Bound-bound transitions occur when the ions and neutral atoms collide, transferring an electron from the atom to the ion. This method predominates at low current-densities, and is responsible for producing the spectral-line emission. Free-bound transitions happen when an ion captures a free electron. This method produces the continuum emission, and is more prominent at higher current-densities. Some of the continuum is also produced when an electron accelerates toward an ion, called free-free transitions, producing bremsstrahlung radiation. Bremsstrahlung radiation increases with increasing energy density, and causes a shift toward the blue and ultraviolet end of the spectrum.