PHYSICAL REVIEW B VOLUME 25, NUMBER 6 15 MARCH 1982 Observation of rotational polarization produced in molecule-surface collisions A. C. Luntz, A. W. Kleyn, and D. J. Auerbach IBM Research Laboratory, San Jose, California 95193 (Received 24 August 1981; revised manuscript received 25 February 1982) The rotational polarization produced by scattering a rotationally cold beam of NO from Ag(111) has been measured by laser-induced fluorescence. A strong rotational po- larization perpendicular to the surface normal is observed. The degree of polarization de- pends strongly on final rotational state, incident energy, and incident angle. Molecular-beam scattering experiments on sur- faces have focused primarily on measurements of angular and velocity distributions of the scattered particles. Quite recently, measurements of final internal state distributions of the scattered particles have become available by laser-induced-fluores- cence (LIF) detection. ' In gas-phase scattering experiments, in addition to these techniques, direc- tional or vector properties are also accessible and provide information on many new features of the dynamics. For example, in gas-phase reactive scattering both the dependence of reactant orienta- tion on reaction probability and the spatial align- ment or polarization of product rotation have been measured. We report here observation of the rotational polarization produced by scattering a ro- tationally cold beam of NO from an Ag(111) sur- face. A strong dependence of the degree of polari- zation is found on final rotational quantum num- ber J and initial angle of incidence 8;. These re- sults clearly demonstrate that many new aspects of the surface-molecule dynamics can be revealed by measuring vector properties. The experiments are based on measuring the dis- tribution of orientations of the final angular momentum vector J, by observing the dependence of LIF intensity on the direction of linear polariza- tion of the exciting radiation. Any anisotropic dis- tribution of a given J can be described classically by the distribution function n (8, $) where 8 and tb are the polar angles of J relative to some axis such as the surface normal n. Since the azimuthal dependence of the surface corrugation is expected to be quite small, we assume that n (8, $) is isotro- pic in P and can be represented generally by the Legendre expansion n(8)= gb~Pt(cos8), 1=p where 0 is the angle between J and n. The polarization properties inherent in LIF detection are capable of measuring several mo- ments of this distribution. ' In the experiments reported here, the detected fluorescence is spectral- ly unresolved, representing an unweighted sum over P, Q, and R branches. In this case, the fluorescence is isotropic, and the LIF intensity I depends only on absorption, i. e. , I- f n(8) ~ u el, i sin8d8, (2) 2 &2 I(8p} bp 1+ P2(cos8p) 5 bp (3) where ep is the angle of eL relative to n. The total population of the state bp is measured when 8p=0. 955, i. e. , the "magic angle" where P2(cos8p) =0. The polarization anisotropy 9' is given by b2 I(0) — 1(m/2) bp I(0)+2I(tr/2) (4) where — 2.5 ( H & 5. 0. These two limits corre- spond to perfect alignment of J perpendicular to and parallel to n, respectively. Thus, measure- ments of I(8p} yield both the population and the polarization anisotropy for the state J. Small J- dependent corrections are necessary for the coeffi- cients —, in Eq. (3) and 5 in Eq. (4) due to the spectral overlap of the weak P2~ or 8 ~2 bands with the Q branch transitions. where u is the transition dipole and eL is the direc- tion of the linear polarized laser. For LIF detec- tion of NO via the X+ — II electronic transition, u is paralllel to J for Q branch absorptions. In the classical limit (J » 1), evaluation of Eq. (2) with the distribution given in Eq. (1) yields for Q branch transitions 25 4273