Quantum Evaporation
What Is Quantum Evaporation?
Quantum Evaporation
(click to enlarge)
Inside a body of He II exist quasiparticles, or phonons and rotons. When energy from other sources is deposited into He II, quasiparticles form. When a quasiparticle travels to the helium/vacuum interface, it is possible for it to interact with helium atoms there. Assuming that one phonon/roton interacts with a single atom and the quasiparticle's energy E exceeds the binding energy per atom, Eb, then the atom would be ejected into the vacuum space with kinetic energy E − Eb. This process is called “quantum evaporation”. It might be helpful to think about it in analogy with the photoelectric effect. Meanwhile, it is worth noting that:
Helium Dispersion Curve
(click to enlarge)
- Phonons with energy < 7.15 K do not meet the energy criterion;
- Lower wavenumber rotons, or R-s—rotons with negative group velocities—have a much lower probability of ejecting atoms;
- Higher wavenumber rotons, or R+s, can eject atoms, provided their incident angles are smaller than a critical angle (~ 17°). Beyond that, total reflection occurs.
Detection of Quantum Evaporation
When a helium atom is evaporated into vacuum, it travels balistically, with speed determined by kinetic energy. Since energy of qusiparticles is typically a few Kelvins, helium atom travels at speed at the order of 100 m/s. If an absorber is placed on its path, the atom will be absorbed to the surface upon impact and release its kinetic energy and more significantly, a binding energy of ~100 K in the form of heat, which effectively amplifies the signal. By monitoring the temperature change of the absorber, one can obtain the original distribution of quasiparticles from signal amplitude and time of flight information.