in pulsed molecular beams

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Abstract
In order to investigate the dependence of van der Waals (vdW) cluster infrared photodissociation spectra on the size and complexity of the cluster, we carried out a series of pulsed molecular beam experiments on vdW clusters of OCS with methane (d4), ethane, butane, and hexane. A CO2 laser was used to excite the first overtone of the OCS ν2 bending mode at frequencies around 1045 cm−1, and absorption followed by dissociation was monitored by observing the attenuation of the mass spectrometer signal due to the cluster. All four clusters display Lorentzian absorption profiles with linewidths between 4.0 and 5.7 cm−1, a remarkably small range considering the variation in the number of vibrational degrees of freedom within this series. It appears that the OCS excited-state decay which is responsible for the photodissociation linewidth does not occur by coupling of the OCS vibration to vibrational degrees of freedom in the other molecule, but by coupling to rotational or librational coordinates which express the relative orientations of the cluster constituents. The absolute absorption intensities of OCS ⋅ ethane and OCS ⋅ hexane are about the same as that of isolated OCS, but those of OCS ⋅ CD4 and OCS ⋅ butane are about a factor of 2 smaller.