✍ Scribed by Yunjie Xu; A.R.W. McKellar
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In the only previous study of the CO -OCS van der Waals complex, is easily accomplished with SPFIT ) . The fit also included the 22 MWFT pure rotational transitions of ( 1 ) , which were given greatly increased Brookes et al. ( 1 ) reported microwave Fourier transform ( MWFT ) pure rotational spectra in the 6 -18 GHz region and infrared diode laser spec-weights corresponding to the squares of the ratios of their estimated precisions ( 0.4 to 1.0 kHz ) to that of the infrared data ( 30 MHz ) . These tra in the 2062 cm 01 region of the O -C stretching vibration of OCS. The infrared results were obtained with a pulsed pinhole-shaped super-rotational transitions involved J-values ranging from 0 to 6 and K avalues of 0, 1, and 2. sonic jet, which gave an effective rotational temperature of about 1 K. In this note, we report a reinvestigation of this same infrared band using
The molecular parameters resulting from the fit are listed in Table 2, and the residuals (observed minus calculated wavenumbers) for the present data a continuous supersonic expansion through a slit-shaped jet. The higher effective temperature ( É10 K ) of the slit jet enabled us to observe a are given in Table 1. The quality of the fit was very good, with an rms deviation of only 0.00031 cm 01 for the infrared data. The ground state wider range of transitions, involving higher levels of J ( 13 as compared parameters were almost completely determined by the very precise MWFT to 10 ) and especially of K a ( 9 as compared to 4 ) . The result is a more data (1), so it is not surprising that they are almost identical to those complete and precise set of molecular parameters which describe this determined by Brookes et al.
(1). However, for the excited state, our more band of CO -OCS.
extensive infrared data allowed a larger and more accurate set of parameters The results of Brookes et al.
( 1 ) showed that CO -OCS has a reasonto be determined. Our band origin agrees almost exactly with that from ably rigid planar T-shaped structure, with the OCS forming the head of (1), and the rotational constants are also in good agreement. Our diagonal the T and the CO forming its leg. The CO carbon atom is closest to the quartic distortion parameters, D, are also in reasonable agreement with OCS, and the sulfur points away from the CO, with an average OCSthose of (1), especially considering that they are affected by our inclusion CO angle of about 106Њ and an average intermolecular distance of 4.17 of higher-order parameters. A ˚. This means that the inertial a-axis is roughly parallel to the CO and Perhaps the most noteworthy aspect of Table 2 is that certain centrifuthe b-axis roughly parallel to the OCS. The transitions of the complex gal distortion parameters ( especially D J , H J , and h 1 ) exhibit very large accompanying the OCS O -C stretch were observed to form a perpendicchanges between the ground and excited states. How can we explain ular ( b-type ) band, with DK a Å {1, in good agreement with this model. such substantial changes? One possibility is that the parameters are not This structure is similar to those of the analogous complexes CO -CO 2 as well determined as they seem, so that the changes are not entirely ( 2, 3 ) and CO -N 2 O (4) .
real. This could conceivably occur because the ground state parameters The present apparatus was the same as that used in our recent studies are essentially determined by the very precise, but limited ( K a £ 2 ) of the complexes Ar -CO ( 5 ) , N 2 -CO ( 6 ) , CO -CO 2 ( 7 ) , and CO -MWFT data, while the excited state parameters are determined by the N 2 O (4) . It consisted of a 2.5 cm 1 50 mm slit nozzle in a chamber extensive, but less precise, infrared data. However, we believe that the evacuated by a large Edwards EH4200 Roots pump, and a liquid-nitrochanges are real, and that they are a reflection of the weakly bound and gen-cooled diode laser infrared source. In the present case, the gas somewhat floppy nature of CO -OCS, together with the fact that the mixture was approximately 75% He, 25% CO, and a trace amount of excited state, which is the carbonyl sulfide O -C stretching vibration at OCS, with source and chamber pressures of about 1000 and 0.3 Torr, 2063 cm 01 , can be perturbed by other nearby vibrational states of the respectively. Wavelength calibration was made using OCS ( 8 ) itself complex. The most likely perturbing states are the carbon monoxide in a room temperature cell as a reference gas.
C -O stretching vibration, located at 2143.3 cm 01 in the isolated CO Our 170 observed infrared lines of 12 C 16 O -16 O 12 C 32 S are listed together with their assignments in Table 1. Some of these lines are blends monomer, and the OCS bending overtone vibration, ( v 1 , v l 2 , v 3 ) Å ( 0, 4 0 , 0 ) , located at 2104.8 cm 01 in free OCS. of more than one transition, as indicated by asterisks, so there are a total of 184 assignments in Table 1. The experimental accuracy of these
In conclusion, we have reexamined the perpendicular carbonyl sulfide O -C stretching band of the CO -OCS van der Waals complex in the measurements should be better than 0.001 cm 01 . In the previous study of this band ( 1 ) , there were 83 observed lines and 98 assignments.
2063 cm 01 region, using a continuous slit-jet supersonic expansion and a tunable infrared diode laser spectrometer. In doing so, we have ex-The most important aspect of the present result is that our data extend to much higher values of K a , namely to the K a Å 9 R 8 subband as tended the coverage of K a -levels from 4 to 9 and have determined a refined and extended set of excited state molecular parameters, includ-compared to the K a Å 4 R 3 subband in ( 1 ) .
We analyzed the observed data of Table 1 with an s-reduced asymmet-ing most of the sextic centrifugal distortion parameters. Some of the distortion parameters show large changes between the ground and ex-ric rotor Hamiltonian ( 9 ) so that the parameters would be directly comparable with those obtained in Ref.
( 1 ) . The fit was accomplished using cited states, which we tentatively ascribe to perturbations of the excited state by other vibrations. While performing this study, we also searched the computer program SPFIT of Pickett ( 10 ) . Unresolved asymmetry doublets, for which the columns K c are blank in Table 1, were fitted to for the carbon monoxide C -O stretching band of CO -OCS in the 2150 cm 01 region where the analogous bands of CO -CO 2 and CO -N 2 O are the mean of the two calculated components, and the other blended lines, indicated by asterisks, were also fitted to intensity-weighted means ( this observed ( 4, 7 ) . We could only observe a few weak lines in the interval 202
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The weakly bound van der Waals complex CO-N 2 O has been studied for the first time. Absorption spectra in the CO stretching region near 2150 cm 01 were recorded using a tunable diode laser spectrometer to probe the continuous supersonic jet from a slit-shaped nozzle. A total of 186 lines were measu