For atomic spectra, atomic gas or vapor at a pressure that is much _ than atmospheric pressure is excited.

Explanation

Atomic spectra are produced when the atoms of an element in a gaseous state are excited to a higher energy level, and then they emit radiation when they return to their original energy state. If the pressure is increased to atmospheric pressure or above, the excited atoms will collide more frequently with other atoms or molecules in the gas, which can cause energy to be lost in the collision instead of being emitted as radiation. This collisional de-excitation process will produce a broad, continuous spectrum, which will obscure the atomic spectral lines. Therefore, atomic spectra are obtained at low-pressure conditions, where the collision frequency is low, and the probability of radiative de-excitation is high.