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Why does thallium produce a green flame in a Bunsen burner?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — boron group element Modern version of the periodic table of the elements (printable).

Thallium produces a brilliant green flame in a Bunsen burner because of the specific spectral line it emits when its atoms are excited by the heat of the flame.1

When a sample containing thallium is introduced into a flame, the thermal energy causes the atoms to become electronically excited. As these excited atoms return to their lower energy states (ground states), they release energy in the form of light at characteristic frequencies. In the case of thallium, this emission occurs at a wavelength that corresponds to a distinct green color. This phenomenon is a fundamental principle of spectrochemical analysis, where the characteristic light emitted by excited atoms is used to identify the elemental composition of a substance.213