The time constant T2 is usually much smaller than T1, and the bandwidth of the absorption spectrum is related to these time constants by the Fourier transform, so the time constant T2 is a main contributor to the bandwidth. The time constant T2 has been measured with ultrafast time-resolved spectroscopy directly, such as in photon echo experiments. What is the dephasing rate of a particle that has an energy E if it is subject to a fluctuating environment that has a temperature T? In particular what is the dephasing rate close to equilibrium (E~T), and what happens in the zero temperature limit? This question has fascinated the mesoscopic community during the last two decades (see references below). See also Dephasing rate SP formula References Other (And references therein.) Wave mechanics Quantum optics Quantum information science Mesoscopic physics