Infrared spectrum of the electron bubble in liquid helium

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Abstract
The energy of the ground-state–to–first-excited-state electronic transition in the electron bubble in liquid helium has been measured at 1.3 K and found to be 0.122 eV at a pressure of 1.1 atm and increasing to 0.209 eV at 18.3 atm. The spherical-square-well model of the electron bubble accounts well for the transition energies if the effective surface tension is taken to be independent of pressure. This model also yields improved values for the electron bubble radius as a function of pressure. The position of a line in the P-T plane where the photoconductivity signal vanishes indicates that trapping of electron bubbles on vorticity plays a role in the detection mechanism.