VOLUME 38, NUMBER 1 PHYSICAL REVIEW LETTERS 3 JANUARY 1977 (1976); R. V. Ambartzumian, Yu. A. Gorokhov, V. S. Letokhov, G. N. Makarov, and A. A. Puretzkii, Pis'ma Zh. Eksp. Teor. Fiz. 23, 26 (1976) [JETP Lett. 23, 22 (1976)]; S. Mukamel and J. Jortner, Chem. Phys. Lett. 40, 150 (1976); C. D. Cantrell and H. W. Galbraith, to be published; V. S. Letokhov and A. Makarov, to be published. 5 K. L. Kompa and co-workers have recently studied multiphoton dissociation with crossed molecular and laser beams. They have found a 30% depletion of SF 6 in the beam by the laser field. However, limited by The measurement of the variation of total scat- tering cross section with polarization in state- selected beams has given information about the angular dependence of intermolecular potentials. 1 * 2 However, the effects of change of polarization up- on the received signal are small. Because long integrating times are necessary, it is difficult to obtain the velocity dependence of the difference in total cross section due to polarization, which gives further information about the radial depen- dency of the angular dependent terms in the inter- molecular potential. This laboratory has recent- ly performed measurements of the velocity vari- ation of the differential cross section for atom- atom 3 and atom-molecule 4 scattering. LiF-Ar was studied 4 in the thermal energy range under conditions of very high resolution in which all features of the quantum mechanical scattering were observable, although the rotational states of the molecule were thermally distributed. Ob- servation of damped high-frequency (diffraction) and low-frequency (rainbow) oscillations 5 permit- ted a determination of the spherically averaged potential and some characteristics of the angular dependent potential. The present experiments combine the observational advantages of both state selection and high-resolution differential their apparatus, they have not been able to carry out the type of measurements reported in this Letter. See K. L. Kompa, in Proceedings of the Conference on La- ser Chemistry, Steamboat Springs, Colorado, Febru- ary, 1976 (to be published). 6 N. K. Isenor and M. C. Richardson, Appl. Phys. Lett. 18, 225 (1971); N. K. Isenor, V. Merchant, R. S. Halls- worth, and Mo C. Richardson, Can. J. Phys. 51, 281 (1973); E. Yablonovitch, to be published. 7 See, for example, K. Shobatake, Y. T. Lee, and S. A. Rice, J. Chem. Phys. ^9,' 6104 (1973). cross sections. The use of a seeded high-temperature super- sonic source to cool the LiF beam translationally ^ and rotationally, and the use of quadrupole fields i- to provide finite-aperture (f/100) optics made possible the observation of the differential cross i section of a state-selected molecular beam at in- tensities of about ^-that of a non-state-selected molecular beam previously used and with no sac- rifice in geometrical angular resolution. A super- :- sonic Ar jet carried 0.5-Torr superheated LiF as it expanded from 300 Torr at 1750 K from an in- directly heated graphite oven through a 0.15-mm thin orifice. It was skimmed by a 50° full angle, 0.5-mm-diam truncated cone. 6 Rotational and translational temperatures ranged from 25 to 85 K. Differential pumping was provided by a 100-1/ sec booster and by 4200- and 200-l/sec diffusion pumps, sequentially, to a pressure of lxlO" 7 Torr in the first refocuser chamber. The cham- ber containing the second refocuser and the scat- tering apparatus was pumped by a 600 l/sec ion pump supplemented by liquid hydrogen cryopump- ing and Ti sublimation pumping to a pressure of 3xl0~ 8 Torr. The state selectors were two 180° refocusing "three-quarter" quadrupoles together with apertures for velocity selection (8.5% stan- Observation of High-Resolution Differential Cross Section of Polarized, Rotationally State-Selected LiF on Art Len-Yuan Tsou, Daniel Auerbach, and Lennard Wharton Department of Chemistry and James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (Received 9 August 1976) We report high-resolution differential cross sections at thermal energy for polarized, rotationally state-selected LiF on Ar. They were 2-8% larger with LiF in the J= \M\ = 1 orientation along the relative velocity than in the J= 1 unpolarized state. The differential cross section in the present case is much more sensitive to change in polarization than is the total cross section. The high-frequency oscillations of the differential cross sec- tions were found substantially independent of J when the present </= 1 data were compared with non-state-selected data for J= 15 or J = 25. 20