VELOCITY DISTRIBUTIONS IN K AND TI BEAMS 1317 + 0.5 percent accuracy of the temperature measure- ment and has no observable effect on the results The oven temperatures were measured with Chromel P-Alumel thermocouples peened directly into the oven O RUN 57 as shown in F all available information and auxiliary checks against a Pt, Pt-Rh thermocouple indicate the temperature measurement to be accurate absolute. The beam intensity is very sensitive to the i temperature of the oven, so a temperature control unit, 2 essentially an on-off switch to control a portion of the oven heater current, was employed to maintain the oven within 0. 25.C of the nominal desired temperature The beam was detected on a tungsten surface ionization detector. In the case of thallium, oxygen as sprayed on the detector wire to increase the detec- ion efficiency. The filament was conditioned so that the detected beam was relatively insensitive to the filament temperature in the neighborhood of the operating temperature. The detection efficiency fc potassium can be made greater than 99 perce while for thallium this may not be true. Preliminary of the velocity distribution as long as the over-all detec- with tres in the ovens are given in itx detection efficiency has no observable effect on the shape press nd 60 with thin tion efficiency remains constant during the run. This shows that if there is a velocity-dependent detection in which the dimensionless variable V, the reduced efficiency, it is not very sensitive to the over-all detec- velocity, is the atom velocity vo, divided by the velocity tion efficiency of the intensity maximum of the distribution. This is RESULTS a very convenient expression for examining the agree- Differentiation of Eq. (5)shows that a?v, or., ment between theoretical and experimental curves since equals two at the intensity maximum, so that Eq. (5) will be the same for all velocity distributions which esult from a single molecular species. To take into I,=8yV4 exp(-2v) (8) account the effect of finite rotor slot width, the actual theoretical distribution used in this work was a < uni- obtained from Eq.(6) with ao replace by Vv2 Figure 4 shows universal plots of typical velocity distributions for potassium; each distribution corre- COPPER sponds to different experimental conditions. To compute he reduced velocity V, the experimental velocities were divided by the theoretical velocity of the intensity maximum calculated from the measured oven tempera ture. Since the uncertainty in temperature measurement HOLes does not exceed +0.5 percent, the velocity correspo dis g to the tribution will be accurate to =0.25 percent. The ob served intensity measurements have been multiplied, in ll cases, by an appropriate factor to give coincidence maximum TAPERED GROOVES Some of the important results are tabulated in Table FOR SUPPORTING PINS II. The velocity at which the maximum intensity occurs oven used for potassium showing can be determined directly from the experimental velocity distribution and may also be calculated from (1948. Cogin and G. E.Kimball, J. Chem. Phys. 16, 1035 the oven temperature. The agreement of the two velocities is one criterion which must be satisfied if the