Why do gaseous molecules produce broad bands of light rather than sharp spectral lines?

Gaseous molecules produce broad bands of light rather than sharp spectral lines because their energy states are more complex than those of individual atoms. While atoms primarily exhibit electronic energy levels, molecules possess additional, closely spaced energy levels resulting from vibrational and rotational motions.123

When a molecule undergoes an electronic transition—moving an electron from one molecular orbital to another—this change is almost always accompanied by simultaneous transitions between these various vibrational and rotational states. Because these levels are numerous and closely spaced, the resulting spectrum consists of a vast number of transitions that appear as a broad band of light rather than the distinct, sharp lines characteristic of atomic spectra. Under high-resolution conditions, these bands can be resolved into fine structure, but under standard conditions, they manifest as continuous bands of electromagnetic radiation.45