The broadening of the ultraviolet absorption bands of xenon under pressure

By a series of experiments reported in a previous publication of the authors, it was shown that for the monatomic gas xenon, the absorption at the resonance wave-length λ 1469 A. (1 S0 —3 P°1 ) exhibited an unusually large broadening with increase of pressure. With the gas at a pressure of 2 mm. mercury, in a cell with fluorite windows spaced 2 mm. apart, the absorption appeared as a sharp line at λ 1469 A. As the pressure of the xenon was increased at a constant temperature of 12° C. the absorption rapidly widened out into a continuous band. At a pressure of 50 atmospheres, the band was 156 Angstroms wide and extended from λ 1484 A. to λ 1428 A. The spreading out was definitely asymmetrical towards the longer wave-lengths. The shift of the long wave-length absorption limit corresponded to an energy change of 0·7 volts, and that of the short wave-length limit to 0·3 volts. The type of absorption obtained with xenon was found to be similar to that obtained with the vapours of zinc, cadmium and mercury. In all spectra the absorption occurred at the resonance lines (1 S0 -3 p°1 ;1 s0 -1 p°1 ) and developed asymmetrically to the longer wave-lengths. It is significant that all these atoms have similar characteristics. Xenon, as well as the metal vapours, is known to approximate closely to the ideal monatomic gas. The normal state of each of the atoms by virtue of completed electron shells, is a1 S0 . The first excited state to which the atoms can be raised by absorption of light is the3 P°1 of the3 P°012 group and the next higher state is the1 P°1 associated with the triplet group. The transitions1 S0 —3 P°1 and1 S0 —1 P°1 from these two states to the normal state give rise to the resonance lines, the latter of which has the shorter wave-length. For the metal vapours the shorter resonance line is the more absorbed, and has a much greater development of an associated absorption band.