Fine-structure resolved measurements of photoelectron angular ...

PHYSICAL REVIEW A

VOLUME 59, NUMBER 6

JUNE 1999

Fine-structure resolved measurements of photoelectron angular distributions of Ge2 at visible photon wavelengths W. W. Williams, D. L. Carpenter, A. M. Covington, and J. S. Thompson Department of Physics and Chemical Physics Program, University of Nevada, Reno, Nevada 89557-0058 ~Received 3 November 1998! The spectral dependence of the angular distributions of photoelectrons produced by the single-photon detachment of Ge2 ions have been measured at four discrete photon wavelengths ranging from 457.9 to 647.1 nm ~2.71–1.92 eV! using a crossed laser-ion beams apparatus. Values of the fine-structure resolved asymmetry parameters have been determined by fits to the photoelectron yield as a function of the angle between the laser polarization vector and the linear momentum vector of the collected photoelectrons. The measured asymmetry parameters for Ge2 are compared to previous asymmetry parameter measurements for C2 and Si2 . The measurements were also fit to a previously reported model for photoelectron asymmetry parameters for photodetaching p-orbital electrons from negative ions. @S1050-2947~99!00306-6# PACS number~s!: 32.80.Gc, 32.10.2f, 33.60.2q

I. INTRODUCTION

Investigations of the photodetachment process for negative ions provide a unique opportunity to examine electrons liberated from a neutral core. Fundamental experimental and theoretical studies of the photodetachment of negative ions @1–3# provide information necessary to characterize the initial and final states of the target species, as well as dynamical information pertaining to the mutual interaction of collision partners @4#. Subtle interactions such as electron correlation in the initial and final states of the photon-negative ion collision, which might be overshadowed by the long-range Coulomb potential present in the photoionization of either neutral or positively charged ionic species, can be studied in photodetachment processes. The most general form of the angular distribution of a collision process for an unpolarized target can be summarized by the following theorem derived by Yang @5#: ‘‘If only incoming waves of orbital angular momentum L contribute appreciably to the reaction, the angular distribution of the outgoing particles in the center of mass system is an even polynomial of cos u with maximum exponent not higher than

b5

2L.’’ Here u is the angle between the incoming and outgoing particles in the center-of-mass system. Cooper and Zare @6,7# developed a negative ion specific form of the differential cross section for the production of photoelectrons detached from a randomly polarized target by incident linearly polarized light. The differential cross section can be written in the dipole approximation as

F

G

ds s b 5 11 ~ 3 cos2 u 21 ! , dV 4 p 2

where s is the total photodetachment cross section at a given photon energy, u is the angle between the polarization vector of the photon and the momentum vector of the photodetached electron, and b is the asymmetry parameter, which completely characterizes the shape of the photoelectron emission pattern. The differential cross section must be nonnegative, which restricts the range of the asymmetry parameter 21< b