The process by which a monochromatic laser excites hydrogen atoms from the n=2 state to the n=5 state is known as two-photon absorption. It is an important component of many experiments, such as those that measure nuclear magnetic resonance or study molecular motion. For these types of experiments, it can be quite difficult to get excited states of hydrogen with a single photon due to energy and momentum conservation laws.
The two-photon absorption process is also used to measure the hyperfine splitting of hydrogen. The laser beam used in the experiment is monochromatic, meaning that it only has a single wavelength. This type of light can be created using an argon-ion or krypton-gas filled lamp, which emits all wavelengths equally well and are stable over long periods of time.
To get hydrogen to absorb energy from this laser, a molecular gas cell containing hydrogen must be placed inside the vacuum chamber before turning on the argon-ion or krypton lamps. The atoms will then absorb photons from both types of sources at once.
Hydrogen’s hyperfine splitting arises because its electron orbitals have different energies for ground state electrons with even (n=even) vs odd (n=odd) quantum numbers n.