Influence of autoionisation and predissociation on the photoelectron parameters in HBr

Abstract

The photoionisation spectrum of HBr is strongly influenced in the 100-85 nm wavelength range by the autoionisation of Rydberg series converging to the A ² Sigma ⁺ ionic state. These Rydberg series are also partly predissociated. Using a unified multichannel quantum-defect theory treatment of autoionisation and predissociation processes, both the photoionisation and photodissociation cross sections are calculated. The photodissociation cross section is predicted to be 10 times larger than for HCl, i.e. about 10 Mb. The photoelectron angular distribution parameter beta is predicted and the influence of predissociation on its value is pointed out. The spin-polarisation parameter A (or P) is calculated. It shows oscillatory structure related to the autoionisation of the ¹ Pi states. Using circularly polarised synchrotron radiation, this parameter has been measured for both fine-structure states HBr⁺ X ² Pi _3/2 (v=0) and X ² Pi _1/2 (v=0). The agreement between calculated and measured values is relatively good.