ON THE PHOTOEXCITATION OF THE 2S-]2P TRANSITION IN LIGHT MUONIC ATOMS

It is known that the energy of the 2s --> 2p transition of light mu-atoms can be precisely measured by laser spectroscopy. Such measurements provide a good test of quantum electrodynamics predictions. Here we consider how the hyperfine muon-nucleus interaction effects on the photoexcitation of this transition and on the subsequent X-rays emission. Besides the obvious change of the transition energy the hyperfine interaction mixes the 2p1/2 and 2p3/2 orbits. This mixing is rather effective in mu-atoms of H-1, Li-7, Be-9, B-10 and B-11. Its taking into account changes the photoexcitation cross-section of electric dipole transitions between the hyperfine components of the 2s and 2p orbits and an angular X-rays distribution as well. These changes prove to be considerable for some transitions. For example, in mu H-2 and mu Li-7 the cross-section is decreased by factors of two and four respectively. In mu Be-9 it is increased by a factor of two. Moreover, in mu 1H the angular X-rays distribution becomes more anisotropic. Besides the above subject one more question is discussed here. The fact is that the laser experiments can give an information on the residual muon spin polarization at the 2s orbit and on the degree of alignment of the angular mu-atom momentum in this state. The polarization can be determined by measuring an angular correlation between the X-rays emission direction and the momentum of a hard mu-decay electron. The alignment degree can be found by measuring an anisotropy of the angular X-rays distribution.