5142
J. Phys. Chem. 1984,88, 5142-5143
Argon Isotope Effect in the Microwave Spectra of ArHF Brian L. Cousins, Sean C. O'Brien,t and James M. Lisy* Department of Chemistry, Uiiiversity of Illinois, Urbana, Illinois 61801 (Received: July 2, 1984)
-
Rotational transitions for the less abundant isotopes of Ar in ArHF have been observed by use of a pulsed Fourier-transform Fabry-Perot microwave spectrometer. The K = 0, J = 1 2 transitions in 36ArHFand 38ArHFhave been measured in natural abundance. The use of the isotopic data to estimate the ArHF well depth and equilibrium structure will be discussed.
Introduction Rotational transitions in A r H F were first reported' over a decade ago. A structural determination of the complex was hampered by two factors. First, 19Flacks a nuclear quadrupole moment, making it impossible to obtain accurate angular information from the rotational spectrum, and second, information on the argon isotope effect was unavailable, limiting study of the radial potential. Recently the use of the HF spin-spin constant has been incorporated in a reanalysis2 of the ArHF rotational spectrum. Improvements in the sensitivity (vide infra) of the Fourier-transform Fabry-Perot microwave spectrometer3 have permitted the observation of rotational transitions in 36ArHFand 38ArHF in natural abundance (0.06% 38Ar, 0.3% 36Ar). Details of the Ar-HF interaction potential have previously been inferred from an analysis of the centrifugal distortion constant1,* using a pseudodiatomic approximation for the complex. Use of the argon isotope effect within the pseudodiatomic approximation can be made to determine the radial portion of the interaction potential independent of centrifugal distortion. Experimental Section The Fourier-transform Fabry-Perot microwave spectrometer and experimental method developed by W. H. Flygare have been described in detail el~ewhere.~-~ The improved sensitivity of the apparatus, which has enabled the measurement of complexes involving the less abundant isotopes of argon, is due to a number of modifications: the use of multiple microwave pulses per gas pulse; an adjustable nozzle position relative to the microwave cavity; improved coupling of the microwave radiation into and out of the Fabry-Perot cavity; leaner gas mixtures (0.1-0.5% H F in Ar) and lower backing pressures (0.25-0.5 atm) to prevent formation of larger clusters; shorter gas pulses, approximately 3-4 ms in duration; higher repetition rates, typically 5 gas pulses/s, to more fully utilize the pumping capacity of the 23-in.-diameter diffusion pump/Stokes Model 212-1 1 roughing pump system. Details of these improvements will be presented elsewheree6 Although the individual contributions of each modification have not been determined, the cumulative effect has increased the sensitivity by at least a factor of 20. The A r H F complexes were prepared by pulsing a room-temperature mixture of 0.5% HF in Ar at 5 pulses/s through a commercial pulsed valve7 fitted with a 0.95-mm aperture. Each gas pulse was polarized with four 0.5-ps microwave pulses separated by 500 ps. The emission from each microwave pulse was digitized a t 0.5 ps per point and averaged. The time domain spectrum was then Fourier transformed to give a 128-point power spectrum having 3.906 kHz per point resolution. Each transition was measured at four slightly different master oscillator frequency settings, and the resulting values were averaged. The uncertainty in our frequency measurements was estimated to be less than 5 kHz. For 36ArHFtypically 300 gas pulses were averaged, while for 38ArHF 1500-2000 gas pulses were needed to fill the signal averager. This roughly correlates with the 5:l isotopic ratio of 36Arto 38Ar. A typical time domain spectrum of 36ArHFis shown in Figure 1. Current address: Department of Chemistry, Rice University, Houston,
TX 77001.
0022-3654/84/2088-5142$01.50/0
TABLE I: Rotational Transition Frequencies in ArHF calcd. difference. obsd, species MHz MHz kHz 40ArHF J = 0 J = 1 6131.1362' 6131.1362 0.0 J=1
38ArHF J = 1 "ArHF J = 1
-
---
J = 2 12260.5707" 12260.5704 J = 2 12470.8353 12470.8359 J = 2 12703.7663 12703.7660
0.3 -0.6 0.3
"These values are based on the fitted spectroscopic constants of ref 2. The J = 0 1 transition does not include the HF spin-spin splittings (-30 kHz) as this splitting is unresolvable (
