This paper reports the first observation of the Jacobi shape transition in ^31P using high energy γ-rays from the decay of giant dipole resonance (GDR) as a probe. The measured GDR spectrum in the decay of ^31P shows a distinct low energy component around 10 MeV, which is a clear signature of Corioli's splitting in a highly deformed rotating nucleus. Interestingly, a self-conjugate α-cluster nucleus ^28Si, populated at similar initial excitation energy and angular momentum, exhibits a vastly

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... ferent GDR line shape. Even though the angular momentum of the compound nucleus ^28Si is higher than the critical angular momentum required for the Jacobi shape transition, the GDR lineshape is akin to a prolate deformed nucleus. Considering the present results for ^28Si and similar observation recently reported in ^32S, it is proposed that the nuclear orbiting phenomenon exhibited by α-cluster nuclei hinders the Jacobi shape transition. The present experimental results suggest a possibility to investigate the nuclear orbiting phenomenon using high energy γ-rays as a probe.