Final answer:
In UV-VIS spectroscopy, a heteronuclear diatomic molecule like HBr likely absorbs light due to a transition between bonding and anti-bonding orbitals, an electronic absorption transition.
Step-by-step explanation:
In UV-VIS spectroscopy, light absorption for a heteronuclear diatomic molecule like HBr is most likely due to a transition between bonding and anti-bonding orbitals. This type of transition involves an electron absorbing energy and moving from a lower energy bonding orbital to a higher energy anti-bonding orbital, also known as an electronic absorption transition. Transitions involving electron spin flips or within the same energy level generally do not lead to light absorption in UV-VIS spectroscopy. Transitions from an excited state to the ground state are typically involved in emission processes rather than absorption.
Understanding that a molecule will absorb specific wavelengths of light that correspond to the energy difference between the involved molecular orbitals is key to molecular spectroscopy. This is why molecules with conjugated systems, where the energy gap is smaller for "π -> π*" transitions, are often studied using UV-VIS spectroscopy as they absorb light in the UV-visible range.