The infrared spectrum of liquid and solid hydrogen

The study of the infrared spectrum of compressed hydrogen gas has been very fruitful in delineating the characteristics of pressure-induced absorption. This type of absorption has its origin in the asymmetric distortion of the electron distributions of molecules during collisions, as a consequence of which vibrations and rotations, inactive in the free molecule for reasons of symmetry, can become active in the pressurized gas. At lower pressures the absorption is conditioned by binary collisions, but at higher pressures ternary and higher order collisions become important and have a marked influence on the spectrum. These effects in hydrogen gas have been studied over a wide range of pressures and temperatures; for example, at room temperature the experiments have been extended up to 5000 arm where the gas density is greater than that of the liquid at its normal boiling point [1,2]. It was then natural to inquire how the collision-induced spectrum changes in going to the condensed phases. The experiments on liquid and solid hydrogen have a partitular interest which arises from the fact that, as "~ consequence of the lower thermal energies at low temperatures, tim spectral lines are relatively sharp and much more detail can be observed than for the gas at room temperature. In the following paragraphs the main results of the obserw~tions on the spectrum of liquid and solid hydrogen will be summarized. The earlier work at low resolution [3-5] has now been supplemented by recent experiments with a high resolution grating spectrometer. The spectra thus obt~ined show a wealth of detail which has not yet been fully interpreted. Fig. l(a) shows the fundamental vibrational band of solid normal hydrogen at 11 ~ obtained with a spectral resolution of 20 cm -1. The designations used for the various components follow, where possible, the nomenclature of mole('ular spectroscopy. For the (2 components the vibrational transition is un