1 Introduction

The physics of highly ionized atoms is not only a field of extraordinary richness in specific physical phenomena, but also a part of atomic physics that cross-correlates with many other scientific disciplines such as astrophysics, plasma physics, heavy ion physics, etc. Apart from its pure scientific importance, the physics of highly charged ions also has strong impact on research in several applied physics and technological areas (for example in fusion research and materials science) which provides a further stimulus for the development of this field.

One of the properties of highly ionized atoms that is a subject of considerable interest for many fields, such astrophysics and fusion plasma research, is the transition probability between different electronic levels and, more specifically, the oscillator strength.

For many years we have applied the Quantum Defect Orbital method, both in its non-relativistic (QDO) (Simons 1974) and its relativistic (RQDO) (Martín & Karwowski 1991) versions, to the calculation of oscillator strengths and photoionization cross sections of a rather large number of atomic species, including several isoelectronic sequences (see, e.g. Charro et al. 1996, 1997).

One of the isoelectronic series of which the heavier species have not been so far studied, to our knowledge, is that of the arsenic atom. As I has itself been the object of great interest in the last few years (see e.g. Bengston et al. 1992), given the detection of its $\rm 4p^2\ 5s\ {}^4P_{3/2}-4p^3\ {}^4S^o_{3/2}$ line in the spectrum of Chi Lupi, a HgMn type of star, through a spectrometer on board the Hubble Space Telescope. Other atomic systems that have been found to exist in different ionization degrees in Chi Lupi are Sr, Y, Zr, Ru and Pd (Leckrone et al. 1996), which are potentially members of the As I isoelectronic sequence, although there are so far no reports on the existence of these elements in As-like form.

For the reasons described above, we have undertaken the study of the intensities of all the allowed lines of $\rm 4p^3- 4p^2\ 5s$ transition array in a number of As-like ions ranging from BrIII (Z = 35) to LaXXV (Z = 57). Two methodologies have been followed: the aforementioned RQDO method and the multiconfiguration Dirac-Fock (MCDF) formalism with the GRASP code written by Grant et al. (1980, 1989).